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Nicer list and iterable assertion messages for pytest

Project description

License: MPL 2.0 Build codecov Code style: black


Have you ever wanted to use all or any in a unit test, but found the assert message to be lacking? Do assertions on class attributes in collections almost make you wish you were coding in Java (with a nice assertion framework)? Then this is the pytest helper for you! pytest-iterassert provides all_match and any_match to give you nice asserts.


The built-in any or all can cause a lot of sadness when tests fail:

    def test_generator_without_iterassert() -> None:
>       assert all(i < 1 for i in range(3))
E       assert False
E        +  where False = all(<genexpr> at 0x10221a250>)

all_match and any_match make debugging easy by hoisting the comparison out, and printing meaningful debug:

    def test_generator_with_iterassert() -> None:
>       assert all_match(range(3)) < 1
E       assert all(0, 1, 2) < 1
E        +  where all(0, 1, 2) = all_match(range(0, 3))
E        +    where range(0, 3) = range(3)

How about a more complex example? Asserting attributes of a class instance is pretty common.

    def test_attr_of_classes_without_iterassert() -> None:
        foos = [Foo(1), Foo(2), Foo(3)]
>       assert all( < 3 for foo in foos)
E       assert False
E        +  where False = all(<genexpr> at 0x10597ca50>)

iterassert makes it easy to apply functions to the iterable, and will convince pytest to show you the result of that function!

    def test_attr_of_classes_with_iterassert_1() -> None:
        foos = [Foo(1), Foo(2), Foo(3)]
>       assert all_match(foos, get_bar) < 3
E       assert all(9001, 9002, 9003) < 3
E        +  where all(9001, 9002, 9003) = all_match([<Foo(1)>, <Foo(2)>, <Foo(3)>], get_bar)

It's also possible to run more complex checks against all items, by doing the checking inside a function:

    def test_attr_of_classes_with_iterassert_2() -> None:
        foos = [Foo(1), Foo(2), Foo(3)]
>       assert all_match(foos, check_bar)
E       assert all(False, False, False)
E        +  where all(False, False, False) = all_match([<Foo(1)>, <Foo(2)>, <Foo(3)>], check_bar)

Note in this case, much like the buildin functions, all_match and any_match take no operator or operand.

And, if you need to incorporate more transformations, but would like to see the intermediary items, capture allows for this, too:

    def test_attr_of_classes_with_iterassert_3() -> None:
        foos = [Foo(1), Foo(2), Foo(3)]
>       assert all_match(capture( for foo in foos), check_val)
E       assert all(False, False, False)
E        +  where all(False, False, False) = all_match([9001, 9002, 9003], check_val)
E        +    where [9001, 9002, 9003] = capture(<genexpr> at 0x1031220d0>)

Even the test summary says it all:

FAILED - assert False
FAILED - assert all(0, 1, 2) < 1
FAILED - assert False
FAILED - assert all(9001, 9002, 9003) < 3
FAILED - assert all(False, False, False)
FAILED - assert all(False, False, False)


pytest-iterassert is on PyPI, so you can simply install via pip install pytest-iterassert (requires Python 3.6 or higher).

(If you're really brave, you can also alias all_match and any_match to the builtin functions on import.)


[0.0.3] - 2020-05-10

  • Add capture, and allow all_match and any_match to not take an operator/operand, for checks inside the mapping function

[0.0.2] - 2020-05-07

  • Initial release


This library uses Poetry for managing dependencies. You just need to run poetry install, and it will create a virtual environment with all developer dependencies installed.

Please run poetry run ./lint before submitting pull requests.


This library is licensed under the Mozilla Public License Version 2.0. For more information, see LICENSE.

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