pytest-compatible "matcher objects"?

[seanh]

Does anyone know of a library of matcher objects or matcher functions that work nicely with pytest's assert statement? Or does anyone have any advice on how to write one?

I'm talking about this sort of thing:

class AnyIterable:
    def __init__(self, containing, only=False, in_order=False, ...):
        ...

    def __eq__(self, other):
        ...

def test_something():
    result = some_function()

    assert result == AnyIterable(containing=[1, 2, 3], only=True)

It wouldn't necessarily have to have this particular interface/API design. But the idea is that it provides a way to assert that result as an iterable (of any type) containing 1, 2 and 3, in any order, but no other values, all in a single easy-to-write assertion.

This particular example is a bit silly. But you can imagine a library with a collection of "matcher objects" like this for matching all sorts of different values (AnyInstanceOf(SomeModel, with_attrs={"foo": 42, "bar": 23}), etc), along with providing the user with the ability to write their own matchers.

The aim is to enable writing complex test assertions more fluently.

PyHamcrest seems to be the leading implementation of this sort of thing in Python but that doesn't look particularly pytest-friendly. It has its own assertion methods like assert_that(theBiscuit, equal_to(myBiscuit)), rather than working with normal assert's and comparisons.

Thanks!

[jwodder]

I believe anys meets your needs.

Full disclosure: I wrote it.

[RonnyPfannschmidt]

@jwodder that looks amazingly close to some ideas we had for matches to integrate in pytest as well

i'd love to collaborate wrt making this a more integral part as the lack of matchers is something that pains since a while

[RonnyPfannschmidt]

@jwodder it seems like there may be a need for better assertrepr_compare hooks to better show the differeces for certain matches