Test parametrization does not affect the fixture dependency tree

[sermoz]

pytest v3.0.5

@pytest.fixture
def fix1(fix2):
   return fix2 + '1'

@pytest.fixture
def fix2(fix3):
   return fix3 + '2'

@pytest.fixture
def fix3():
   print("fix3 instantiated!!")
   return '3'

@pytest.mark.parametrize('fix2', ['2'])
def test_it(fix1):
   assert fix1 == '21'

This example demonstrates that fix3 is still instantiated, although fix2 - fix3 dependency should have been broken because of parametrization.

[nicoddemus]

Thanks for the report! 👍

[sermoz]

Can I expect this to be fixed by some date? Is this issue included in any future release?

[The-Compiler]

Nobody is currently working on this - so unless you submit a fix yourself (or get a support contract with Merlinux), there isn't really a guarantee things will get fixed at some point. Keep in mind we're all volunteers 😉

[timofurrer]

What is the expected behavior for the above example?
I might be able to work on it.

[nicoddemus]

@timofurrer

Here's a more complete example:

import pytest

@pytest.fixture
def fix1(fix2):
   return fix2 + '1'

@pytest.fixture
def fix2(fix3):
    print('fix2 instantiated!!')
    return fix3 + '2'

@pytest.fixture
def fix3():
    print("fix3 instantiated!!")
    return '3'

@pytest.mark.parametrize('fix2', ['2'])
def test_it(fix1):
   assert fix1 == '21'

Running pytest -s:

fix3 instantiated!!
.
1 passed in 0.01 seconds

As can be seen, even though fix2 was not instantiated because it was shadowed by the parametrize call in test_it, fix3 was still instantiated because of the fix2 dependency to fix3.

At least that's how I understood it, @egnartsms feel free to correct me.

[timofurrer]

fix1 is instantiated, too. Which of the fixtures should be instantiated? Non of them?

[nicoddemus]

@timofurrer yeah it is a little confusing at first, here's a summary of what's happening:

• fixture fix1 should be instantiated because test_it explicitly asks for it in its parameter list;
• parameter fix2 is injected into test_it because of the parametrize call;
• fixture fix2 is never instantiated because it is not part of the parameter list of test_it;
• fixture fix3 should never be instantiated because, while it is a dependency of fixture fix2, fix2 itself was not instantiated and so shouldn't fix3.

It seems fixture fix2 is at some point being considered a dependency of test_it (which implicitly also makes fix3 a dependency), and then later fix2 is replaced by the parameter fix2 but fix3 is kept. It warrants more investigation for sure. 😅

[nicoddemus]

Fixed by #3629, thanks @egnartsms!