Your project

(intro_hypothesis) jack@localhost:~/Repos/intro-to-hypothesis$ tree -L 2
.
├── my_package
│   ├── __init__.py
│   └── my_module.py
└── tests
    ├── __init__.py
    └── test_my_module.py

Your code

# my_package/my_module.py

def add_numbers(a, b):
    return a + b

# tests/test_my_module.py

from my_package.my_module import add_numbers


def test_add_numers_is_commutative():
    assert add_numbers(1.23, 4.56) == add_numbers(4.56, 1.23)

Your tests

pytest -v

# or...
python -m pytest -v

==================================== test session starts =====================================
platform linux -- Python 3.6.3, pytest-3.5.0, py-1.5.3, pluggy-0.6.0
               -- /home/jack/.virtualenvs/intro_hypothesis/bin/python3.6
cachedir: .pytest_cache
rootdir: /home/jack/Repos/intro-to-hypothesis, inifile:
collected 1 item                                                                             

tests/test_my_module.py::test_add_numers_is_commutative PASSED                         [100%]

================================== 1 passed in 0.01 seconds ==================================

Success?

You didn't prove that the commutative property holds in general. You just proved that such property holds for this specific case.

The combination of 1.23 and 4.56 is a very tiny subset of the entire input space of numbers that your function can receive...

# tests/test_my_module.py

from my_package.my_module import add_numbers


def test_add_numers_is_commutative():
    assert add_numbers(1.23, 4.56) == add_numbers(4.56, 1.23)

So...?

The lazy solution

# tests/test_my_module.py

from my_package.my_module import add_numbers


def test_add_numers_is_commutative():
    assert add_numbers(1.23, 4.56) == add_numbers(4.56, 1.23)

def test_add_numers_is_commutative_another_case(self):
    assert add_numbers(0.789, 321) == add_numbers(321, 0.789)

# more tests here...

You simply write more test cases.

For these other specific cases the commutative property holds. But you don't want to write a million test cases by hand...

The risky solution

# tests/test_my_module.py
import random
import unittest
from ddt import ddt, idata, unpack
from my_package.my_module import add_numbers

def float_pairs_generator():
    num_test_cases = 100
    for i in range(num_test_cases):
        a = random.random() * 10.0
        b = random.random() * 10.0
        yield (a, b)

@ddt
class TestAddNumbers(unittest.TestCase):
    @idata(float_pairs_generator())
    @unpack
    def test_add_floats_ddt(self, a, b):
        self.assertEqual(add_numbers(a, b), add_numbers(b, a))

You use fuzzing to create random test cases at every run.

You use ddt to multiply the test cases (works with pytest too).

100 test cases

==================================== test session starts =====================================
platform linux -- Python 3.6.3, pytest-3.5.0, py-1.5.3, pluggy-0.6.0
               -- /home/jack/.virtualenvs/intro_hypothesis/bin/python3.6
cachedir: .pytest_cache
rootdir: /home/jack/Repos/intro-to-hypothesis, inifile:
collected 100 items 

tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00001__1_5626962926374943__1_9960917540401857_ PASSED [  1%]
tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00002__8_826169800117212__0_46531523690026333_ PASSED [  2%]
tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00003__5_9016174415787415__9_626363288868493_ PASSED [  3%]
tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00004__4_2946227013991685__6_73085683629837_ PASSED [  4%]
tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00005__5_758774597260805__9_72994743211482_ PASSED [  5%]

...

tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00096__7_322612946947605__3_3536474120855377_ PASSED [ 96%]
tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00097__1_075369396293605__4_872490525884292_ PASSED [ 97%]
tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00098__1_5173664261532571__1_2611556220323872_ PASSED [ 98%]
tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00099__6_727606012779317__2_4322871197800144_ PASSED [ 99%]
tests/test_my_module.py::TestAddNumbers::test_add_floats_ddt_00100__9_319277865106583__3_9858815547475537_ PASSED [100%]

================================= 100 passed in 0.15 seconds =================================

Still not enough

If you think about it, we are still testing some random combinations of numbers between 0.0 and 10.0 here...

It's not a very extensive portion of the input domain of the function add_numbers.

What to do?

1. find a way to generate domain objects that your function can accept. In this case the domain objects are the floats that add_numbers can receive.
2. use hypothesis

The best solution

# tests/test_my_module.py

from hypothesis import given
from hypothesis.strategies import floats
from my_package.my_module import add_numbers


@given(a=floats(), b=floats())
def test_add_numbers(a, b):
    assert add_numbers(a, b) == add_numbers(b, a)

Run the tests and...

What happened?

Increase the verbosity level of your test by using the @settings decorator to find it out.

# tests/test_my_module.py

from hypothesis import given, settings, Verbosity
from hypothesis.strategies import floats
from my_package.my_module import add_numbers


@settings(verbosity=Verbosity.verbose)
@given(a=floats(), b=floats())
def test_add_numbers(a, b):
    assert add_numbers(a, b) == add_numbers(b, a)

A falsifying example

Trying example: test_add_numbers(a=nan, b=0.0)
Traceback (most recent call last):
  [...]
  File "/home/jack/Repos/intro-to-hypothesis/tests/test_my_module_hypothesis.py", line 10,
  in test_add_numbers
    assert add_numbers(a, b) == add_numbers(b, a)
AssertionError: assert nan == nan
 +  where nan = add_numbers(nan, 0.0)
 +  and   nan = add_numbers(0.0, nan)

Trying example: test_add_numbers(a=6.4348585852518236e-232, b=0.0)
Trying example: test_add_numbers(a=-0.99999, b=3.402823466e+38)
Trying example: test_add_numbers(a=-inf, b=-1.175494351e-38)

Falsifying example: test_add_numbers(a=0.0, b=nan)

Some test cases are fine.

Some other ones are not.

Fail? It depends...

from hypothesis import given
from hypothesis.strategies import floats
from my_package.my_module import add_numbers

@given(
  a=floats(allow_nan=False, allow_infinity=False),
  b=floats(allow_nan=False, allow_infinity=False))
def test_add_numbers(a, b):
    assert add_numbers(a, b) == add_numbers(b, a)

The test fails because in Python nan is a valid float.

Is nan a valid input for your application? What about inf?

For example, if you are absolutely sure that add_numbers will never receive a nan or a inf as inputs, you can write a test that never generates either nan or inf.

Test exceptions 1/4

What if add_numbers could in fact receive nan or inf as (invalid) inputs?

• the test should be able to generate nan or inf as (invalid) inputs
• add_numbers should raise a specific exception for each (invalid) input
• the test should reject that failure (i.e. the test should not consider that specific exception a failure)

Test exceptions 2/4

# my_package/my_module.py

import math

class NaNIsNotAllowed(ValueError):
    pass

class InfIsNotAllowed(ValueError):
    pass

def add_numbers(a, b):
    if math.isnan(a) or math.isnan(b):
        raise NaNIsNotAllowed('nan is not a valid input')
    elif math.isinf(a) or math.isinf(b):
        raise InfIsNotAllowed('inf is not a valid input')
    return a + b

Test exceptions 3/4

========================================== FAILURES ==========================================
______________________________________ test_add_numbers ______________________________________

    @given(a=floats(), b=floats())
>   def test_add_numbers(a, b):
E   hypothesis.errors.MultipleFailures: Hypothesis found 2 distinct failures.

tests/test_my_module_hypothesis.py:7: MultipleFailures
----------------------------------------- Hypothesis -----------------------------------------
Falsifying example: test_add_numbers(a=0.0, b=nan)
Traceback (most recent call last):
  [...]
    raise NaNIsNotAllowed('nan is not a valid input')
my_package.my_module.NaNIsNotAllowed: nan is not a valid input

Falsifying example: test_add_numbers(a=0.0, b=inf)
Traceback (most recent call last):
  [...]
    raise InfIsNotAllowed('inf is not a valid input')
my_package.my_module.InfIsNotAllowed: inf is not a valid input

Test exceptions 4/4

# tests/test_my_module.py

from hypothesis import given, reject
from hypothesis.strategies import floats
from my_package.my_module import add_numbers, NaNIsNotAllowed, InfIsNotAllowed


@given(a=floats(), b=floats())
def test_add_numbers_invalid_inputs(a, b):
    try:
        assert add_numbers(a, b) == add_numbers(b, a)
    except (NaNIsNotAllowed, InfIsNotAllowed):
        reject()

Explicit examples

# tests/test_my_module.py

from hypothesis import given, example
from hypothesis.strategies import floats
from my_package.my_module import add_numbers


@example(a=1.23, b=4.56)
@given(
  a=floats(allow_nan=False, allow_infinity=False),
  b=floats(allow_nan=False, allow_infinity=False))
def test_add_numbers_explicit_example(a, b):
    assert add_numbers(a, b) == add_numbers(b, a)

Test all the things!

# tests/test_my_module.py

from hypothesis import given, example, reject
from hypothesis.strategies import floats
from my_package.my_module import add_numbers, NaNIsNotAllowed, InfIsNotAllowed

@example(a=1.23, b=4.56)
@given(
  a=floats(allow_nan=False, allow_infinity=False),
  b=floats(allow_nan=False, allow_infinity=False))
def test_add_numbers_explicit_example(a, b):
    assert add_numbers(a, b) == add_numbers(b, a)

@given(
  a=floats(allow_nan=False, allow_infinity=False),
  b=floats(allow_nan=False, allow_infinity=False))
def test_add_numbers_valid_inputs(a, b):
    assert add_numbers(a, b) == add_numbers(b, a)

@given(a=floats(), b=floats())
def test_add_numbers_invalid_inputs(a, b):
    try:
        assert add_numbers(a, b) == add_numbers(b, a)
    except (NaNIsNotAllowed, InfIsNotAllowed):
        reject()

Success!

==================================== test session starts =====================================
platform linux -- Python 3.6.3, pytest-3.5.0, py-1.5.3, pluggy-0.6.0
               -- /home/jack/.virtualenvs/intro_hypothesis/bin/python3.6
cachedir: .pytest_cache
rootdir: /home/jack/Repos/intro-to-hypothesis, inifile:
plugins: hypothesis-3.55.1
collected 3 items                                                                            

tests/test_my_module_hypothesis.py::test_add_numbers_explicit_example PASSED           [ 33%]
tests/test_my_module_hypothesis.py::test_add_numbers_valid_inputs PASSED               [ 66%]
tests/test_my_module_hypothesis.py::test_add_numbers_invalid_inputs PASSED             [100%]

================================== 3 passed in 0.51 seconds ==================================

Reference

• What is property based testing?
• Getting started with Hypothesis
• Anatomy of a Hypothesis based test
• Evolving toward property-based testing with Hypothesis
• List of articles on Hypothesis
• Rule based stateful testing
• Unit tests that write themselves
• These slides as a blog post

@jackdbd

giacomodebidda.com