What you can generate and how

The general philosophy of Hypothesis data generation is that everything should be possible to generate and most things should be easy. Most things in the standard library is more aspirational than achieved, the state of the art is already pretty good.

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.

Functions for building strategies are all available in the hypothesis.strategies module. The salient functions from it are as follows:


This strategy never successfully draws a value and will always reject on an attempt to draw.


Return a strategy which only generates value.

Note: value is not copied. Be wary of using mutable values.


Return a strategy which only generates None.


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) and one_of(*x) are equivalent.

hypothesis.strategies.integers(min_value=None, max_value=None)[source]

Returns a strategy which generates integers (in Python 2 these may be ints or longs).

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


Returns a strategy which generates instances of bool.

hypothesis.strategies.floats(min_value=None, max_value=None, allow_nan=None, allow_infinity=None)[source]

Returns a strategy which generates floats.

  • If min_value is not None, all values will be >= min_value.
  • If max_value is not None, all values will be <= max_value.
  • 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.


Returns a strategy that generates complex numbers.


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.


Returns a strategy which generates any value present in the iterable elements.

Note that as with just, values will not be copied and thus you should be careful of using mutable data.

hypothesis.strategies.lists(elements=None, min_size=None, average_size=None, max_size=None, unique_by=None, unique=False)[source]

Returns a list containing values drawn from elements length in the interval [min_size, max_size] (no bounds in that direction if these are None). If max_size is 0 then elements may be None and only the empty list will be drawn.

average_size may be used as a size hint to roughly control the size of list but it may not be the actual average of sizes you get, due to a variety of factors.

If unique is True (or something that evaluates to True), we compare direct object equality, as if unique_by was lambda x: x. This comparison only works for hashable types.

if unique_by is not None it must be a function returning a hashable type when given a value drawn from elements. The resulting list will satisfy the condition that for i != j, unique_by(result[i]) != unique_by(result[j]).

hypothesis.strategies.sets(elements=None, min_size=None, average_size=None, 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.

hypothesis.strategies.frozensets(elements=None, min_size=None, average_size=None, max_size=None)[source]

This is identical to the sets function but instead returns frozensets.


Generate 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.

hypothesis.strategies.dictionaries(keys, values, dict_class=<type 'dict'>, min_size=None, average_size=None, max_size=None)[source]

Generates dictionaries of type dict_class with keys drawn from the keys argument and values drawn from the values argument.

The size parameters have the same interpretation as for lists.


Generates an infinite stream of values where each value is drawn from elements.

The result is iterable (the iterator will never terminate) and indexable.

hypothesis.strategies.characters(whitelist_categories=None, blacklist_categories=None, blacklist_characters=None, min_codepoint=None, max_codepoint=None)[source]

Generates unicode text type (unicode on python 2, str on python 3) characters following specified filtering rules.

This strategy accepts lists of Unicode categories, characters of which should (whitelist_categories) or should not (blacklist_categories) be produced.

Also there could be applied limitation by minimal and maximal produced code point of the characters.

If you know what exactly characters you don’t want to be produced, pass them with blacklist_characters argument.

hypothesis.strategies.text(alphabet=None, min_size=None, average_size=None, max_size=None)[source]

Generates values of a unicode text type (unicode on python 2, str on python 3) with values drawn from alphabet, which should be an iterable of length one strings or a strategy generating such. If it is None it will default to generating the full unicode range. If it is an empty collection this will only generate empty strings.

min_size, max_size and average_size have the usual interpretations.

hypothesis.strategies.binary(min_size=None, average_size=None, max_size=None)[source]

Generates the appropriate binary type (str in python 2, bytes in python 3).

min_size, average_size and max_size have the usual interpretations.


Generates instances of Random (actually a Hypothesis specific RandomWithSeed class which displays what it was initially seeded with)


If your code depends on the global random module then you need to use this.

It will explicitly seed the random module at the start of your test so that tests are reproducible. The value it passes you is an opaque object whose only useful feature is that its repr displays the random seed. It is not itself a random number generator. If you want a random number generator you should use the randoms() strategy which will give you one.

hypothesis.strategies.builds(target, *args, **kwargs)[source]

Generates values by drawing from args and kwargs and passing them to target in the appropriate argument position.

e.g. builds(target, integers(), flag=booleans()) would draw an integer i and a boolean b and call target(i, flag=b).

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 than min_value.

If max_value is not None then all generated values are no greater than max_value.

If max_denominator is not None then the absolute value of the denominator of any generated values is no greater than max_denominator. Note that max_denominator must be at least 1.

hypothesis.strategies.decimals(min_value=None, max_value=None, allow_nan=None, allow_infinity=None)[source]

Generates instances of decimals.Decimal.

  • If min_value is not None, all values will be >= min_value.
  • If max_value is not None, all values will be <= max_value.
  • 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.

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 that S = extend(base | S). That is, values maybe 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, []], [[[[True]]]], are all valid values to be drawn from that strategy.


Return a strategy which returns permutations of the collection “values”.


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.

hypothesis.strategies.shared(base, key=None)[source]

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:

>>> 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")

Strategy that generates a function that behaves like random.choice.

Will note choices made for reproducibility.


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.


Sometimes you need an input to be from a known set of items. hypothesis gives you 2 ways to do this, choice() and sampled_from().

Examples on how to use them both are below. First up choice:

from hypothesis import given, strategies as st

@given(user=st.text(min_size=1), service=st.text(min_size=1), choice=st.choices())
def test_tickets(user, service, choice):
    t=choice(('ST', 'LT', 'TG', 'CT'))
    # asserts go here.

This means t will randomly be one of the items in the list (‘ST’, ‘LT’, ‘TG’, ‘CT’). just like if you were calling random.choice() on the list.

A different, and probably better way to do this, is to use sampled_from:

from hypothesis import given, strategies as st

    user=st.text(min_size=1), service=st.text(min_size=1),
    t=st.sampled_from(('ST', 'LT', 'TG', 'CT')))
def test_tickets(user, service, t):
    # asserts and test code go here.

Values from sampled_from will not be copied and thus you should be careful of using mutable data. Which makes it great for the above use case, but may not always work out.

Infinite streams

Sometimes you need examples of a particular type to keep your test going but you’re not sure how many you’ll need in advance. For this, we have streaming types.

>>> from hypothesis.strategies import streaming, integers
>>> x = streaming(integers()).example()
>>> x
>>> x[2]
>>> x
Stream(32, 132, 209, ...)
>>> x[10]
>>> x
Stream(32, 132, 209, 843, -19, 58, 141, -1046, 37, 243, 130, ...)

Think of a Stream as an infinite list where we’ve only evaluated as much as we need to. As per above, you can index into it and the stream will be evaluated up to that index and no further.

You can iterate over it too (warning: iter on a stream given to you by Hypothesis in this way will never terminate):

>>> it = iter(x)
>>> next(it)
>>> next(it)
>>> next(it)
>>> next(it)

Slicing will also work, and will give you back Streams. If you set an upper bound then iter on those streams will terminate:

>>> list(x[:5])
[32, 132, 209, 843, -19]
>>> y = x[1::2]
>>> y
>>> y[0]
>>> y[1]
>>> y
Stream(132, 843, ...)

You can also apply a function to transform a stream:

>>> t = streaming(int).example()
>>> tm = t.map(lambda n: n * 2)
>>> tm[0]
>>> t[0]
>>> tm
Stream(26, ...)
>>> t
Stream(13, ...)

map creates a new stream where each element of the stream is the function applied to the corresponding element of the original stream. Evaluating the new stream will force evaluating the original stream up to that index.

(Warning: This isn’t the map builtin. In Python 3 the builtin map should do more or less the right thing, but in Python 2 it will never terminate and will just eat up all your memory as it tries to build an infinitely long list)

These are the only operations a Stream supports. There are a few more internal ones, but you shouldn’t rely on them.

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.


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.


>>> lists(integers()).map(sorted).example()
[1, 5, 17, 21, 24, 30, 45, 82, 88, 88, 90, 96, 105]

Note that many things that you might use mapping for can also be done with the builds function in hypothesis.strategies.


filter lets you reject some examples. s.filter(f).example() is some example of s such that f(s) is truthy.

>>> integers().filter(lambda x: x > 11).example()
>>> integers().filter(lambda x: x > 11).example()

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):
  File "<stdin>", line 1, in <module>
  File "/home/david/projects/hypothesis/src/hypothesis/searchstrategy/strategies.py", line 175, in example
    'Could not find any valid examples in 20 tries'
hypothesis.errors.NoExamples: 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(
... lambda x: tuple(sorted(x))).filter(lambda x: x[0] != x[1]).example()
(42, 1281698)

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 eachother.

For example suppose we wanted to generate a list of lists of the same length:

>>> from hypothesis.strategies import integers, lists
>>> from hypothesis import find
>>> rectangle_lists = integers(min_value=0, max_value=10).flatmap(lambda n:
... lists(lists(integers(), min_size=n, max_size=n)))
>>> find(rectangle_lists, lambda x: True)
>>> find(rectangle_lists, lambda x: len(x) >= 10)
[[], [], [], [], [], [], [], [], [], []]
>>> find(rectangle_lists, lambda t: len(t) >= 3 and len(t[0])  >= 3)
[[0, 0, 0], [0, 0, 0], [0, 0, 0]]
>>> find(rectangle_lists, lambda t: sum(len(s) for s in t) >= 10)
[[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:

  1. Any float, any boolean, any unicode string.
  2. Any list of valid JSON data
  3. 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 way Hypothesis handles this is with the ‘recursive’ function in hypothesis.strategies 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:

>>> import hypothesis.strategies as st
>>> json = st.recursive(st.floats() | st.booleans() | st.text() | st.none(),
... lambda children: st.lists(children) | st.dictionaries(st.text(), children))
>>> json.example()
{'': None, '\U000b3407\U000b3407\U000b3407': {
    '': '"é""é\x11', '\x13': 1.6153068016570349e-282,
    '\x00': '\x11\x11\x11"\x11"é"éé\x11""éé"\x11"éé\x11éé\x11é\x11',
  '\x80': 'é\x11\x11\x11\x11\x11\x11', '\x13\x13\x00\x80\x80\x00': 4.643602465868519e-144
  }, '\U000b3407': None}
>>> json.example()
>>> json.example()

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 = st.recursive(st.booleans(), st.lists, max_leaves=5)
>>> small_lists.example()
>>> small_lists.example()
[[False], [], [], [], [], []]
>>> small_lists.example()
>>> small_lists.example()

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 giving you a function as the first argument that you can use to draw examples from other strategies. For example, the following gives you a list and an index into it:

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().example()
([5585, 4073], 1)

>>> list_and_index(booleans())
>>> list_and_index(booleans()).example()
([False, False, True], 1)

Note that the repr will work exactly like it does for all the built-in 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:

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.

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:

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:

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