# Details and advanced features¶

This is an account of slightly less common Hypothesis features that you don’t need to get started but will nevertheless make your life easier.

## Additional test output¶

Normally the output of a failing test will look something like:

```
Falsifying example: test_a_thing(x=1, y="foo")
```

With the `repr`

of each keyword argument being printed.

Sometimes this isn’t enough, either because you have values with a `repr`

that
isn’t very descriptive or because you need to see the output of some
intermediate steps of your test. That’s where the `note`

function comes in:

```
>>> from hypothesis import given, note, strategies as st
>>> @given(st.lists(st.integers()), st.randoms())
... def test_shuffle_is_noop(ls, r):
... ls2 = list(ls)
... r.shuffle(ls2)
... note("Shuffle: %r" % (ls2))
... assert ls == ls2
...
>>> test_shuffle_is_noop()
Falsifying example: test_shuffle_is_noop(ls=[0, 0, 1], r=RandomWithSeed(0))
Shuffle: [0, 1, 0]
Traceback (most recent call last):
...
AssertionError
```

The note is printed in the final run of the test in order to include any additional information you might need in your test.

## Test Statistics¶

If you are using py.test you can see a number of statistics about the executed tests
by passing the command line argument `--hypothesis-show-statistics`

. This will include
some general statistics about the test:

For example if you ran the following with `--hypothesis-show-statistics`

:

```
from hypothesis import given, strategies as st
@given(st.integers())
def test_integers(i):
pass
```

You would see:

```
test_integers:
- 200 passing examples, 0 failing examples, 0 invalid examples
- Typical runtimes: < 1ms
- Stopped because settings.max_examples=200
```

The final “Stopped because” line is particularly important to note: It tells you the setting value that determined when the test should stop trying new examples. This can be useful for understanding the behaviour of your tests. Ideally you’d always want this to be max_examples.

In some cases (such as filtered and recursive strategies) you will see events mentioned which describe some aspect of the data generation:

```
from hypothesis import given, strategies as st
@given(st.integers().filter(lambda x: x % 2 == 0))
def test_even_integers(i):
pass
```

You would see something like:

```
test_even_integers:
- 200 passing examples, 0 failing examples, 16 invalid examples
- Typical runtimes: < 1ms
- Stopped because settings.max_examples=200
- Events:
* 30.56%, Retried draw from integers().filter(lambda x: x % 2 == 0) to satisfy filter
* 7.41%, Aborted test because unable to satisfy integers().filter(lambda x: x % 2 == 0)
```

You can also mark custom events in a test using the ‘event’ function:

```
from hypothesis import given, event, strategies as st
@given(st.integers().filter(lambda x: x % 2 == 0))
def test_even_integers(i):
event("i mod 3 = %d" % (i % 3,))
```

You will then see output like:

```
test_even_integers:
- 200 passing examples, 0 failing examples, 28 invalid examples
- Typical runtimes: < 1ms
- Stopped because settings.max_examples=200
- Events:
* 47.81%, Retried draw from integers().filter(lambda x: x % 2 == 0) to satisfy filter
* 31.14%, i mod 3 = 2
* 28.95%, i mod 3 = 1
* 27.63%, i mod 3 = 0
* 12.28%, Aborted test because unable to satisfy integers().filter(lambda x: x % 2 == 0)
```

Arguments to event() can be any hashable type, but two events will be considered the same if they are the same when converted to a string with str().

## Making assumptions¶

Sometimes Hypothesis doesn’t give you exactly the right sort of data you want - it’s
mostly of the right shape, but some examples won’t work and you don’t want to care about
them. You *can* just ignore these by aborting the test early, but this runs the risk of
accidentally testing a lot less than you think you are. Also it would be nice to spend
less time on bad examples - if you’re running 200 examples per test (the default) and
it turns out 150 of those examples don’t match your needs, that’s a lot of wasted time.

The way Hypothesis handles this is to let you specify things which you *assume* to be
true. This lets you abort a test in a way that marks the example as bad rather than
failing the test. Hypothesis will use this information to try to avoid similar examples
in future.

For example suppose had the following test:

```
from hypothesis import given
from hypothesis.strategies import floats
@given(floats())
def test_negation_is_self_inverse(x):
assert x == -(-x)
```

Running this gives us:

```
Falsifying example: test_negation_is_self_inverse(x=float('nan'))
AssertionError
```

This is annoying. We know about NaN and don’t really care about it, but as soon as Hypothesis finds a NaN example it will get distracted by that and tell us about it. Also the test will fail and we want it to pass.

So lets block off this particular example:

```
from hypothesis import given, assume
from hypothesis.strategies import floats
from math import isnan
@given(floats())
def test_negation_is_self_inverse_for_non_nan(x):
assume(not isnan(x))
assert x == -(-x)
```

And this passes without a problem.

`assume()`

throws an exception which
terminates the test when provided with a false argument.
It’s essentially an assert, except that
the exception it throws is one that Hypothesis
identifies as meaning that this is a bad example, not a failing test.

In order to avoid the easy trap where you assume a lot more than you intended, Hypothesis will fail a test when it can’t find enough examples passing the assumption.

If we’d written:

```
from hypothesis import given, assume
from hypothesis.strategies import floats
@given(floats())
def test_negation_is_self_inverse_for_non_nan(x):
assume(False)
assert x == -(-x)
```

Then on running we’d have got the exception:

```
Unsatisfiable: Unable to satisfy assumptions of hypothesis test_negation_is_self_inverse_for_non_nan. Only 0 examples found after 0.0791318 seconds
```

### How good is assume?¶

Hypothesis has an adaptive exploration strategy to try to avoid things which falsify assumptions, which should generally result in it still being able to find examples in hard to find situations.

Suppose we had the following:

```
@given(lists(integers()))
def test_sum_is_positive(xs):
assert sum(xs) > 0
```

Unsurprisingly this fails and gives the falsifying example [].

Adding `assume(xs)`

to this removes the trivial empty example and gives us [0].

Adding `assume(all(x > 0 for x in xs))`

and it passes: A sum of a list of
positive integers is positive.

The reason that this should be surprising is not that it doesn’t find a counter-example, but that it finds enough examples at all.

In order to make sure something interesting is happening, suppose we wanted to try this for long lists. e.g. suppose we added an assume(len(xs) > 10) to it. This should basically never find an example: A naive strategy would find fewer than one in a thousand examples, because if each element of the list is negative with probability half, you’d have to have ten of these go the right way by chance. In the default configuration Hypothesis gives up long before it’s tried 1000 examples (by default it tries 200).

Here’s what happens if we try to run this:

```
@given(lists(integers()))
def test_sum_is_positive(xs):
assume(len(xs) > 10)
assume(all(x > 0 for x in xs))
print(xs)
assert sum(xs) > 0
In: test_sum_is_positive()
[17, 12, 7, 13, 11, 3, 6, 9, 8, 11, 47, 27, 1, 31, 1]
[6, 2, 29, 30, 25, 34, 19, 15, 50, 16, 10, 3, 16]
[25, 17, 9, 19, 15, 2, 2, 4, 22, 10, 10, 27, 3, 1, 14, 17, 13, 8, 16, 9, 2...
[17, 65, 78, 1, 8, 29, 2, 79, 28, 18, 39]
[13, 26, 8, 3, 4, 76, 6, 14, 20, 27, 21, 32, 14, 42, 9, 24, 33, 9, 5, 15, ...
[2, 1, 2, 2, 3, 10, 12, 11, 21, 11, 1, 16]
```

As you can see, Hypothesis doesn’t find *many* examples here, but it finds some - enough to
keep it happy.

In general if you *can* shape your strategies better to your tests you should - for example
`integers_in_range(1, 1000)`

is a lot better than `assume(1 <= x <= 1000)`

, but assume will take
you a long way if you can’t.

## Defining strategies¶

The type of object that is used to explore the examples given to your test
function is called a `SearchStrategy`

.
These are created using the functions
exposed in the `hypothesis.strategies`

module.

Many of these strategies expose a variety of arguments you can use to customize
generation. For example for integers you can specify `min`

and `max`

values of
integers you want:

```
>>> from hypothesis.strategies import integers
>>> integers()
RandomGeometricIntStrategy() | WideRangeIntStrategy()
>>> integers(min_value=0)
IntegersFromStrategy(0)
>>> integers(min_value=0, max_value=10)
BoundedIntStrategy(0, 10)
```

If you want to see exactly what a strategy produces you can ask for an example:

```
>>> integers(min_value=0, max_value=10).example()
7
```

Many strategies are build out of other strategies. For example, if you want to define a tuple you need to say what goes in each element:

```
>>> from hypothesis.strategies import tuples
>>> tuples(integers(), integers()).example()
(-1953, 85733644253897814191482551773726674360154905303788466954)
```

Further details are available in a separate document.

## The gory details of given parameters¶

The `@given`

decorator may be used
to specify what arguments of a function should
be parametrized over. You can use either positional or keyword arguments or a mixture
of the two.

For example all of the following are valid uses:

```
@given(integers(), integers())
def a(x, y):
pass
@given(integers())
def b(x, y):
pass
@given(y=integers())
def c(x, y):
pass
@given(x=integers())
def d(x, y):
pass
@given(x=integers(), y=integers())
def e(x, **kwargs):
pass
@given(x=integers(), y=integers())
def f(x, *args, **kwargs):
pass
class SomeTest(TestCase):
@given(integers())
def test_a_thing(self, x):
pass
```

The following are not:

```
@given(integers(), integers(), integers())
def g(x, y):
pass
@given(integers())
def h(x, *args):
pass
@given(integers(), x=integers())
def i(x, y):
pass
@given()
def j(x, y):
pass
```

The rules for determining what are valid uses of given are as follows:

- You may pass any keyword argument to given.
- Positional arguments to given are equivalent to the rightmost named arguments for the test function.
- positional arguments may not be used if the underlying test function has varargs or arbitrary keywords.
- Functions tested with given may not have any defaults.

The reason for the “rightmost named arguments” behaviour is so that
using `@given`

with instance methods works: self
will be passed to the function as normal and not be parametrized over.

The function returned by given has all the arguments that the original test did , minus the ones that are being filled in by given.

## Custom function execution¶

Hypothesis provides you with a hook that lets you control how it runs examples.

This lets you do things like set up and tear down around each example, run examples in a subprocess, transform coroutine tests into normal tests, etc.

The way this works is by introducing the concept of an executor. An executor is essentially a function that takes a block of code and run it. The default executor is:

```
def default_executor(function):
return function()
```

You define executors by defining a method execute_example on a class. Any
test methods on that class with `@given`

used on them will use
`self.execute_example`

as an executor with which to run tests. For example,
the following executor runs all its code twice:

```
from unittest import TestCase
class TestTryReallyHard(TestCase):
@given(integers())
def test_something(self, i):
perform_some_unreliable_operation(i)
def execute_example(self, f):
f()
return f()
```

Note: The functions you use in map, etc. will run *inside* the executor. i.e.
they will not be called until you invoke the function passed to setup_example.

An executor must be able to handle being passed a function which returns None, otherwise it won’t be able to run normal test cases. So for example the following executor is invalid:

```
from unittest import TestCase
class TestRunTwice(TestCase):
def execute_example(self, f):
return f()()
```

and should be rewritten as:

```
from unittest import TestCase
import inspect
class TestRunTwice(TestCase):
def execute_example(self, f):
result = f()
if inspect.isfunction(result):
result = result()
return result
```

Methods of a BasicStrategy however will typically be called whenever. This may happen inside your executor or outside. This is why they have a “Warning you have no control over the lifecycle of these values” attached.

## Using Hypothesis to find values¶

You can use Hypothesis’s data exploration features to find values satisfying some predicate:

```
>>> from hypothesis import find
>>> from hypothesis.strategies import sets, lists, integers
>>> find(lists(integers()), lambda x: sum(x) >= 10)
[10]
>>> find(lists(integers()), lambda x: sum(x) >= 10 and len(x) >= 3)
[0, 0, 10]
>>> find(sets(integers()), lambda x: sum(x) >= 10 and len(x) >= 3)
{0, 1, 9}
```

The first argument to `find()`

describes data in the usual way for an argument to
given, and supports all the same data types. The second is a
predicate it must satisfy.

Of course not all conditions are satisfiable. If you ask Hypothesis for an example to a condition that is always false it will raise an error:

```
>>> find(integers(), lambda x: False)
Traceback (most recent call last):
...
hypothesis.errors.NoSuchExample: No examples of condition lambda x: <unknown>
>>> from hypothesis.strategies import booleans
>>> find(booleans(), lambda x: False)
Traceback (most recent call last):
...
hypothesis.errors.NoSuchExample: No examples of condition lambda x: <unknown>
```

(The “lambda x: unknown” is because Hypothesis can’t retrieve the source code of lambdas from the interactive python console. It gives a better error message most of the time which contains the actual condition)

The reason for the two different types of errors is that there are only a small number of booleans, so it is feasible for Hypothesis to enumerate all of them and simply check that your condition is never true.

## Providing explicit examples¶

You can explicitly ask Hypothesis to try a particular example as follows:

```
from hypothesis import given, example
from hypothesis.strategies import text
@given(text())
@example("Hello world")
@example(x="Some very long string")
def test_some_code(x):
assert True
```

Hypothesis will run all examples you’ve asked for first. If any of them fail it will not go on to look for more examples.

It doesn’t matter whether you put the example decorator before or after given. Any permutation of the decorators in the above will do the same thing.

Note that examples can be positional or keyword based. If they’re positional then they will be filled in from the right when calling, so things like the following will also work:

```
from unittest import TestCase
from hypothesis import given, example
from hypothesis.strategies import text
class TestThings(TestCase):
@given(text())
@example("Hello world")
@example(x="Some very long string")
def test_some_code(self, x):
assert True
```

It is *not* permitted for a single example to be a mix of positional and
keyword arguments. Either are fine, and you can use one in one example and the
other in another example if for some reason you really want to, but a single
example must be consistent.