What you can generate and how¶
Most things should be easy to generate and everything should be possible.
To support this principle Hypothesis provides strategies for most builtin types with arguments to constrain or adjust the output, as well as higherorder strategies that can be composed to generate more complex types.
This document is a guide to what strategies are available for generating data and how to build them. Strategies have a variety of other important internal features, such as how they simplify, but the data they can generate is the only public part of their API.
Core strategies¶
Functions for building strategies are all available in the hypothesis.strategies module. The salient functions from it are as follows:

hypothesis.strategies.
binary
(*, min_size=0, max_size=None)[source]¶ Generates
bytes
.The generated
bytes
will have a length of at leastmin_size
and at mostmax_size
. Ifmax_size
is None there is no upper limit.Examples from this strategy shrink towards smaller strings and lower byte values.

hypothesis.strategies.
booleans
()[source]¶ Returns a strategy which generates instances of
bool
.Examples from this strategy will shrink towards
False
(i.e. shrinking will replaceTrue
withFalse
where possible).

hypothesis.strategies.
builds
(target, /, *args, **kwargs)[source]¶ Generates values by drawing from
args
andkwargs
and passing them to the callable (provided as the first positional argument) in the appropriate argument position.e.g.
builds(target, integers(), flag=booleans())
would draw an integeri
and a booleanb
and calltarget(i, flag=b)
.If the callable has type annotations, they will be used to infer a strategy for required arguments that were not passed to builds. You can also tell builds to infer a strategy for an optional argument by passing the special value
hypothesis.infer
as a keyword argument to builds, instead of a strategy for that argument to the callable.If the callable is a class defined with attrs, missing required arguments will be inferred from the attribute on a besteffort basis, e.g. by checking attrs standard validators. Dataclasses are handled natively by the inference from type hints.
Examples from this strategy shrink by shrinking the argument values to the callable.

hypothesis.strategies.
characters
(*, whitelist_categories=None, blacklist_categories=None, blacklist_characters=None, min_codepoint=None, max_codepoint=None, whitelist_characters=None)[source]¶ Generates characters, lengthone
str
ings, following specified filtering rules.When no filtering rules are specified, any character can be produced.
If
min_codepoint
ormax_codepoint
is specified, then only characters having a codepoint in that range will be produced.If
whitelist_categories
is specified, then only characters from those Unicode categories will be produced. This is a further restriction, characters must also satisfymin_codepoint
andmax_codepoint
.If
blacklist_categories
is specified, then any character from those categories will not be produced. Any overlap betweenwhitelist_categories
andblacklist_categories
will raise an exception, as each character can only belong to a single class.If
whitelist_characters
is specified, then any additional characters in that list will also be produced.If
blacklist_characters
is specified, then any characters in that list will be not be produced. Any overlap betweenwhitelist_characters
andblacklist_characters
will raise an exception.
The
_codepoint
arguments must be integers between zero andsys.maxunicode
. The_characters
arguments must be collections of lengthone unicode strings, such as a unicode string.The
_categories
arguments must be used to specify either the oneletter Unicode major category or the twoletter Unicode general category. For example,('Nd', 'Lu')
signifies “Number, decimal digit” and “Letter, uppercase”. A single letter (‘major category’) can be given to match all corresponding categories, for example'P'
for characters in any punctuation category.Examples from this strategy shrink towards the codepoint for
'0'
, or the first allowable codepoint after it if'0'
is excluded.

hypothesis.strategies.
complex_numbers
(*, min_magnitude=0, max_magnitude=None, allow_infinity=None, allow_nan=None)[source]¶ Returns a strategy that generates complex numbers.
This strategy draws complex numbers with constrained magnitudes. The
min_magnitude
andmax_magnitude
parameters should be nonnegativeReal
numbers; a value ofNone
corresponds an infinite upper bound.If
min_magnitude
is nonzero ormax_magnitude
is finite, it is an error to enableallow_nan
. Ifmax_magnitude
is finite, it is an error to enableallow_infinity
.The magnitude contraints are respected up to a relative error of (around) floatingpoint epsilon, due to implementation via the system
sqrt
function.Examples from this strategy shrink by shrinking their real and imaginary parts, as
floats()
.If you need to generate complex numbers with particular real and imaginary parts or relationships between parts, consider using
builds(complex, ...)
or@composite
respectively.

hypothesis.strategies.
composite
(f)[source]¶ Defines a strategy that is built out of potentially arbitrarily many other strategies.
This is intended to be used as a decorator. See the full documentation for more details about how to use this function.
Examples from this strategy shrink by shrinking the output of each draw call.

hypothesis.strategies.
data
()[source]¶ This isn’t really a normal strategy, but instead gives you an object which can be used to draw data interactively from other strategies.
See the rest of the documentation for more complete information.
Examples from this strategy do not shrink (because there is only one), but the result of calls to each draw() call shrink as they normally would.

class
hypothesis.strategies.
DataObject
(data)[source]¶ This type only exists so that you can write type hints for tests using the
data()
strategy. Do not use it directly!

hypothesis.strategies.
dates
(min_value=datetime.date.min, max_value=datetime.date.max)[source]¶ A strategy for dates between
min_value
andmax_value
.Examples from this strategy shrink towards January 1st 2000.

hypothesis.strategies.
datetimes
(min_value=datetime.datetime.min, max_value=datetime.datetime.max, *, timezones=none(), allow_imaginary=True)[source]¶ A strategy for generating datetimes, which may be timezoneaware.
This strategy works by drawing a naive datetime between
min_value
andmax_value
, which must both be naive (have no timezone).timezones
must be a strategy that generates eitherNone
, for naive datetimes, ortzinfo
objects for ‘aware’ datetimes. You can construct your own, though we recommend using the dateutil package andhypothesis.extra.dateutil.timezones()
strategy, and also providehypothesis.extra.pytz.timezones()
.You may pass
allow_imaginary=False
to filter out “imaginary” datetimes which did not (or will not) occur due to daylight savings, leap seconds, timezone and calendar adjustments, etc. Imaginary datetimes are allowed by default, because malformed timestamps are a common source of bugs. Note that because pytz predates PEP 495, this does not work correctly with timezones that use a negative DST offset (such as"Europe/Dublin"
).Examples from this strategy shrink towards midnight on January 1st 2000, local time.

hypothesis.strategies.
decimals
(min_value=None, max_value=None, *, allow_nan=None, allow_infinity=None, places=None)[source]¶ Generates instances of
decimal.Decimal
, which may be:A finite rational number, between
min_value
andmax_value
.Not a Number, if
allow_nan
is True. None means “allow NaN, unlessmin_value
andmax_value
are not None”.Positive or negative infinity, if
max_value
andmin_value
respectively are None, andallow_infinity
is not False. None means “allow infinity, unless excluded by the min and max values”.
Note that where floats have one
NaN
value, Decimals have four: signed, and either quiet or signalling. See the decimal module docs for more information on special values.If
places
is not None, all finite values drawn from the strategy will have that number of digits after the decimal place.Examples from this strategy do not have a well defined shrink order but try to maximize human readability when shrinking.

hypothesis.strategies.
deferred
(definition)[source]¶ A deferred strategy allows you to write a strategy that references other strategies that have not yet been defined. This allows for the easy definition of recursive and mutually recursive strategies.
The definition argument should be a zeroargument function that returns a strategy. It will be evaluated the first time the strategy is used to produce an example.
Example usage:
>>> import hypothesis.strategies as st >>> x = st.deferred(lambda: st.booleans()  st.tuples(x, x)) >>> x.example() (((False, (True, True)), (False, True)), (True, True)) >>> x.example() True
Mutual recursion also works fine:
>>> a = st.deferred(lambda: st.booleans()  b) >>> b = st.deferred(lambda: st.tuples(a, a)) >>> a.example() True >>> b.example() (False, (False, ((False, True), False)))
Examples from this strategy shrink as they normally would from the strategy returned by the definition.

hypothesis.strategies.
dictionaries
(keys, values, *, dict_class=<class 'dict'>, min_size=0, max_size=None)[source]¶ Generates dictionaries of type
dict_class
with keys drawn from thekeys
argument and values drawn from thevalues
argument.The size parameters have the same interpretation as for
lists()
.Examples from this strategy shrink by trying to remove keys from the generated dictionary, and by shrinking each generated key and value.

hypothesis.strategies.
emails
()[source]¶ A strategy for generating email addresses as unicode strings. The address format is specified in RFC 5322#section3.4.1. Values shrink towards shorter localparts and host domains.
This strategy is useful for generating “user data” for tests, as mishandling of email addresses is a common source of bugs.

hypothesis.strategies.
fixed_dictionaries
(mapping, *, optional=None)[source]¶ Generates a dictionary of the same type as mapping with a fixed set of keys mapping to strategies.
mapping
must be a dict subclass.Generated values have all keys present in mapping, with the corresponding values drawn from mapping[key]. If mapping is an instance of OrderedDict the keys will also be in the same order, otherwise the order is arbitrary.
If
optional
is passed, the generated value may or may not contain each key fromoptional
and a value drawn from the corresponding strategy.Examples from this strategy shrink by shrinking each individual value in the generated dictionary, and omitting optional keyvalue pairs.

hypothesis.strategies.
floats
(min_value=None, max_value=None, *, allow_nan=None, allow_infinity=None, width=64, exclude_min=False, exclude_max=False)[source]¶ Returns a strategy which generates floats.
If min_value is not None, all values will be
>= min_value
(or> min_value
ifexclude_min
).If max_value is not None, all values will be
<= max_value
(or< max_value
ifexclude_max
).If min_value or max_value is not None, it is an error to enable allow_nan.
If both min_value and max_value are not None, it is an error to enable allow_infinity.
Where not explicitly ruled out by the bounds, all of infinity, infinity and NaN are possible values generated by this strategy.
The width argument specifies the maximum number of bits of precision required to represent the generated float. Valid values are 16, 32, or 64. Passing
width=32
will still use the builtin 64bitfloat
class, but always for values which can be exactly represented as a 32bit float.The exclude_min and exclude_max argument can be used to generate numbers from open or halfopen intervals, by excluding the respective endpoints. Excluding either signed zero will also exclude the other. Attempting to exclude an endpoint which is None will raise an error; use
allow_infinity=False
to generate finite floats. You can however use e.g.min_value=math.inf, exclude_min=True
to exclude only one infinite endpoint.Examples from this strategy have a complicated and hard to explain shrinking behaviour, but it tries to improve “human readability”. Finite numbers will be preferred to infinity and infinity will be preferred to NaN.

hypothesis.strategies.
fractions
(min_value=None, max_value=None, *, max_denominator=None)[source]¶ Returns a strategy which generates Fractions.
If
min_value
is not None then all generated values are no less thanmin_value
. Ifmax_value
is not None then all generated values are no greater thanmax_value
.min_value
andmax_value
may be anything accepted by theFraction
constructor.If
max_denominator
is not None then the denominator of any generated values is no greater thanmax_denominator
. Note thatmax_denominator
must be None or a positive integer.Examples from this strategy shrink towards smaller denominators, then closer to zero.

hypothesis.strategies.
from_regex
(regex, *, fullmatch=False)[source]¶ Generates strings that contain a match for the given regex (i.e. ones for which
re.search()
will return a nonNone result).regex
may be a pattern orcompiled regex
. Both bytestrings and unicode strings are supported, and will generate examples of the same type.You can use regex flags such as
re.IGNORECASE
orre.DOTALL
to control generation. Flags can be passed either in compiled regex or inside the pattern with a(?iLmsux)
group.Some regular expressions are only partly supported  the underlying strategy checks local matching and relies on filtering to resolve contextdependent expressions. Using too many of these constructs may cause healthcheck errors as too many examples are filtered out. This mainly includes (positive or negative) lookahead and lookbehind groups.
If you want the generated string to match the whole regex you should use boundary markers. So e.g.
r"\A.\Z"
will return a single character string, while"."
will return any string, andr"\A.$"
will return a single character optionally followed by a"\n"
. Alternatively, passingfullmatch=True
will ensure that the whole string is a match, as if you had used the\A
and\Z
markers.Examples from this strategy shrink towards shorter strings and lower character values, with exact behaviour that may depend on the pattern.

hypothesis.strategies.
from_type
(thing)[source]¶ Looks up the appropriate search strategy for the given type.
from_type
is used internally to fill in missing arguments tobuilds()
and can be used interactively to explore what strategies are available or to debug type resolution.You can use
register_type_strategy()
to handle your custom types, or to globally redefine certain strategies  for example excluding NaN from floats, or use timezoneaware instead of naive time and datetime strategies.The resolution logic may be changed in a future version, but currently tries these five options:
If
thing
is in the default lookup mapping or userregistered lookup, return the corresponding strategy. The default lookup covers all types with Hypothesis strategies, including extras where possible.If
thing
is from thetyping
module, return the corresponding strategy (special logic).If
thing
has one or more subtypes in the merged lookup, return the union of the strategies for those types that are not subtypes of other elements in the lookup.Finally, if
thing
has type annotations for all required arguments, and is not an abstract class, it is resolved viabuilds()
.Because
abstract types
cannot be instantiated, we treat abstract types as the union of their concrete subclasses. Note that this lookup works via inheritance but not viaregister
, so you may still need to useregister_type_strategy()
.
There is a valuable recipe for leveraging
from_type()
to generate “everything except” values from a specified type. I.e.def everything_except(excluded_types): return ( from_type(type).flatmap(from_type) .filter(lambda x: not isinstance(x, excluded_types)) )
For example,
everything_except(int)
returns a strategy that can generate anything thatfrom_type()
can ever generate, except for instances ofint
, and excluding instances of types added viaregister_type_strategy()
.This is useful when writing tests which check that invalid input is rejected in a certain way.

hypothesis.strategies.
frozensets
(elements, *, min_size=0, max_size=None)[source]¶ This is identical to the sets function but instead returns frozensets.

hypothesis.strategies.
functions
(*, like=lambda : ..., returns=none(), pure=False)[source]¶ A strategy for functions, which can be used in callbacks.
The generated functions will mimic the interface of
like
, which must be a callable (including a class, method, or function). The return value for the function is drawn from thereturns
argument, which must be a strategy.If
pure=True
, all arguments passed to the generated function must be hashable, and if passed identical arguments the original return value will be returned again  not regenerated, so beware mutable values.If
pure=False
, generated functions do not validate their arguments, and may return a different value if called again with the same arguments.Generated functions can only be called within the scope of the
@given
which created them. This strategy does not support.example()
.

hypothesis.strategies.
integers
(min_value=None, max_value=None)[source]¶ Returns a strategy which generates integers.
If min_value is not None then all values will be >= min_value. If max_value is not None then all values will be <= max_value
Examples from this strategy will shrink towards zero, and negative values will also shrink towards positive (i.e. n may be replaced by +n).

hypothesis.strategies.
ip_addresses
(*, v=None, network=None)[source]¶ Generate IP addresses 
v=4
forIPv4Address
es,v=6
forIPv6Address
es, or leave unspecified to allow both versions.network
may be anIPv4Network
orIPv6Network
, or a string representing a network such as"127.0.0.0/24"
or"2001:db8::/32"
. As well as generating addresses within a particular routable network, this can be used to generate addresses from a reserved range listed in the IANA registries.If you pass both
v
andnetwork
, they must be for the same version.

hypothesis.strategies.
iterables
(elements, *, min_size=0, max_size=None, unique_by=None, unique=False)[source]¶ This has the same behaviour as lists, but returns iterables instead.
Some iterables cannot be indexed (e.g. sets) and some do not have a fixed length (e.g. generators). This strategy produces iterators, which cannot be indexed and do not have a fixed length. This ensures that you do not accidentally depend on sequence behaviour.

hypothesis.strategies.
just
(value)[source]¶ Return a strategy which only generates
value
.Note:
value
is not copied. Be wary of using mutable values.If
value
is the result of a callable, you can usebuilds(callable)
instead ofjust(callable())
to get a fresh value each time.Examples from this strategy do not shrink (because there is only one).

hypothesis.strategies.
lists
(elements, *, min_size=0, max_size=None, unique_by=None, unique=False)[source]¶ Returns a list containing values drawn from elements with length in the interval [min_size, max_size] (no bounds in that direction if these are None). If max_size is 0, only the empty list will be drawn.
If
unique
is True (or something that evaluates to True), we compare direct object equality, as if unique_by waslambda x: x
. This comparison only works for hashable types.If
unique_by
is not None it must be a callable or tuple of callables returning a hashable type when given a value drawn from elements. The resulting list will satisfy the condition that fori
!=j
,unique_by(result[i])
!=unique_by(result[j])
.If
unique_by
is a tuple of callables the uniqueness will be respective to each callable.For example, the following will produce two columns of integers with both columns being unique respectively.
>>> twoints = st.tuples(st.integers(), st.integers()) >>> st.lists(twoints, unique_by=(lambda x: x[0], lambda x: x[1]))
Examples from this strategy shrink by trying to remove elements from the list, and by shrinking each individual element of the list.

hypothesis.strategies.
none
()[source]¶ Return a strategy which only generates None.
Examples from this strategy do not shrink (because there is only one).

hypothesis.strategies.
nothing
()[source]¶ This strategy never successfully draws a value and will always reject on an attempt to draw.
Examples from this strategy do not shrink (because there are none).

hypothesis.strategies.
one_of
(*args)[source]¶ Return a strategy which generates values from any of the argument strategies.
This may be called with one iterable argument instead of multiple strategy arguments, in which case
one_of(x)
andone_of(*x)
are equivalent.Examples from this strategy will generally shrink to ones that come from strategies earlier in the list, then shrink according to behaviour of the strategy that produced them. In order to get good shrinking behaviour, try to put simpler strategies first. e.g.
one_of(none(), text())
is better thanone_of(text(), none())
.This is especially important when using recursive strategies. e.g.
x = st.deferred(lambda: st.none()  st.tuples(x, x))
will shrink well, butx = st.deferred(lambda: st.tuples(x, x)  st.none())
will shrink very badly indeed.

hypothesis.strategies.
permutations
(values)[source]¶ Return a strategy which returns permutations of the ordered collection
values
.Examples from this strategy shrink by trying to become closer to the original order of values.

hypothesis.strategies.
random_module
()[source]¶ The Hypothesis engine handles PRNG state for the stdlib and Numpy random modules internally, always seeding them to zero and restoring the previous state after the test.
If having a fixed seed would unacceptably weaken your tests, and you cannot use a
random.Random
instance provided byrandoms()
, this strategy callsrandom.seed()
with an arbitrary integer and passes you an opaque object whose repr displays the seed value for debugging. Ifnumpy.random
is available, that state is also managed.Examples from these strategy shrink to seeds closer to zero.

hypothesis.strategies.
randoms
(*, note_method_calls=False, use_true_random=None)[source]¶ Generates instances of
random.Random
. The generated Random instances are of a special HypothesisRandom subclass.If
note_method_calls
is set toTrue
, Hypothesis will print the randomly drawn values in any falsifying test case. This can be helpful for debugging the behaviour of randomized algorithms.If
use_true_random
is set toTrue
then values will be drawn from their usual distribution, otherwise they will actually be Hypothesis generated values (and will be shrunk accordingly for any failing test case). Settinguse_true_random=False
will tend to expose bugs that would occur with very low probability when it is set to True, and this flag should only be set to True when your code relies on the distribution of values for correctness.

hypothesis.strategies.
recursive
(base, extend, *, max_leaves=100)[source]¶ base: A strategy to start from.
extend: A function which takes a strategy and returns a new strategy.
max_leaves: The maximum number of elements to be drawn from base on a given run.
This returns a strategy
S
such thatS = extend(base  S)
. That is, values may be drawn from base, or from any strategy reachable by mixing applications of  and extend.An example may clarify:
recursive(booleans(), lists)
would return a strategy that may return arbitrarily nested and mixed lists of booleans. So e.g.False
,[True]
,[False, []]
, and[[[[True]]]]
are all valid values to be drawn from that strategy.Examples from this strategy shrink by trying to reduce the amount of recursion and by shrinking according to the shrinking behaviour of base and the result of extend.

hypothesis.strategies.
register_type_strategy
(custom_type, strategy)[source]¶ Add an entry to the global typetostrategy lookup.
This lookup is used in
builds()
and@given
.builds()
will be used automatically for classes with type annotations on__init__
, so you only need to register a strategy if one or more arguments need to be more tightly defined than their typebased default, or if you want to supply a strategy for an argument with a default value.strategy
may be a search strategy, or a function that takes a type and returns a strategy (useful for generic types).Note that you may not register a parametrised generic type (such as
MyCollection[int]
) directly, because the resolution logic does not handle this case correctly. Instead, you may register a function forMyCollection
and inspect the type parameters within that function.

hypothesis.strategies.
runner
(*, default=not_set)[source]¶ A strategy for getting “the current test runner”, whatever that may be. The exact meaning depends on the entry point, but it will usually be the associated ‘self’ value for it.
If there is no current test runner and a default is provided, return that default. If no default is provided, raises InvalidArgument.
Examples from this strategy do not shrink (because there is only one).

hypothesis.strategies.
sampled_from
(elements)[source]¶ Returns a strategy which generates any value present in
elements
.Note that as with
just()
, values will not be copied and thus you should be careful of using mutable data.sampled_from
supports ordered collections, as well asEnum
objects.Flag
objects may also generate any combination of their members.Examples from this strategy shrink by replacing them with values earlier in the list. So e.g.
sampled_from([10, 1])
will shrink by trying to replace 1 values with 10, andsampled_from([1, 10])
will shrink by trying to replace 10 values with 1.

hypothesis.strategies.
sets
(elements, *, min_size=0, max_size=None)[source]¶ This has the same behaviour as lists, but returns sets instead.
Note that Hypothesis cannot tell if values are drawn from elements are hashable until running the test, so you can define a strategy for sets of an unhashable type but it will fail at test time.
Examples from this strategy shrink by trying to remove elements from the set, and by shrinking each individual element of the set.
Returns a strategy that draws a single shared value per run, drawn from base. Any two shared instances with the same key will share the same value, otherwise the identity of this strategy will be used. That is:
>>> s = integers() # or any other strategy >>> x = shared(s) >>> y = shared(s)
In the above x and y may draw different (or potentially the same) values. In the following they will always draw the same:
>>> x = shared(s, key="hi") >>> y = shared(s, key="hi")
Examples from this strategy shrink as per their base strategy.

hypothesis.strategies.
slices
(size)[source]¶ Generates slices that will select indices up to the supplied size
Generated slices will have start and stop indices that range from size to size  1 and will step in the appropriate direction. Slices should only produce an empty selection if the start and end are the same.
Examples from this strategy shrink toward 0 and smaller values

hypothesis.strategies.
text
(alphabet=characters(blacklist_categories=('Cs')), *, min_size=0, max_size=None)[source]¶ Generates strings with characters drawn from
alphabet
, which should be a collection of length one strings or a strategy generating such strings.The default alphabet strategy can generate the full unicode range but excludes surrogate characters because they are invalid in the UTF8 encoding. You can use
characters()
without arguments to find surrogaterelated bugs such as bpo34454.min_size
andmax_size
have the usual interpretations. Note that Python measures string length by counting codepoints: U+00C5Å
is a single character, while U+0041 U+030AÅ
is two  theA
, and a combining ring above.Examples from this strategy shrink towards shorter strings, and with the characters in the text shrinking as per the alphabet strategy. This strategy does not
normalize()
examples, so generated strings may be in any or none of the ‘normal forms’.

hypothesis.strategies.
timedeltas
(min_value=datetime.timedelta.min, max_value=datetime.timedelta.max)[source]¶ A strategy for timedeltas between
min_value
andmax_value
.Examples from this strategy shrink towards zero.

hypothesis.strategies.
times
(min_value=datetime.time.min, max_value=datetime.time.max, *, timezones=none())[source]¶ A strategy for times between
min_value
andmax_value
.The
timezones
argument is handled as fordatetimes()
.Examples from this strategy shrink towards midnight, with the timezone component shrinking as for the strategy that provided it.

hypothesis.strategies.
tuples
(*args)[source]¶ Return a strategy which generates a tuple of the same length as args by generating the value at index i from args[i].
e.g. tuples(integers(), integers()) would generate a tuple of length two with both values an integer.
Examples from this strategy shrink by shrinking their component parts.

hypothesis.strategies.
uuids
(*, version=None)[source]¶ Returns a strategy that generates
UUIDs
.If the optional version argument is given, value is passed through to
UUID
and only UUIDs of that version will be generated.All returned values from this will be unique, so e.g. if you do
lists(uuids())
the resulting list will never contain duplicates.Examples from this strategy don’t have any meaningful shrink order.
Provisional strategies¶
This module contains various provisional APIs and strategies.
It is intended for internal use, to ease code reuse, and is not stable. Point releases may move or break the contents at any time!
Internet strategies should conform to RFC 3986 or the authoritative definitions it links to. If not, report the bug!
Shrinking¶
When using strategies it is worth thinking about how the data shrinks. Shrinking is the process by which Hypothesis tries to produce human readable examples when it finds a failure  it takes a complex example and turns it into a simpler one.
Each strategy defines an order in which it shrinks  you won’t usually need to care about this much, but it can be worth being aware of as it can affect what the best way to write your own strategies is.
The exact shrinking behaviour is not a guaranteed part of the API, but it doesn’t change that often and when it does it’s usually because we think the new way produces nicer examples.
Possibly the most important one to be aware of is
one_of()
, which has a preference for values
produced by strategies earlier in its argument list. Most of the others should
largely “do the right thing” without you having to think about it.
Adapting strategies¶
Often it is the case that a strategy doesn’t produce exactly what you want it to and you need to adapt it. Sometimes you can do this in the test, but this hurts reuse because you then have to repeat the adaption in every test.
Hypothesis gives you ways to build strategies from other strategies given functions for transforming the data.
Mapping¶
map
is probably the easiest and most useful of these to use. If you have a
strategy s
and a function f
, then an example s.map(f).example()
is
f(s.example())
, i.e. we draw an example from s
and then apply f
to it.
e.g.:
>>> lists(integers()).map(sorted).example()
[25527, 24245, 23118, 93, 70, 7, 0, 39, 40, 65, 88, 112, 6189, 9480, 19469, 27256, 32526, 1566924430]
Note that many things that you might use mapping for can also be done with
builds()
.
Filtering¶
filter
lets you reject some examples. s.filter(f).example()
is some
example of s
such that f(example)
is truthy.
>>> integers().filter(lambda x: x > 11).example()
26126
>>> integers().filter(lambda x: x > 11).example()
23324
It’s important to note that filter
isn’t magic and if your condition is too
hard to satisfy then this can fail:
>>> integers().filter(lambda x: False).example()
Traceback (most recent call last):
...
hypothesis.errors.Unsatisfiable: Could not find any valid examples in 20 tries
In general you should try to use filter
only to avoid corner cases that you
don’t want rather than attempting to cut out a large chunk of the search space.
A technique that often works well here is to use map to first transform the data
and then use filter
to remove things that didn’t work out. So for example if
you wanted pairs of integers (x,y) such that x < y you could do the following:
>>> tuples(integers(), integers()).map(sorted).filter(lambda x: x[0] < x[1]).example()
[8543729478746591815, 3760495307320535691]
Chaining strategies together¶
Finally there is flatmap
. flatmap
draws an example, then turns that
example into a strategy, then draws an example from that strategy.
It may not be obvious why you want this at first, but it turns out to be quite useful because it lets you generate different types of data with relationships to each other.
For example suppose we wanted to generate a list of lists of the same length:
>>> rectangle_lists = integers(min_value=0, max_value=10).flatmap(
... lambda n: lists(lists(integers(), min_size=n, max_size=n)))
>>> rectangle_lists.example()
[]
>>> rectangle_lists.filter(lambda x: len(x) >= 10).example()
[[], [], [], [], [], [], [], [], [], []]
>>> rectangle_lists.filter(lambda t: len(t) >= 3 and len(t[0]) >= 3).example()
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> rectangle_lists.filter(lambda t: sum(len(s) for s in t) >= 10).example()
[[0], [0], [0], [0], [0], [0], [0], [0], [0], [0]]
In this example we first choose a length for our tuples, then we build a strategy which generates lists containing lists precisely of that length. The finds show what simple examples for this look like.
Most of the time you probably don’t want flatmap
, but unlike filter
and
map
which are just conveniences for things you could just do in your tests,
flatmap
allows genuinely new data generation that you wouldn’t otherwise be
able to easily do.
(If you know Haskell: Yes, this is more or less a monadic bind. If you don’t know Haskell, ignore everything in these parentheses. You do not need to understand anything about monads to use this, or anything else in Hypothesis).
Recursive data¶
Sometimes the data you want to generate has a recursive definition. e.g. if you wanted to generate JSON data, valid JSON is:
Any float, any boolean, any unicode string.
Any list of valid JSON data
Any dictionary mapping unicode strings to valid JSON data.
The problem is that you cannot call a strategy recursively and expect it to not just blow up and eat all your memory. The other problem here is that not all unicode strings display consistently on different machines, so we’ll restrict them in our doctest.
The way Hypothesis handles this is with the recursive()
strategy which you pass in a base case and a function that, given a strategy
for your data type, returns a new strategy for it. So for example:
>>> from string import printable; from pprint import pprint
>>> json = recursive(none()  booleans()  floats()  text(printable),
... lambda children: lists(children, 1)  dictionaries(text(printable), children, min_size=1))
>>> pprint(json.example())
[[1.175494351e38, ']', 1.9, True, False, '.M}Xl', ''], True]
>>> pprint(json.example())
{'de(l': None,
'nK': {'(Rt)': None,
'+hoZh1YU]gy8': True,
'8z]EIFA06^li^': 'LFE{Q',
'9,': 'l{cA=/'}}
That is, we start with our leaf data and then we augment it by allowing lists and dictionaries of anything we can generate as JSON data.
The size control of this works by limiting the maximum number of values that can be drawn from the base strategy. So for example if we wanted to only generate really small JSON we could do this as:
>>> small_lists = recursive(booleans(), lists, max_leaves=5)
>>> small_lists.example()
True
>>> small_lists.example()
[False]
Composite strategies¶
The @composite
decorator lets
you combine other strategies in more or less
arbitrary ways. It’s probably the main thing you’ll want to use for
complicated custom strategies.
The composite decorator works by converting a function that returns one
example into a function that returns a strategy that produces such
examples  which you can pass to @given
, modify
with .map
or .filter
, and generally use like any other strategy.
It does this by giving you a special function draw
as the first
argument, which can be used just like the corresponding method of the
data()
strategy within a test. In fact,
the implementation is almost the same  but defining a strategy with
@composite
makes code reuse
easier, and usually improves the display of failing examples.
For example, the following gives you a list and an index into it:
>>> @composite
... def list_and_index(draw, elements=integers()):
... xs = draw(lists(elements, min_size=1))
... i = draw(integers(min_value=0, max_value=len(xs)  1))
... return (xs, i)
draw(s)
is a function that should be thought of as returning s.example()
,
except that the result is reproducible and will minimize correctly. The
decorated function has the initial argument removed from the list, but will
accept all the others in the expected order. Defaults are preserved.
>>> list_and_index()
list_and_index()
>>> list_and_index().example()
([15949, 35, 21764, 8167, 1607867656, 41, 104, 19, 90, 520116744169390387, 7107438879249457973], 0)
>>> list_and_index(booleans())
list_and_index(elements=booleans())
>>> list_and_index(booleans()).example()
([True, False], 0)
Note that the repr will work exactly like it does for all the builtin strategies: it will be a function that you can call to get the strategy in question, with values provided only if they do not match the defaults.
You can use assume
inside composite functions:
@composite
def distinct_strings_with_common_characters(draw):
x = draw(text(min_size=1))
y = draw(text(alphabet=x))
assume(x != y)
return (x, y)
This works as assume
normally would, filtering out any examples for which the
passed in argument is falsey.
Take care that your function can cope with adversarial draws, or explicitly rejects
them using the .filter()
method or assume()
 our mutation
and shrinking logic can do some strange things, and a naive implementation might
lead to serious performance problems. For example:
@composite
def reimplementing_sets_strategy(draw, elements=st.integers(), size=5):
# The bad way: if Hypothesis keeps generating e.g. zero,
# we'll keep looping for a very long time.
result = set()
while len(result) < size:
result.add(draw(elements))
# The good way: use a filter, so Hypothesis can tell what's valid!
for _ in range(size):
result.add(draw(elements.filter(lambda x: x not in result)))
return result
If @composite
is used to decorate a
method or classmethod, the draw
argument must come before self
or cls
.
While we therefore recommend writing strategies as standalone functions and using
the register_type_strategy()
function to associate
them with a class, methods are supported and the @composite
decorator may be
applied either before or after @classmethod
or @staticmethod
.
See issue #2578 and pull request #2634 for more details.
Drawing interactively in tests¶
There is also the data()
strategy, which gives you a means of using
strategies interactively. Rather than having to specify everything up front in
@given
you can draw from strategies in the body of your test.
This is similar to @composite
, but
even more powerful as it allows you to mix test code with example generation.
The downside of this power is that data()
is
incompatible with explicit @example(...)
s 
and the mixed code is often harder to debug when something goes wrong.
If you need values that are affected by previous draws but which don’t depend
on the execution of your test, stick to the simpler
@composite
.
@given(data())
def test_draw_sequentially(data):
x = data.draw(integers())
y = data.draw(integers(min_value=x))
assert x < y
If the test fails, each draw will be printed with the falsifying example. e.g. the above is wrong (it has a boundary condition error), so will print:
Falsifying example: test_draw_sequentially(data=data(...))
Draw 1: 0
Draw 2: 0
As you can see, data drawn this way is simplified as usual.
Optionally, you can provide a label to identify values generated by each call
to data.draw()
. These labels can be used to identify values in the output
of a falsifying example.
For instance:
@given(data())
def test_draw_sequentially(data):
x = data.draw(integers(), label="First number")
y = data.draw(integers(min_value=x), label="Second number")
assert x < y
will produce the output:
Falsifying example: test_draw_sequentially(data=data(...))
Draw 1 (First number): 0
Draw 2 (Second number): 0