Should pytest copy the input parameters to a parameterized fixture?

[lithomas1]

In pytest, when using a parameterized fixture, it looks like pytest does not copy the input parameters to the fixture.
This means that the inputs to the parameterized fixture are essentially cached if request.param is directly returned from the function, making it possible to modify the fixture values (even if the scope is equal to function).

Example test case. 

import io import pytest @pytest.fixture( params=[io.StringIO()], ) def bufs(request): return request.param def test1(bufs): bufs.write("hi!") assert bufs.getvalue() == "hi!" def test2(bufs): bufs.write("bye!") # Fails!!! since there is already stuff in bufs assert bufs.getvalue() == "bye!" 

To prevent this, should pytest copy the input parameters to fixtures?
(The above test case will pass if you do copy.copy on request.param)

[RonnyPfannschmidt]

No, Parameters ough@t to be immutable

If it's something mutable, make it in a fixture

[lithomas1]

Thanks, is this documented somewhere?

I skimmed through https://docs.pytest.org/en/7.1.x/how-to/fixtures.html#parametrizing-fixtures, and didn't see anything about mutability in there.