[ENH] pytest conditional fixtures

[fkiraly]

This PR contains a plugin for pytest that allows the user to easily specify conditional/nested fixtures over combinations of variables.

The utility is in utils._testing._conditional_fixtures, the function conditional_fixtures_and_names. It takes conditional fixture generation functions and allows arbitrary nesting of conditionals (as long as they are non-circular).

As a proof-of-concept of how this simplifies things, I've used it to refactor test_all_estimators.

This will allow us to specify conditional fixtures in the future, and pytest_generate_tests will always be the same. This will also unlock easy inheritance of tests when wrapped into classes.

Closely related to discussion in #1819, especially the part about the loops at the end. The conditional fixtures utility allows to easily specify the loops, by moving modular conditional functions around.

[fkiraly]

FYI @Lovkush-A, related to our design discussion.

I think this the "right" form of the loops.

[lmmentel]

@fkiraly would you able able to give a more accessible explanation of what this PR is trying to achieve, fix, enhance?

[fkiraly]

Sure. From the docstring, an example, which may help:

    Example: we want to loop over two fixture variables, "number" and "multiples"
        "number" are integers from 1 to 10,
        "multiples" are multiples of "number" up to "number"-squared
        we then write a generator_dict with two entries
        generator_dict["number"] is a function (test_name, **kwargs) -> list
            that returns [1, 2, ..., 10]
        generator_dict["multiples"] is a function (test_name, number, **kwargs) -> list
            that returns [number, 2* number, ..., number*number]

In pytest, how would you do that? You would do a mark.parameterize, and manually generate the combination of fixtures.
If you have more than two fixture variables, this gets tedious. So we decompose this into conditional fixtuers, and write only the logic that generates fixtures for variable i given fixtures for variables i-1, i-2, ..., 1. The rest is done by the framework (see docstring for the syntax).

[fkiraly]

@fkiraly would you able able to give a more accessible explanation of what this PR is trying to achieve, fix, enhance?

now in #1922 which describes the "target state", hope that is accessible...

[lmmentel]

I don't really get what's happening in get_fixtures or how to test it, I trust that it's doing something useful.

[fkiraly]

I don't really get what's happening in get_fixtures or how to test it, I trust that it's doing something useful.

it's just a wrapper around an invocation of generator_dict values (which are functions generating fixture lists and optionally names), and adds names if these are not returned by that function.

I'll add clarification to the docstring.

[fkiraly]

approvals from @chrisholder, @lmmentel up to non-blocking comments which have been addressed (I hope) - merging