Why does @pytest.fixture(autouse=True) not populate namespace of test function?

[ego-thales]

Hi everyone 👋

I'm quite new to pytest and I love it, so... thanks!! I recently understood -- or so I though -- how to use @pytest.fixture(autouse=True) with the setup and teeardown logics separated with yield .

However, I was surprised to discover that the yield statement does not add the fixture to the namespace of the test function. For example, I expected the following to work.

# conftest.py
@pytest.fixture(autouse=True)
def tmp():
    print("setup")
    yield "tmp"
    print("teardown")
    return

# test_file.py
def test_autouse_in_namespace():
    assert tmp == "tmp"

But as you probably know, it fails.

_______________ test_autouse_in_namespace _______________

    def test_autouse_in_namespace():
>       assert tmp == "tmp"
E       NameError: name 'tmp' is not defined

test/test_file.py:2: NameError
----------------- Captured stdout setup -----------------
setup
--------------- Captured stdout teardown ----------------
teardown

My questions are then:

1. Is there a good reason for pytest not to make this work?
2. Is there an elegant pythonic way to emulate this?

Thanks for your time!

[The-Compiler]

Moving this to a discussion, please use those for questions and only use issues for things that are actionable in some way.

pytest doesn't do this because there is no way to magically "add something to the namespace" when you call a function. If you need the value of a fixture, you will need to have it as an argument.

[ego-thales]

Thank you. I will say though that I specifically opened a "feature request" issue because it could be actionable.

there is no way to magically "add something to the namespace" when you call a function

Are you definitive about this? I'm not entirely sold. For example, I don't know all pytest magic, but it doesn't seem too far fetched to use ast and exec to manually edit the source code of test functions and append said fixtures. Furthermore, there may also be approaches manipulating frames and f_locals.

No?

[The-Compiler]

Okay, there is no sane way to do that. I'm sure there is a way somehow, but that will definitely bring more trouble than it's worth.

For starters, pytest currently works by importing the test file and then calling the functions in it, and it will need to import the file to discover the fixtures in it.

[ego-thales]

Thanks for your reply! So when pytest is calling the functions, it could very well do it using exec for example, no?
I don't think this is particularly insane, even the standard library dataclasses uses this for example.

Could you please consider moving this back to "feature request" issues? I think it makes A LOT of sense for a package that already manipulates namespaces and scopes such as pytest, and it would make testing so much more elegant in many cases (for example, imagine have a null autouse fixture, no need to have it in every test function ssignature anymore).

Thanks in advance :)

[The-Compiler]

This has been discussed many times before, see e.g. #9955. How does pytest "already manipulate namespaces and scopes", and what would that null fixture do?

[ego-thales]

My point is not specifically about null. I would personally need it for a fixture used in 50+% of my tests.

Often, null = object() acts as a local sentinel value to check return values for functions that accept a default return on fail (e.g. getattr(obj, attr, null) is null, though this is a bad example since hasattr exists). See this python doc for example. It avoid the need to raise and catch many errors sometimes.

I will take a look at your referenced discussion soon, thanks!

[ego-thales]

Very simplistic example.

>>> def foo():
...     print(bar)
... 
... exec(foo.__code__, {"bar": "baz"})
... 
baz

[eliegoudout]

@The-Compiler
It looks to me that adding the following two lines to pytest_pyfunc_call at solves the problem almost perfectly and is pretty sane.

autoused = {arg: funcargs[arg] for arg in funcargs if arg not in testargs}
testfunction = types.FunctionType(testfunction.__code__, testfunction.__globals__ | autoused)

Two caveats:

• It is still not possible to directly use, for example, autoused_int_fixture += 1 without a prior global autosed_int_fixture statement in test definition. But this is fine as long as documented (we're only adding functionality here)
• These two lines make the test work with a copy of the globals dict, so if the test actually modifies its globals() (which looks like a terrible idea and is probably discouraged somewhere in pytest docs), we may run into trouble.

See #13180.