This is a record of all past Hypothesis releases and what went into them, in reverse chronological order. All previous releases should still be available on pip.

Hypothesis APIs come in three flavours:

  • Public: Hypothesis releases since 1.0 are semantically versioned with respect to these parts of the API. These will not break except between major version bumps. All APIs mentioned in this documentation are public unless explicitly noted otherwise.
  • Semi-public: These are APIs that are considered ready to use but are not wholly nailed down yet. They will not break in patch releases and will usually not break in minor releases, but when necessary minor releases may break semi-public APIs.
  • Internal: These may break at any time and you really should not use them at all.

You should generally assume that an API is internal unless you have specific information to the contrary.

3.18.3 - 2017-08-17

This release should improve the performance of some tests which experienced a slow down as a result of the 3.13.0 release.

Tests most likely to benefit from this are ones that make extensive use of min_size parameters, but others may see some improvement as well.

3.18.2 - 2017-08-16

This release fixes a bug introduced in 3.18.0. If the arguments whitelist_characters and blacklist_characters to characters() both contained elements, then an InvalidArgument exception would be raised.

Thanks to Zac Hatfield-Dodds for reporting and fixing this.

3.18.1 - 2017-08-14

This is a bug fix release to fix issue #780, where sets() and similar would trigger health check errors if their element strategy could only produce one element (e.g. if it was just()).

3.18.0 - 2017-08-13

This is a feature release:

  • characters() now accepts whitelist_characters, particular characters which will be added to those it produces. (issue #668)
  • A bug fix for the internal function _union_interval_lists(), and a rename to _union_intervals(). It now correctly handles all cases where intervals overlap, and it always returns the result as a tuple for tuples.

Thanks to Alex Willmer for these.

3.17.0 - 2017-08-07

This release documents the previously undocumented phases feature”, making it part of the official public API. It also updates how the example database is used. Principally:

  • A Phases.reuse argument will now correctly control whether examples from the database are run (it previously did exactly the wrong thing and controlled whether examples would be saved).
  • Hypothesis will no longer try to rerun all previously failing examples. Instead it will replay the smallest previously failing example and a selection of other examples that are likely to trigger any other bugs that will found. This prevents a previous failure from dominating your tests unnecessarily.
  • As a result of the previous change, Hypothesis will be slower about clearing out old examples from the database that are no longer failing (because it can only clear out ones that it actually runs).

3.16.1 - 2017-08-07

This release makes an implementation change to how Hypothesis handles certain internal constructs.

The main effect you should see is improvement to the behaviour and performance of collection types, especially ones with a min_size parameter. Many cases that would previously fail due to being unable to generate enough valid examples will now succeed, and other cases should run slightly faster.

3.16.0 - 2017-08-04

This release introduces a deprecation of the timeout feature. This results in the following changes:

  • Creating a settings object with an explicit timeout will emit a deprecation warning.
  • If your test stops because it hits the timeout (and has not found a bug) then it will emit a deprecation warning.
  • There is a new value unlimited which you can import from hypothesis. settings(timeout=unlimited) will not cause a deprecation warning.
  • There is a new health check, hung_test, which will trigger after a test has been running for five minutes if it is not suppressed.

3.15.0 - 2017-08-04

This release deprecates two strategies, choices() and streaming().

Both of these are somewhat confusing to use and are entirely redundant since the introduction of the data() strategy for interactive drawing in tests, and their use should be replaced with direct use of data() instead.

3.14.2 - 2017-08-03

This fixes a bug where Hypothesis would not work correctly on Python 2.7 if you had the typing module backport installed.

3.14.1 - 2017-08-02

This raises the maximum depth at which Hypothesis starts cutting off data generation to a more reasonable value which it is harder to hit by accident.

This resolves (issue #751), in which some examples which previously worked would start timing out, but it will also likely improve the data generation quality for complex data types.

3.14.0 - 2017-07-23

Hypothesis now understands inline type annotations (issue #293):

  • If the target of builds() has type annotations, a default strategy for missing required arguments is selected based on the type. Type-based strategy selection will only override a default if you pass hypothesis.infer as a keyword argument.
  • If @given wraps a function with type annotations, you can pass infer as a keyword argument and the appropriate strategy will be substituted.
  • You can check what strategy will be inferred for a type with the new from_type() function.
  • register_type_strategy() teaches Hypothesis which strategy to infer for custom or unknown types. You can provide a strategy, or for more complex cases a function which takes the type and returns a strategy.

3.13.1 - 2017-07-20

This is a bug fix release for issue #514 - Hypothesis would continue running examples after a SkipTest exception was raised, including printing a falsifying example. Skip exceptions from the standard unittest module, and pytest, nose, or unittest2 modules now abort the test immediately without printing output.

3.13.0 - 2017-07-16

This release has two major aspects to it: The first is the introduction of deferred(), which allows more natural definition of recursive (including mutually recursive) strategies.

The second is a number of engine changes designed to support this sort of strategy better. These should have a knock-on effect of also improving the performance of any existing strategies that currently generate a lot of data or involve heavy nesting by reducing their typical example size.

3.12.0 - 2017-07-07

This release makes some major internal changes to how Hypothesis represents data internally, as a prelude to some major engine changes that should improve data quality. There are no API changes, but it’s a significant enough internal change that a minor version bump seemed warranted.

User facing impact should be fairly mild, but includes:

  • All existing examples in the database will probably be invalidated. Hypothesis handles this automatically, so you don’t need to do anything, but if you see all your examples disappear that’s why.
  • Almost all data distributions have changed significantly. Possibly for the better, possibly for the worse. This may result in new bugs being found, but it may also result in Hypothesis being unable to find bugs it previously did.
  • Data generation may be somewhat faster if your existing bottleneck was in draw_bytes (which is often the case for large examples).
  • Shrinking will probably be slower, possibly significantly.

If you notice any effects you consider to be a significant regression, please open an issue about them.

3.11.6 - 2017-06-19

This release involves no functionality changes, but is the first to ship wheels as well as an sdist.

3.11.5 - 2017-06-18

This release provides a performance improvement to shrinking. For cases where there is some non-trivial “boundary” value (e.g. the bug happens for all values greater than some other value), shrinking should now be substantially faster. Other types of bug will likely see improvements too.

This may also result in some changes to the quality of the final examples - it may sometimes be better, but is more likely to get slightly worse in some edge cases. If you see any examples where this happens in practice, please report them.

3.11.4 - 2017-06-17

This is a bugfix release: Hypothesis now prints explicit examples when running in verbose mode. (issue #313)

3.11.3 - 2017-06-11

This is a bugfix release: Hypothesis no longer emits a warning if you try to use sampled_from() with collections.OrderedDict. (issue #688)

3.11.2 - 2017-06-10

This is a documentation release. Several outdated snippets have been updated or removed, and many cross-references are now hyperlinks.

3.11.1 - 2017-05-28

This is a minor ergonomics release. Tracebacks shown by pytest no longer include Hypothesis internals for test functions decorated with @given.

3.11.0 - 2017-05-23

This is a feature release, adding datetime-related strategies to the core strategies.

timezones() allows you to sample pytz timezones from the Olsen database. Use directly in a recipe for tz-aware datetimes, or compose with none() to allow a mix of aware and naive output.

The new dates(), times(), datetimes(), and timedeltas() strategies are all constrained by objects of their type. This means that you can generate dates bounded by a single day (i.e. a single date), or datetimes constrained to the microsecond.

times() and datetimes() take an optional timezones= argument, which defaults to none() for naive times. You can use our extra strategy based on pytz, or roll your own timezones strategy with dateutil or even the standard library.

The old dates, times, and datetimes strategies in hypothesis.extra.datetimes are deprecated in favor of the new core strategies, which are more flexible and have no dependencies.

3.10.0 - 2017-05-22

Hypothesis now uses inspect.getfullargspec() internally. On Python 2, there are no visible changes.

On Python 3 @given and @composite now preserve PEP 3107 annotations on the decorated function. Keyword-only arguments are now either handled correctly (e.g. @composite), or caught in validation instead of silently discarded or raising an unrelated error later (e.g. @given).

3.9.1 - 2017-05-22

This is a bugfix release: the default field mapping for a DateTimeField in the Django extra now respects the USE_TZ setting when choosing a strategy.

3.9.0 - 2017-05-19

This is feature release, expanding the capabilities of the decimals() strategy.

  • The new (optional) places argument allows you to generate decimals with a certain number of places (e.g. cents, thousandths, satoshis).
  • If allow_infinity is None, setting min_bound no longer excludes positive infinity and setting max_value no longer excludes negative infinity.
  • All of NaN, -Nan, sNaN, and -sNaN may now be drawn if allow_nan is True, or if allow_nan is None and min_value or max_value is None.
  • min_value and max_value may be given as decimal strings, e.g. "1.234".

3.8.5 - 2017-05-16

Hypothesis now imports sqlite3 when a SQLite database is used, rather than at module load, improving compatibility with Python implementations compiled without SQLite support (such as BSD or Jython).

3.8.4 - 2017-05-16

This is a compatibility bugfix release. sampled_from no longer raises a deprecation warning when sampling from an Enum, as all enums have a reliable iteration order.

3.8.3 - 2017-05-09

This release removes a version check for older versions of pytest when using the Hypothesis pytest plugin. The pytest plugin will now run unconditionally on all versions of pytest. This breaks compatibility with any version of pytest prior to 2.7.0 (which is more than two years old).

The primary reason for this change is that the version check was a frequent source of breakage when pytest change their versioning scheme. If you are not working on pytest itself and are not running a very old version of it, this release probably doesn’t affect you.

3.8.2 - 2017-04-26

This is a code reorganisation release that moves some internal test helpers out of the main source tree so as to not have changes to them trigger releases in future.

3.8.1 - 2017-04-26

This is a documentation release. Almost all code examples are now doctests checked in CI, eliminating stale examples.

3.8.0 - 2017-04-23

This is a feature release, adding the iterables() strategy, equivalent to lists(...).map(iter) but with a much more useful repr. You can use this strategy to check that code doesn’t accidentally depend on sequence properties such as indexing support or repeated iteration.

3.7.4 - 2017-04-22

This is a bug fix release for a single bug:

  • In 3.7.3, using @example and a pytest fixture in the same test could cause the test to fail to fill the arguments, and throw a TypeError.

3.7.3 - 2017-04-21

This release should include no user visible changes and is purely a refactoring release. This modularises the behaviour of the core given() function, breaking it up into smaller and more accessible parts, but its actual behaviour should remain unchanged.

3.7.2 - 2017-04-21

This reverts an undocumented change in 3.7.1 which broke installation on debian stable: The specifier for the hypothesis[django] extra_requires had introduced a wild card, which was not supported on the default version of pip.

3.7.1 - 2017-04-21

This is a bug fix and internal improvements release.

  • In particular Hypothesis now tracks a tree of where it has already explored. This allows it to avoid some classes of duplicate examples, and significantly improves the performance of shrinking failing examples by allowing it to skip some shrinks that it can determine can’t possibly work.
  • Hypothesis will no longer seed the global random arbitrarily unless you have asked it to using random_module()
  • Shrinking would previously have not worked correctly in some special cases on Python 2, and would have resulted in suboptimal examples.

3.7.0 - 2017-03-20

This is a feature release.

New features:

  • Rule based stateful testing now has an @invariant decorator that specifies methods that are run after init and after every step, allowing you to encode properties that should be true at all times. Thanks to Tom Prince for this feature.
  • The decimals() strategy now supports allow_nan and allow_infinity flags.
  • There are significantly more strategies available for numpy, including for generating arbitrary data types. Thanks to Zac Hatfield Dodds for this feature.
  • When using the data() strategy you can now add a label as an argument to draw(), which will be printed along with the value when an example fails. Thanks to Peter Inglesby for this feature.

Bug fixes:

  • Bug fix: composite() now preserves functions’ docstrings.
  • The build is now reproducible and doesn’t depend on the path you build it from. Thanks to Chris Lamb for this feature.
  • numpy strategies for the void data type did not work correctly. Thanks to Zac Hatfield Dodds for this fix.

There have also been a number of performance optimizations:

  • The permutations() strategy is now significantly faster to use for large lists (the underlying algorithm has gone from O(n^2) to O(n)).
  • Shrinking of failing test cases should have got significantly faster in some circumstances where it was previously struggling for a long time.
  • Example generation now involves less indirection, which results in a small speedup in some cases (small enough that you won’t really notice it except in pathological cases).

3.6.1 - 2016-12-20

This release fixes a dependency problem and makes some small behind the scenes improvements.

  • The fake-factory dependency was renamed to faker. If you were depending on it through hypothesis[django] or hypothesis[fake-factory] without pinning it yourself then it would have failed to install properly. This release changes it so that hypothesis[fakefactory] (which can now also be installed as hypothesis[faker]) will install the renamed faker package instead.
  • This release also removed the dependency of hypothesis[django] on hypothesis[fakefactory] - it was only being used for emails. These now use a custom strategy that isn’t from fakefactory. As a result you should also see performance improvements of tests which generated User objects or other things with email fields, as well as better shrinking of email addresses.
  • The distribution of code using nested calls to one_of() or the | operator for combining strategies has been improved, as branches are now flattened to give a more uniform distribution.
  • Examples using composite() or .flatmap should now shrink better. In particular this will affect things which work by first generating a length and then generating that many items, which have historically not shrunk very well.

3.6.0 - 2016-10-31

This release reverts Hypothesis to its old pretty printing of lambda functions based on attempting to extract the source code rather than decompile the bytecode. This is unfortunately slightly inferior in some cases and may result in you occasionally seeing things like lambda x: <unknown> in statistics reports and strategy reprs.

This removes the dependencies on uncompyle6, xdis and spark-parser.

The reason for this is that the new functionality was based on uncompyle6, which turns out to introduce a hidden GPLed dependency - it in turn depended on xdis, and although the library was licensed under the MIT license, it contained some GPL licensed source code and thus should have been released under the GPL.

My interpretation is that Hypothesis itself was never in violation of the GPL (because the license it is under, the Mozilla Public License v2, is fully compatible with being included in a GPL licensed work), but I have not consulted a lawyer on the subject. Regardless of the answer to this question, adding a GPLed dependency will likely cause a lot of users of Hypothesis to inadvertently be in violation of the GPL.

As a result, if you are running Hypothesis 3.5.x you really should upgrade to this release immediately.

3.5.3 - 2016-10-05

This is a bug fix release.

Bugs fixed:

  • If the same test was running concurrently in two processes and there were examples already in the test database which no longer failed, Hypothesis would sometimes fail with a FileNotFoundError (IOError on Python 2) because an example it was trying to read was deleted before it was read. (issue #372).
  • Drawing from an integers() strategy with both a min_value and a max_value would reject too many examples needlessly. Now it repeatedly redraws until satisfied. (pull request #366. Thanks to Calen Pennington for the contribution).

3.5.2 - 2016-09-24

This is a bug fix release.

  • The Hypothesis pytest plugin broke pytest support for doctests. Now it doesn’t.

3.5.1 - 2016-09-23

This is a bug fix release.

  • Hypothesis now runs cleanly in -B and -BB modes, avoiding mixing bytes and unicode.
  • unittest.TestCase tests would not have shown up in the new statistics mode. Now they do.
  • Similarly, stateful tests would not have shown up in statistics and now they do.
  • Statistics now print with pytest node IDs (the names you’d get in pytest verbose mode).

3.5.0 - 2016-09-22

This is a feature release.

  • fractions() and decimals() strategies now support min_value and max_value parameters. Thanks go to Anne Mulhern for the development of this feature.
  • The Hypothesis pytest plugin now supports a –hypothesis-show-statistics parameter that gives detailed statistics about the tests that were run. Huge thanks to Jean-Louis Fuchs and Adfinis-SyGroup for funding the development of this feature.
  • There is a new event() function that can be used to add custom statistics.

Additionally there have been some minor bug fixes:

  • In some cases Hypothesis should produce fewer duplicate examples (this will mostly only affect cases with a single parameter).
  • py.test command line parameters are now under an option group for Hypothesis (thanks to David Keijser for fixing this)
  • Hypothesis would previously error if you used PEP 3107 function annotations on your tests under Python 3.4.
  • The repr of many strategies using lambdas has been improved to include the lambda body (this was previously supported in many but not all cases).

3.4.2 - 2016-07-13

This is a bug fix release, fixing a number of problems with the settings system:

  • Test functions defined using @given can now be called from other threads (issue #337)
  • Attempting to delete a settings property would previously have silently done the wrong thing. Now it raises an AttributeError.
  • Creating a settings object with a custom database_file parameter was silently getting ignored and the default was being used instead. Now it’s not.

3.4.1 - 2016-07-07

This is a bug fix release for a single bug:

  • On Windows when running two Hypothesis processes in parallel (e.g. using pytest-xdist) they could race with each other and one would raise an exception due to the non-atomic nature of file renaming on Windows and the fact that you can’t rename over an existing file. This is now fixed.

3.4.0 - 2016-05-27

This release is entirely provided by Lucas Wiman:

Strategies constructed by models() will now respect much more of Django’s validations out of the box. Wherever possible full_clean() should succeed.

In particular:

  • The max_length, blank and choices kwargs are now respected.
  • Add support for DecimalField.
  • If a field includes validators, the list of validators are used to filter the field strategy.

3.3.0 - 2016-05-27

This release went wrong and is functionally equivalent to 3.2.0. Ignore it.

3.2.0 - 2016-05-19

This is a small single-feature release:

  • All tests using @given now fix the global random seed. This removes the health check for that. If a non-zero seed is required for the final falsifying example, it will be reported. Otherwise Hypothesis will assume randomization was not a significant factor for the test and be silent on the subject. If you use random_module() this will continue to work and will always display the seed.

3.1.3 - 2016-05-01

Single bug fix release

  • Another charmap problem. In 3.1.2 text() and characters() would break on systems which had /tmp mounted on a different partition than the Hypothesis storage directory (usually in home). This fixes that.

3.1.2 - 2016-04-30

Single bug fix release:

  • Anything which used a text() or characters() strategy was broken on Windows and I hadn’t updated appveyor to use the new repository location so I didn’t notice. This is now fixed and windows support should work correctly.

3.1.1 - 2016-04-29

Minor bug fix release.

  • Fix concurrency issue when running tests that use text() from multiple processes at once (issue #302, thanks to Alex Chan).
  • Improve performance of code using lists() with max_size (thanks to Cristi Cobzarenco).
  • Fix install on Python 2 with ancient versions of pip so that it installs the enum34 backport (thanks to Donald Stufft for telling me how to do this).
  • Remove duplicated __all__ exports from hypothesis.strategies (thanks to Piët Delport).
  • Update headers to point to new repository location.
  • Allow use of strategies that can’t be used in find() (e.g. choices()) in stateful testing.

3.1.0 - 2016-03-06

  • Add a nothing() strategy that never successfully generates values.
  • sampled_from() and one_of() can both now be called with an empty argument list, in which case they also never generate any values.
  • one_of() may now be called with a single argument that is a collection of strategies as well as as varargs.
  • Add a runner() strategy which returns the instance of the current test object if there is one.
  • ‘Bundle’ for RuleBasedStateMachine is now a normal(ish) strategy and can be used as such.
  • Tests using RuleBasedStateMachine should now shrink significantly better.
  • Hypothesis now uses a pretty-printing library internally, compatible with IPython’s pretty printing protocol (actually using the same code). This may improve the quality of output in some cases.
  • As a ‘phases’ setting that allows more fine grained control over which parts of the process Hypothesis runs
  • Add a suppress_health_check setting which allows you to turn off specific health checks in a fine grained manner.
  • Fix a bug where lists of non fixed size would always draw one more element than they included. This mostly didn’t matter, but if would cause problems with empty strategies or ones with side effects.
  • Add a mechanism to the Django model generator to allow you to explicitly request the default value (thanks to Jeremy Thurgood for this one).

3.0.5 - 2016-02-25

  • Fix a bug where Hypothesis would now error on py.test development versions.

3.0.4 - 2016-02-24

  • Fix a bug where Hypothesis would error when running on Python 2.7.3 or earlier because it was trying to pass a bytearray object to struct.unpack() (which is only supported since 2.7.4).

3.0.3 - 2016-02-23

  • Fix version parsing of py.test to work with py.test release candidates
  • More general handling of the health check problem where things could fail because of a cache miss - now one “free” example is generated before the start of the health check run.

3.0.2 - 2016-02-18

  • Under certain circumstances, strategies involving text() buried inside some other strategy (e.g. text().filter(...) or recursive(text(), ...)) would cause a test to fail its health checks the first time it ran. This was caused by having to compute some related data and cache it to disk. On travis or anywhere else where the .hypothesis directory was recreated this would have caused the tests to fail their health check on every run. This is now fixed for all the known cases, although there could be others lurking.

3.0.1 - 2016-02-18

  • Fix a case where it was possible to trigger an “Unreachable” assertion when running certain flaky stateful tests.
  • Improve shrinking of large stateful tests by eliminating a case where it was hard to delete early steps.
  • Improve efficiency of drawing binary(min_size=n, max_size=n) significantly by provide a custom implementation for fixed size blocks that can bypass a lot of machinery.
  • Set default home directory based on the current working directory at the point Hypothesis is imported, not whenever the function first happens to be called.

3.0.0 - 2016-02-17

Codename: This really should have been 2.1.

Externally this looks like a very small release. It has one small breaking change that probably doesn’t affect anyone at all (some behaviour that never really worked correctly is now outright forbidden) but necessitated a major version bump and one visible new feature.

Internally this is a complete rewrite. Almost nothing other than the public API is the same.

New features:

  • Addition of data() strategy which allows you to draw arbitrary data interactively within the test.
  • New “exploded” database format which allows you to more easily check the example database into a source repository while supporting merging.
  • Better management of how examples are saved in the database.
  • Health checks will now raise as errors when they fail. It was too easy to have the warnings be swallowed entirely.

New limitations:

Feature removal:

  • The ForkingTestCase executor has gone away. It may return in some more working form at a later date.

Performance improvements:

  • A new model which allows flatmap, composite strategies and stateful testing to perform much better. They should also be more reliable.
  • Filtering may in some circumstances have improved significantly. This will help especially in cases where you have lots of values with individual filters on them, such as lists(x.filter(...)).
  • Modest performance improvements to the general test runner by avoiding expensive operations

In general your tests should have got faster. If they’ve instead got significantly slower, I’m interested in hearing about it.

Data distribution:

The data distribution should have changed significantly. This may uncover bugs the previous version missed. It may also miss bugs the previous version could have uncovered. Hypothesis is now producing less strongly correlated data than it used to, but the correlations are extended over more of the structure.


Shrinking quality should have improved. In particular Hypothesis can now perform simultaneous shrinking of separate examples within a single test (previously it was only able to do this for elements of a single collection). In some cases performance will have improved, in some cases it will have got worse but generally shouldn’t have by much.

2.0.0 - 2016-01-10

Codename: A new beginning

This release cleans up all of the legacy that accrued in the course of Hypothesis 1.0. These are mostly things that were emitting deprecation warnings in 1.19.0, but there were a few additional changes.

In particular:

  • non-strategy values will no longer be converted to strategies when used in given or find.
  • FailedHealthCheck is now an error and not a warning.
  • Handling of non-ascii reprs in user types have been simplified by using raw strings in more places in Python 2.
  • given no longer allows mixing positional and keyword arguments.
  • given no longer works with functions with defaults.
  • given no longer turns provided arguments into defaults - they will not appear in the argspec at all.
  • the basic() strategy no longer exists.
  • the n_ary_tree strategy no longer exists.
  • the average_list_length setting no longer exists. Note: If you’re using using recursive() this will cause you a significant slow down. You should pass explicit average_size parameters to collections in recursive calls.
  • @rule can no longer be applied to the same method twice.
  • Python 2.6 and 3.3 are no longer officially supported, although in practice they still work fine.

This also includes two non-deprecation changes:

  • given’s keyword arguments no longer have to be the rightmost arguments and can appear anywhere in the method signature.
  • The max_shrinks setting would sometimes not have been respected.

1.19.0 - 2016-01-09

Codename: IT COMES

This release heralds the beginning of a new and terrible age of Hypothesis 2.0.

It’s primary purpose is some final deprecations prior to said release. The goal is that if your code emits no warnings under this release then it will probably run unchanged under Hypothesis 2.0 (there are some caveats to this: 2.0 will drop support for some Python versions, and if you’re using internal APIs then as usual that may break without warning).

It does have two new features:

  • New @seed() decorator which allows you to manually seed a test. This may be harmlessly combined with and overrides the derandomize setting.
  • settings objects may now be used as a decorator to fix those settings to a particular @given test.

API changes (old usage still works but is deprecated):

  • Settings has been renamed to settings (lower casing) in order to make the decorator usage more natural.
  • Functions for the storage directory that were in hypothesis.settings are now in a new hypothesis.configuration module.

Additional deprecations:

  • the average_list_length setting has been deprecated in favour of being explicit.
  • the basic() strategy has been deprecated as it is impossible to support it under a Conjecture based model, which will hopefully be implemented at some point in the 2.x series.
  • the n_ary_tree strategy (which was never actually part of the public API) has been deprecated.
  • Passing settings or random as keyword arguments to given is deprecated (use the new functionality instead)

Bug fixes:

  • No longer emit PendingDeprecationWarning for __iter__ and StopIteration in streaming() values.
  • When running in health check mode with non strict, don’t print quite so many errors for an exception in reify.
  • When an assumption made in a test or a filter is flaky, tests will now raise Flaky instead of UnsatisfiedAssumption.

1.18.1 - 2015-12-22

Two behind the scenes changes:

  • Hypothesis will no longer write generated code to the file system. This will improve performance on some systems (e.g. if you’re using PythonAnywhere which is running your code from NFS) and prevent some annoying interactions with auto-restarting systems.
  • Hypothesis will cache the creation of some strategies. This can significantly improve performance for code that uses flatmap or composite and thus has to instantiate strategies a lot.

1.18.0 - 2015-12-21


  • Tests and find are now explicitly seeded off the global random module. This means that if you nest one inside the other you will now get a health check error. It also means that you can control global randomization by seeding random.
  • There is a new random_module() strategy which seeds the global random module for you and handles things so that you don’t get a health check warning if you use it inside your tests.
  • floats() now accepts two new arguments: allow_nan and allow_infinity. These default to the old behaviour, but when set to False will do what the names suggest.

Bug fixes:

  • Fix a bug where tests that used text() on Python 3.4+ would not actually be deterministic even when explicitly seeded or using the derandomize mode, because generation depended on dictionary iteration order which was affected by hash randomization.
  • Fix a bug where with complicated strategies the timing of the initial health check could affect the seeding of the subsequent test, which would also render supposedly deterministic tests non-deterministic in some scenarios.
  • In some circumstances flatmap() could get confused by two structurally similar things it could generate and would produce a flaky test where the first time it produced an error but the second time it produced the other value, which was not an error. The same bug was presumably also possible in composite().
  • flatmap() and composite() initial generation should now be moderately faster. This will be particularly noticeable when you have many values drawn from the same strategy in a single run, e.g. constructs like lists(s.flatmap(f)). Shrinking performance may have suffered, but this didn’t actually produce an interestingly worse result in any of the standard scenarios tested.

1.17.1 - 2015-12-16

A small bug fix release, which fixes the fact that the ‘note’ function could not be used on tests which used the @example decorator to provide explicit examples.

1.17.0 - 2015-12-15

This is actually the same release as 1.16.1, but 1.16.1 has been pulled because it contains the following additional change that was not intended to be in a patch release (it’s perfectly stable, but is a larger change that should have required a minor version bump):

  • Hypothesis will now perform a series of “health checks” as part of running your tests. These detect and warn about some common error conditions that people often run into which wouldn’t necessarily have caused the test to fail but would cause e.g. degraded performance or confusing results.

1.16.1 - 2015-12-14

Note: This release has been removed.

A small bugfix release that allows bdists for Hypothesis to be built under 2.7 - the file which had Python 3 syntax wasn’t intended to be loaded under Python 2, but when building a bdist it was. In particular this would break running test.

1.16.0 - 2015-12-08

There are no public API changes in this release but it includes a behaviour change that I wasn’t comfortable putting in a patch release.

  • Functions from hypothesis.strategies will no longer raise InvalidArgument on bad arguments. Instead the same errors will be raised when a test using such a strategy is run. This may improve startup time in some cases, but the main reason for it is so that errors in strategies won’t cause errors in loading, and it can interact correctly with things like pytest.mark.skipif.
  • Errors caused by accidentally invoking the legacy API are now much less confusing, although still throw NotImplementedError.
  • hypothesis.extra.django is 1.9 compatible.
  • When tests are run with max_shrinks=0 this will now still rerun the test on failure and will no longer print “Trying example:” before each run. Additionally note() will now work correctly when used with max_shrinks=0.

1.15.0 - 2015-11-24

A release with two new features.

  • A ‘characters’ strategy for more flexible generation of text with particular character ranges and types, kindly contributed by Alexander Shorin.
  • Add support for preconditions to the rule based stateful testing. Kindly contributed by Christopher Armstrong

1.14.0 - 2015-11-01

New features:

  • Add ‘note’ function which lets you include additional information in the final test run’s output.
  • Add ‘choices’ strategy which gives you a choice function that emulates random.choice.
  • Add ‘uuid’ strategy that generates UUIDs’
  • Add ‘shared’ strategy that lets you create a strategy that just generates a single shared value for each test run


  • Using strategies of the form streaming(x.flatmap(f)) with find or in stateful testing would have caused InvalidArgument errors when the resulting values were used (because code that expected to only be called within a test context would be invoked).

1.13.0 - 2015-10-29

This is quite a small release, but deprecates some public API functions and removes some internal API functionality so gets a minor version bump.

  • All calls to the ‘strategy’ function are now deprecated, even ones which pass just a SearchStrategy instance (which is still a no-op).
  • Never documented hypothesis.extra entry_points mechanism has now been removed ( it was previously how hypothesis.extra packages were loaded and has been deprecated and unused for some time)
  • Some corner cases that could previously have produced an OverflowError when simplifying failing cases using hypothesis.extra.datetimes (or dates or times) have now been fixed.
  • Hypothesis load time for first import has been significantly reduced - it used to be around 250ms (on my SSD laptop) and now is around 100-150ms. This almost never matters but was slightly annoying when using it in the console.
  • hypothesis.strategies.randoms was previously missing from __all__.

1.12.0 - 2015-10-18

  • Significantly improved performance of creating strategies using the functions from the hypothesis.strategies module by deferring the calculation of their repr until it was needed. This is unlikely to have been an performance issue for you unless you were using flatmap, composite or stateful testing, but for some cases it could be quite a significant impact.
  • A number of cases where the repr of strategies build from lambdas is improved
  • Add dates() and times() strategies to hypothesis.extra.datetimes
  • Add new ‘profiles’ mechanism to the settings system
  • Deprecates mutability of Settings, both the Settings.default top level property and individual settings.
  • A Settings object may now be directly initialized from a parent Settings.
  • @given should now give a better error message if you attempt to use it with a function that uses destructuring arguments (it still won’t work, but it will error more clearly),
  • A number of spelling corrections in error messages
  • py.test should no longer display the intermediate modules Hypothesis generates when running in verbose mode
  • Hypothesis should now correctly handle printing objects with non-ascii reprs on python 3 when running in a locale that cannot handle ascii printing to stdout.
  • Add a unique=True argument to lists(). This is equivalent to unique_by=lambda x: x, but offers a more convenient syntax.

1.11.4 - 2015-09-27

  • Hide modifications Hypothesis needs to make to sys.path by undoing them after we’ve imported the relevant modules. This is a workaround for issues cryptography experienced on windows.
  • Slightly improved performance of drawing from sampled_from on large lists of alternatives.
  • Significantly improved performance of drawing from one_of or strategies using | (note this includes a lot of strategies internally - floats() and integers() both fall into this category). There turned out to be a massive performance regression introduced in 1.10.0 affecting these which probably would have made tests using Hypothesis significantly slower than they should have been.

1.11.3 - 2015-09-23

  • Better argument validation for datetimes() strategy - previously setting max_year < datetime.MIN_YEAR or min_year > datetime.MAX_YEAR would not have raised an InvalidArgument error and instead would have behaved confusingly.
  • Compatibility with being run on pytest < 2.7 (achieved by disabling the plugin).

1.11.2 - 2015-09-23

Bug fixes:

  • Settings(database=my_db) would not be correctly inherited when used as a default setting, so that newly created settings would use the database_file setting and create an SQLite example database.
  • Settings.default.database = my_db would previously have raised an error and now works.
  • Timeout could sometimes be significantly exceeded if during simplification there were a lot of examples tried that didn’t trigger the bug.
  • When loading a heavily simplified example using a basic() strategy from the database this could cause Python to trigger a recursion error.
  • Remove use of deprecated API in pytest plugin so as to not emit warning


  • hypothesis-pytest is now part of hypothesis core. This should have no externally visible consequences, but you should update your dependencies to remove hypothesis-pytest and depend on only Hypothesis.
  • Better repr for hypothesis.extra.datetimes() strategies.
  • Add .close() method to abstract base class for Backend (it was already present in the main implementation).

1.11.1 - 2015-09-16

Bug fixes:

  • When running Hypothesis tests in parallel (e.g. using pytest-xdist) there was a race condition caused by code generation.
  • Example databases are now cached per thread so as to not use sqlite connections from multiple threads. This should make Hypothesis now entirely thread safe.
  • floats() with only min_value or max_value set would have had a very bad distribution.
  • Running on 3.5, Hypothesis would have emitted deprecation warnings because of use of inspect.getargspec

1.11.0 - 2015-08-31

  • text() with a non-string alphabet would have used the repr() of the the alphabet instead of its contexts. This is obviously silly. It now works with any sequence of things convertible to unicode strings.
  • @given will now work on methods whose definitions contains no explicit positional arguments, only varargs (bug #118). This may have some knock on effects because it means that @given no longer changes the argspec of functions other than by adding defaults.
  • Introduction of new @composite feature for more natural definition of strategies you’d previously have used flatmap for.

1.10.6 - 2015-08-26

Fix support for fixtures on Django 1.7.

1.10.4 - 2015-08-21

Tiny bug fix release:

  • If the database_file setting is set to None, this would have resulted in an error when running tests. Now it does the same as setting database to None.

1.10.3 - 2015-08-19

Another small bug fix release.

  • lists(elements, unique_by=some_function, min_size=n) would have raised a ValidationError if n > Settings.default.average_list_length because it would have wanted to use an average list length shorter than the minimum size of the list, which is impossible. Now it instead defaults to twice the minimum size in these circumstances.
  • basic() strategy would have only ever produced at most ten distinct values per run of the test (which is bad if you e.g. have it inside a list). This was obviously silly. It will now produce a much better distribution of data, both duplicated and non duplicated.

1.10.2 - 2015-08-19

This is a small bug fix release:

  • star imports from hypothesis should now work correctly.
  • example quality for examples using flatmap will be better, as the way it had previously been implemented was causing problems where Hypothesis was erroneously labelling some examples as being duplicates.

1.10.0 - 2015-08-04

This is just a bugfix and performance release, but it changes some semi-public APIs, hence the minor version bump.

  • Significant performance improvements for strategies which are one_of() many branches. In particular this included recursive() strategies. This should take the case where you use one recursive() strategy as the base strategy of another from unusably slow (tens of seconds per generated example) to reasonably fast.
  • Better handling of just() and sampled_from() for values which have an incorrect __repr__ implementation that returns non-ASCII unicode on Python 2.
  • Better performance for flatmap from changing the internal morpher API to be significantly less general purpose.
  • Introduce a new semi-public BuildContext/cleanup API. This allows strategies to register cleanup activities that should run once the example is complete. Note that this will interact somewhat weirdly with find.
  • Better simplification behaviour for streaming strategies.
  • Don’t error on lambdas which use destructuring arguments in Python 2.
  • Add some better reprs for a few strategies that were missing good ones.
  • The Random instances provided by randoms() are now copyable.
  • Slightly more debugging information about simplify when using a debug verbosity level.
  • Support using given for functions with varargs, but not passing arguments to it as positional.

1.9.0 - 2015-07-27

Codename: The great bundling.

This release contains two fairly major changes.

The first is the deprecation of the hypothesis-extra mechanism. From now on all the packages that were previously bundled under it other than hypothesis-pytest (which is a different beast and will remain separate). The functionality remains unchanged and you can still import them from exactly the same location, they just are no longer separate packages.

The second is that this introduces a new way of building strategies which lets you build up strategies recursively from other strategies.

It also contains the minor change that calling .example() on a strategy object will give you examples that are more representative of the actual data you’ll get. There used to be some logic in there to make the examples artificially simple but this proved to be a bad idea.

1.8.5 - 2015-07-24

This contains no functionality changes but fixes a mistake made with building the previous package that would have broken installation on Windows.

1.8.4 - 2015-07-20

Bugs fixed:

  • When a call to floats() had endpoints which were not floats but merely convertible to one (e.g. integers), these would be included in the generated data which would cause it to generate non-floats.
  • Splitting lambdas used in the definition of flatmap, map or filter over multiple lines would break the repr, which would in turn break their usage.

1.8.3 - 2015-07-20

“Falsifying example” would not have been printed when the failure came from an explicit example.

1.8.2 - 2015-07-18

Another small bugfix release:

  • When using ForkingTestCase you would usually not get the falsifying example printed if the process exited abnormally (e.g. due to os._exit).
  • Improvements to the distribution of characters when using text() with a default alphabet. In particular produces a better distribution of ascii and whitespace in the alphabet.

1.8.1 - 2015-07-17

This is a small release that contains a workaround for people who have bad reprs returning non ascii text on Python 2.7. This is not a bug fix for Hypothesis per se because that’s not a thing that is actually supposed to work, but Hypothesis leans more heavily on repr than is typical so it’s worth having a workaround for.

1.8.0 - 2015-07-16

New features:

  • Much more sensible reprs for strategies, especially ones that come from hypothesis.strategies. These should now have as reprs python code that would produce the same strategy.
  • lists() accepts a unique_by argument which forces the generated lists to be only contain elements unique according to some function key (which must return a hashable value).
  • Better error messages from flaky tests to help you debug things.

Mostly invisible implementation details that may result in finding new bugs in your code:

  • Sets and dictionary generation should now produce a better range of results.
  • floats with bounds now focus more on ‘critical values’, trying to produce values at edge cases.
  • flatmap should now have better simplification for complicated cases, as well as generally being (I hope) more reliable.

Bug fixes:

  • You could not previously use assume() if you were using the forking executor.

1.7.2 - 2015-07-10

This is purely a bug fix release:

  • When using floats() with stale data in the database you could sometimes get values in your tests that did not respect min_value or max_value.
  • When getting a Flaky error from an unreliable test it would have incorrectly displayed the example that caused it.
  • 2.6 dependency on backports was incorrectly specified. This would only have caused you problems if you were building a universal wheel from Hypothesis, which is not how Hypothesis ships, so unless you’re explicitly building wheels for your dependencies and support Python 2.6 plus a later version of Python this probably would never have affected you.
  • If you use flatmap in a way that the strategy on the right hand side depends sensitively on the left hand side you may have occasionally seen Flaky errors caused by producing unreliable examples when minimizing a bug. This use case may still be somewhat fraught to be honest. This code is due a major rearchitecture for 1.8, but in the meantime this release fixes the only source of this error that I’m aware of.

1.7.1 - 2015-06-29

Codename: There is no 1.7.0.

A slight technical hitch with a premature upload means there’s was a yanked 1.7.0 release. Oops.

The major feature of this release is Python 2.6 support. Thanks to Jeff Meadows for doing most of the work there.

Other minor features

  • strategies now has a permutations() function which returns a strategy yielding permutations of values from a given collection.
  • if you have a flaky test it will print the exception that it last saw before failing with Flaky, even if you do not have verbose reporting on.
  • Slightly experimental git merge script available as “python -m”. Instructions on how to use it in the docstring of that file.

Bug fixes:

  • Better performance from use of filter. In particular tests which involve large numbers of heavily filtered strategies should perform a lot better.
  • floats() with a negative min_value would not have worked correctly (worryingly, it would have just silently failed to run any examples). This is now fixed.
  • tests using sampled_from would error if the number of sampled elements was smaller than min_satisfying_examples.

1.6.2 - 2015-06-08

This is just a few small bug fixes:

  • Size bounds were not validated for values for a binary() strategy when reading examples from the database.
  • sampled_from is now in __all__ in hypothesis.strategies
  • floats no longer consider negative integers to be simpler than positive non-integers
  • Small floating point intervals now correctly count members, so if you have a floating point interval so narrow there are only a handful of values in it, this will no longer cause an error when Hypothesis runs out of values.

1.6.1 - 2015-05-21

This is a small patch release that fixes a bug where 1.6.0 broke the use of flatmap with the deprecated API and assumed the passed in function returned a SearchStrategy instance rather than converting it to a strategy.

1.6.0 - 2015-05-21

This is a smallish release designed to fix a number of bugs and smooth out some weird behaviours.

  • Fix a critical bug in flatmap where it would reuse old strategies. If all your flatmap code was pure you’re fine. If it’s not, I’m surprised it’s working at all. In particular if you want to use flatmap with django models, you desperately need to upgrade to this version.
  • flatmap simplification performance should now be better in some cases where it previously had to redo work.
  • Fix for a bug where invalid unicode data with surrogates could be generated during simplification (it was already filtered out during actual generation).
  • The Hypothesis database is now keyed off the name of the test instead of the type of data. This makes much more sense now with the new strategies API and is generally more robust. This means you will lose old examples on upgrade.
  • The database will now not delete values which fail to deserialize correctly, just skip them. This is to handle cases where multiple incompatible strategies share the same key.
  • find now also saves and loads values from the database, keyed off a hash of the function you’re finding from.
  • Stateful tests now serialize and load values from the database. They should have before, really. This was a bug.
  • Passing a different verbosity level into a test would not have worked entirely correctly, leaving off some messages. This is now fixed.
  • Fix a bug where derandomized tests with unicode characters in the function body would error on Python 2.7.

1.5.0 - 2015-05-14

Codename: Strategic withdrawal.

The purpose of this release is a radical simplification of the API for building strategies. Instead of the old approach of @strategy.extend and things that get converted to strategies, you just build strategies directly.

The old method of defining strategies will still work until Hypothesis 2.0, because it’s a major breaking change, but will now emit deprecation warnings.

The new API is also a lot more powerful as the functions for defining strategies give you a lot of dials to turn. See the updated data section for details.

Other changes:

  • Mixing keyword and positional arguments in a call to @given is deprecated as well.
  • There is a new setting called ‘strict’. When set to True, Hypothesis will raise warnings instead of merely printing them. Turning it on by default is inadvisable because it means that Hypothesis minor releases can break your code, but it may be useful for making sure you catch all uses of deprecated APIs.
  • max_examples in settings is now interpreted as meaning the maximum number of unique (ish) examples satisfying assumptions. A new setting max_iterations which defaults to a larger value has the old interpretation.
  • Example generation should be significantly faster due to a new faster parameter selection algorithm. This will mostly show up for simple data types - for complex ones the parameter selection is almost certainly dominated.
  • Simplification has some new heuristics that will tend to cut down on cases where it could previously take a very long time.
  • timeout would previously not have been respected in cases where there were a lot of duplicate examples. You probably wouldn’t have previously noticed this because max_examples counted duplicates, so this was very hard to hit in a way that mattered.
  • A number of internal simplifications to the SearchStrategy API.
  • You can now access the current Hypothesis version as hypothesis.__version__.
  • A top level function is provided for running the stateful tests without the TestCase infrastructure.

1.4.0 - 2015-05-04

Codename: What a state.

The big feature of this release is the new and slightly experimental stateful testing API. You can read more about that in the appropriate section.

Two minor features the were driven out in the course of developing this:

  • You can now set settings.max_shrinks to limit the number of times Hypothesis will try to shrink arguments to your test. If this is set to <= 0 then Hypothesis will not rerun your test and will just raise the failure directly. Note that due to technical limitations if max_shrinks is <= 0 then Hypothesis will print every example it calls your test with rather than just the failing one. Note also that I don’t consider settings max_shrinks to zero a sensible way to run your tests and it should really be considered a debug feature.
  • There is a new debug level of verbosity which is even more verbose than verbose. You probably don’t want this.

Breakage of semi-public SearchStrategy API:

  • It is now a required invariant of SearchStrategy that if u simplifies to v then it is not the case that strictly_simpler(u, v). i.e. simplifying should not increase the complexity even though it is not required to decrease it. Enforcing this invariant lead to finding some bugs where simplifying of integers, floats and sets was suboptimal.
  • Integers in basic data are now required to fit into 64 bits. As a result python integer types are now serialized as strings, and some types have stopped using quite so needlessly large random seeds.

Hypothesis Stateful testing was then turned upon Hypothesis itself, which lead to an amazing number of minor bugs being found in Hypothesis itself.

Bugs fixed (most but not all from the result of stateful testing) include:

  • Serialization of streaming examples was flaky in a way that you would probably never notice: If you generate a template, simplify it, serialize it, deserialize it, serialize it again and then deserialize it you would get the original stream instead of the simplified one.
  • If you reduced max_examples below the number of examples already saved in the database, you would have got a ValueError. Additionally, if you had more than max_examples in the database all of them would have been considered.
  • @given will no longer count duplicate examples (which it never called your function with) towards max_examples. This may result in your tests running slower, but that’s probably just because they’re trying more examples.
  • General improvements to example search which should result in better performance and higher quality examples. In particular parameters which have a history of producing useless results will be more aggressively culled. This is useful both because it decreases the chance of useless examples and also because it’s much faster to not check parameters which we were unlikely to ever pick!
  • integers_from and lists of types with only one value (e.g. [None]) would previously have had a very high duplication rate so you were probably only getting a handful of examples. They now have a much lower duplication rate, as well as the improvements to search making this less of a problem in the first place.
  • You would sometimes see simplification taking significantly longer than your defined timeout. This would happen because timeout was only being checked after each successful simplification, so if Hypothesis was spending a lot of time unsuccessfully simplifying things it wouldn’t stop in time. The timeout is now applied for unsuccessful simplifications too.
  • In Python 2.7, integers_from strategies would have failed during simplification with an OverflowError if their starting point was at or near to the maximum size of a 64-bit integer.
  • flatmap and map would have failed if called with a function without a __name__ attribute.
  • If max_examples was less than min_satisfying_examples this would always error. Now min_satisfying_examples is capped to max_examples. Note that if you have assumptions to satisfy here this will still cause an error.

Some minor quality improvements:

  • Lists of streams, flatmapped strategies and basic strategies should now now have slightly better simplification.

1.3.0 - 2015-05-22

New features:

  • New verbosity level API for printing intermediate results and exceptions.
  • New specifier for strings generated from a specified alphabet.
  • Better error messages for tests that are failing because of a lack of enough examples.

Bug fixes:

  • Fix error where use of ForkingTestCase would sometimes result in too many open files.
  • Fix error where saving a failing example that used flatmap could error.
  • Implement simplification for sampled_from, which apparently never supported it previously. Oops.

General improvements:

  • Better range of examples when using one_of or sampled_from.
  • Fix some pathological performance issues when simplifying lists of complex values.
  • Fix some pathological performance issues when simplifying examples that require unicode strings with high codepoints.
  • Random will now simplify to more readable examples.

1.2.1 - 2015-04-16

A small patch release for a bug in the new executors feature. Tests which require doing something to their result in order to fail would have instead reported as flaky.

1.2.0 - 2015-04-15

Codename: Finders keepers.

A bunch of new features and improvements.

  • Provide a mechanism for customizing how your tests are executed.
  • Provide a test runner that forks before running each example. This allows better support for testing native code which might trigger a segfault or a C level assertion failure.
  • Support for using Hypothesis to find examples directly rather than as just as a test runner.
  • New streaming type which lets you generate infinite lazily loaded streams of data - perfect for if you need a number of examples but don’t know how many.
  • Better support for large integer ranges. You can now use integers_in_range with ranges of basically any size. Previously large ranges would have eaten up all your memory and taken forever.
  • Integers produce a wider range of data than before - previously they would only rarely produce integers which didn’t fit into a machine word. Now it’s much more common. This percolates to other numeric types which build on integers.
  • Better validation of arguments to @given. Some situations that would previously have caused silently wrong behaviour will now raise an error.
  • Include +/- sys.float_info.max in the set of floating point edge cases that Hypothesis specifically tries.
  • Fix some bugs in floating point ranges which happen when given +/- sys.float_info.max as one of the endpoints... (really any two floats that are sufficiently far apart so that x, y are finite but y - x is infinite). This would have resulted in generating infinite values instead of ones inside the range.

1.1.1 - 2015-04-07

Codename: Nothing to see here

This is just a patch release put out because it fixed some internal bugs that would block the Django integration release but did not actually affect anything anyone could previously have been using. It also contained a minor quality fix for floats that I’d happened to have finished in time.

  • Fix some internal bugs with object lifecycle management that were impossible to hit with the previously released versions but broke hypothesis-django.
  • Bias floating point numbers somewhat less aggressively towards very small numbers

1.1.0 - 2015-04-06

Codename: No-one mention the M word.

  • Unicode strings are more strongly biased towards ascii characters. Previously they would generate all over the space. This is mostly so that people who try to shape their unicode strings with assume() have less of a bad time.
  • A number of fixes to data deserialization code that could theoretically have caused mysterious bugs when using an old version of a Hypothesis example database with a newer version. To the best of my knowledge a change that could have triggered this bug has never actually been seen in the wild. Certainly no-one ever reported a bug of this nature.
  • Out of the box support for Decimal and Fraction.
  • new dictionary specifier for dictionaries with variable keys.
  • Significantly faster and higher quality simplification, especially for collections of data.
  • New filter() and flatmap() methods on Strategy for better ways of building strategies out of other strategies.
  • New BasicStrategy class which allows you to define your own strategies from scratch without needing an existing matching strategy or being exposed to the full horror or non-public nature of the SearchStrategy interface.

1.0.0 - 2015-03-27

Codename: Blast-off!

There are no code changes in this release. This is precisely the 0.9.2 release with some updated documentation.

0.9.2 - 2015-03-26

Codename: T-1 days.

  • floats_in_range would not actually have produced floats_in_range unless that range happened to be (0, 1). Fix this.

0.9.1 - 2015-03-25

Codename: T-2 days.

  • Fix a bug where if you defined a strategy using map on a lambda then the results would not be saved in the database.
  • Significant performance improvements when simplifying examples using lists, strings or bounded integer ranges.

0.9.0 - 2015-03-23

Codename: The final countdown

This release could also be called 1.0-RC1.

It contains a teeny tiny bugfix, but the real point of this release is to declare feature freeze. There will be zero functionality changes between 0.9.0 and 1.0 unless something goes really really wrong. No new features will be added, no breaking API changes will occur, etc. This is the final shakedown before I declare Hypothesis stable and ready to use and throw a party to celebrate.

Bug bounty for any bugs found between now and 1.0: I will buy you a drink (alcoholic, caffeinated, or otherwise) and shake your hand should we ever find ourselves in the same city at the same time.

The one tiny bugfix:

  • Under pypy, databases would fail to close correctly when garbage collected, leading to a memory leak and a confusing error message if you were repeatedly creating databases and not closing them. It is very unlikely you were doing this and the chances of you ever having noticed this bug are very low.

0.7.2 - 2015-03-22

Codename: Hygienic macros or bust

  • You can now name an argument to @given ‘f’ and it won’t break (issue #38)
  • strategy_test_suite is now named strategy_test_suite as the documentation claims and not in fact strategy_test_suitee
  • Settings objects can now be used as a context manager to temporarily override the default values inside their context.

0.7.1 - 2015-03-21

Codename: Point releases go faster

  • Better string generation by parametrizing by a limited alphabet
  • Faster string simplification - previously if simplifying a string with high range unicode characters it would try every unicode character smaller than that. This was pretty pointless. Now it stops after it’s a short range (it can still reach smaller ones through recursive calls because of other simplifying operations).
  • Faster list simplification by first trying a binary chop down the middle
  • Simultaneous simplification of identical elements in a list. So if a bug only triggers when you have duplicates but you drew e.g. [-17, -17], this will now simplify to [0, 0].

0.7.0, - 2015-03-20

Codename: Starting to look suspiciously real

This is probably the last minor release prior to 1.0. It consists of stability improvements, a few usability things designed to make Hypothesis easier to try out, and filing off some final rough edges from the API.

  • Significant speed and memory usage improvements
  • Add an example() method to strategy objects to give an example of the sort of data that the strategy generates.
  • Remove .descriptor attribute of strategies
  • Rename descriptor_test_suite to strategy_test_suite
  • Rename the few remaining uses of descriptor to specifier (descriptor already has a defined meaning in Python)

0.6.0 - 2015-03-13

Codename: I’m sorry, were you using that API?

This is primarily a “simplify all the weird bits of the API” release. As a result there are a lot of breaking changes. If you just use @given with core types then you’re probably fine.

In particular:

  • Stateful testing has been removed from the API
  • The way the database is used has been rendered less useful (sorry). The feature for reassembling values saved from other tests doesn’t currently work. This will probably be brought back in post 1.0.
  • SpecificationMapper is no longer a thing. Instead there is an ExtMethod called strategy which you extend to specify how to convert other types to strategies.
  • Settings are now extensible so you can add your own for configuring a strategy
  • MappedSearchStrategy no longer needs an unpack method
  • Basically all the SearchStrategy internals have changed massively. If you implemented SearchStrategy directly rather than using MappedSearchStrategy talk to me about fixing it.
  • Change to the way extra packages work. You now specify the package. This must have a load() method. Additionally any modules in the package will be loaded in under hypothesis.extra

Bug fixes:

  • Fix for a bug where calling falsify on a lambda with a non-ascii character in its body would error.

Hypothesis Extra:

hypothesis-fakefactory: An extension for using faker data in hypothesis. Depends
on fake-factory.

0.5.0 - 2015-02-10

Codename: Read all about it.

Core hypothesis:

  • Add support back in for pypy and python 3.2
  • @given functions can now be invoked with some arguments explicitly provided. If all arguments that hypothesis would have provided are passed in then no falsification is run.
  • Related to the above, this means that you can now use pytest fixtures and mark.parametrize with Hypothesis without either interfering with the other.
  • Breaking change: @given no longer works for functions with varargs (varkwargs are fine). This might be added back in at a later date.
  • Windows is now fully supported. A limited version (just the tests with none of the extras) of the test suite is run on windows with each commit so it is now a first class citizen of the Hypothesis world.
  • Fix a bug for fuzzy equality of equal complex numbers with different reprs (this can happen when one coordinate is zero). This shouldn’t affect users - that feature isn’t used anywhere public facing.
  • Fix generation of floats on windows and 32-bit builds of python. I was using some struct.pack logic that only worked on certain word sizes.
  • When a test times out and hasn’t produced enough examples this now raises a Timeout subclass of Unfalsifiable.
  • Small search spaces are better supported. Previously something like a @given(bool, bool) would have failed because it couldn’t find enough examples. Hypothesis is now aware of the fact that these are small search spaces and will not error in this case.
  • Improvements to parameter search in the case of hard to satisfy assume. Hypothesis will now spend less time exploring parameters that are unlikely to provide anything useful.
  • Increase chance of generating “nasty” floats
  • Fix a bug that would have caused unicode warnings if you had a sampled_from that was mixing unicode and byte strings.
  • Added a standard test suite that you can use to validate a custom strategy you’ve defined is working correctly.

Hypothesis extra:

First off, introducing Hypothesis extra packages!

These are packages that are separated out from core Hypothesis because they have one or more dependencies. Every hypothesis-extra package is pinned to a specific point release of Hypothesis and will have some version requirements on its dependency. They use entry_points so you will usually not need to explicitly import them, just have them installed on the path.

This release introduces two of them:


Does what it says on the tin: Generates datetimes for Hypothesis. Just install the package and datetime support will start working.

Depends on pytz for timezone support


A very rudimentary pytest plugin. All it does right now is hook the display of falsifying examples into pytest reporting.

Depends on pytest.

0.4.3 - 2015-02-05

Codename: TIL narrow Python builds are a thing

This just fixes the one bug.

  • Apparently there is such a thing as a “narrow python build” and OS X ships with these by default for python 2.7. These are builds where you only have two bytes worth of unicode. As a result, generating unicode was completely broken on OS X. Fix this by only generating unicode codepoints in the range supported by the system.

0.4.2 - 2015-02-04

Codename: O(dear)

This is purely a bugfix release:

  • Provide sensible external hashing for all core types. This will significantly improve performance of tracking seen examples which happens in literally every falsification run. For Hypothesis fixing this cut 40% off the runtime of the test suite. The behaviour is quadratic in the number of examples so if you’re running the default configuration this will be less extreme (Hypothesis’s test suite runs at a higher number of examples than default), but you should still see a significant improvement.
  • Fix a bug in formatting of complex numbers where the string could get incorrectly truncated.

0.4.1 - 2015-02-03

Codename: Cruel and unusual edge cases

This release is mostly about better test case generation.


  • Has a cool release name
  • text_type (str in python 3, unicode in python 2) example generation now actually produces interesting unicode instead of boring ascii strings.
  • floating point numbers are generated over a much wider range, with particular attention paid to generating nasty numbers - nan, infinity, large and small values, etc.
  • examples can be generated using pieces of examples previously saved in the database. This allows interesting behaviour that has previously been discovered to be propagated to other examples.
  • improved parameter exploration algorithm which should allow it to more reliably hit interesting edge cases.
  • Timeout can now be disabled entirely by setting it to any value <= 0.

Bug fixes:

  • The descriptor on a OneOfStrategy could be wrong if you had descriptors which were equal but should not be coalesced. e.g. a strategy for one_of((frozenset({int}), {int})) would have reported its descriptor as {int}. This is unlikely to have caused you any problems
  • If you had strategies that could produce NaN (which float previously couldn’t but e.g. a Just(float(‘nan’)) could) then this would have sent hypothesis into an infinite loop that would have only been terminated when it hit the timeout.
  • Given elements that can take a long time to minimize, minimization of floats or tuples could be quadratic or worse in the that value. You should now see much better performance for simplification, albeit at some cost in quality.


  • A lot of internals have been been rewritten. This shouldn’t affect you at all, but it opens the way for certain of hypothesis’s oddities to be a lot more extensible by users. Whether this is a good thing may be up for debate...

0.4.0 - 2015-01-21

FLAGSHIP FEATURE: Hypothesis now persists examples for later use. It stores data in a local SQLite database and will reuse it for all tests of the same type.

LICENSING CHANGE: Hypothesis is now released under the Mozilla Public License 2.0. This applies to all versions from 0.4.0 onwards until further notice. The previous license remains applicable to all code prior to 0.4.0.


  • Printing of failing examples. I was finding that the pytest runner was not doing a good job of displaying these, and that Hypothesis itself could do much better.
  • Drop dependency on six for cross-version compatibility. It was easy enough to write the shim for the small set of features that we care about and this lets us avoid a moderately complex dependency.
  • Some improvements to statistical distribution of selecting from small (<= 3 elements)
  • Improvements to parameter selection for finding examples.

Bugs fixed:

  • could_have_produced for lists, dicts and other collections would not have examined the elements and thus when using a union of different types of list this could result in Hypothesis getting confused and passing a value to the wrong strategy. This could potentially result in exceptions being thrown from within simplification.
  • sampled_from would not work correctly on a single element list.
  • Hypothesis could get very confused by values which are equal despite having different types being used in descriptors. Hypothesis now has its own more specific version of equality it uses for descriptors and tracking. It is always more fine grained than Python equality: Things considered != are not considered equal by hypothesis, but some things that are considered == are distinguished. If your test suite uses both frozenset and set tests this bug is probably affecting you.

0.3.2 - 2015-01-16

  • Fix a bug where if you specified floats_in_range with integer arguments Hypothesis would error in example simplification.
  • Improve the statistical distribution of the floats you get for the floats_in_range strategy. I’m not sure whether this will affect users in practice but it took my tests for various conditions from flaky to rock solid so it at the very least improves discovery of the artificial cases I’m looking for.
  • Improved repr() for strategies and RandomWithSeed instances.
  • Add detection for flaky test cases where hypothesis managed to find an example which breaks it but on the final invocation of the test it does not raise an error. This will typically happen with too much recursion errors but could conceivably happen in other circumstances too.
  • Provide a “derandomized” mode. This allows you to run hypothesis with zero real randomization, making your build nice and deterministic. The tests run with a seed calculated from the function they’re testing so you should still get a good distribution of test cases.
  • Add a mechanism for more conveniently defining tests which just sample from some collection.
  • Fix for a really subtle bug deep in the internals of the strategy table. In some circumstances if you were to define instance strategies for both a parent class and one or more of its subclasses you would under some circumstances get the strategy for the wrong superclass of an instance. It is very unlikely anyone has ever encountered this in the wild, but it is conceivably possible given that a mix of namedtuple and tuple are used fairly extensively inside hypothesis which do exhibit this pattern of strategy.

0.3.1 - 2015-01-13

  • Support for generation of frozenset and Random values
  • Correct handling of the case where a called function mutates it argument. This involved introducing a notion of a strategies knowing how to copy their argument. The default method should be entirely acceptable and the worst case is that it will continue to have the old behaviour if you don’t mark your strategy as mutable, so this shouldn’t break anything.
  • Fix for a bug where some strategies did not correctly implement could_have_produced. It is very unlikely that any of these would have been seen in the wild, and the consequences if they had been would have been minor.
  • Re-export the @given decorator from the main hypothesis namespace. It’s still available at the old location too.
  • Minor performance optimisation for simplifying long lists.

0.3.0 - 2015-01-12

  • Complete redesign of the data generation system. Extreme breaking change for anyone who was previously writing their own SearchStrategy implementations. These will not work any more and you’ll need to modify them.
  • New settings system allowing more global and modular control of Verifier behaviour.
  • Decouple SearchStrategy from the StrategyTable. This leads to much more composable code which is a lot easier to understand.
  • A significant amount of internal API renaming and moving. This may also break your code.
  • Expanded available descriptors, allowing for generating integers or floats in a specific range.
  • Significantly more robust. A very large number of small bug fixes, none of which anyone is likely to have ever noticed.
  • Deprecation of support for pypy and python 3 prior to 3.3. 3.3 and 3.4. Supported versions are 2.7.x, 3.3.x, 3.4.x. I expect all of these to remain officially supported for a very long time. I would not be surprised to add pypy support back in later but I’m not going to do so until I know someone cares about it. In the meantime it will probably still work.

0.2.2 - 2015-01-08

  • Fix an embarrassing complete failure of the installer caused by my being bad at version control

0.2.1 - 2015-01-07

  • Fix a bug in the new stateful testing feature where you could make __init__ a @requires method. Simplification would not always work if the prune method was able to successfully shrink the test.

0.2.0 - 2015-01-07

  • It’s aliiive.
  • Improve python 3 support using six.
  • Distinguish between byte and unicode types.
  • Fix issues where FloatStrategy could raise.
  • Allow stateful testing to request constructor args.
  • Fix for issue where test annotations would timeout based on when the module was loaded instead of when the test started

0.1.4 - 2013-12-14

  • Make verification runs time bounded with a configurable timeout

0.1.3 - 2013-05-03

  • Bugfix: Stateful testing behaved incorrectly with subclassing.
  • Complex number support
  • support for recursive strategies
  • different error for hypotheses with unsatisfiable assumptions

0.1.2 - 2013-03-24

  • Bugfix: Stateful testing was not minimizing correctly and could throw exceptions.
  • Better support for recursive strategies.
  • Support for named tuples.
  • Much faster integer generation.

0.1.1 - 2013-03-24

  • Python 3.x support via 2to3.
  • Use new style classes (oops).

0.1.0 - 2013-03-23

  • Introduce stateful testing.
  • Massive rewrite of internals to add flags and strategies.

0.0.5 - 2013-03-13

  • No changes except trying to fix packaging

0.0.4 - 2013-03-13

  • No changes except that I checked in a failing test case for 0.0.3 so had to replace the release. Doh

0.0.3 - 2013-03-13

  • Improved a few internals.
  • Opened up creating generators from instances as a general API.
  • Test integration.

0.0.2 - 2013-03-12

  • Starting to tighten up on the internals.
  • Change API to allow more flexibility in configuration.
  • More testing.

0.0.1 - 2013-03-10

  • Initial release.
  • Basic working prototype. Demonstrates idea, probably shouldn’t be used.