pytest_generate_tests can't override fixture parameters?

This is supposed to work right?

import pytest

@pytest.fixture(params=[10,20])
def foo(request):
    return request.param

def test_foo(foo):
    assert foo % 10 == 0

def pytest_generate_tests(metafunc):
    metafunc.parametrize('foo', [100,200,300])

What I get is:

e/lib/python3.8/site-packages/_pytest/fixtures.py:1581: in pytest_generate_tests
    metafunc.parametrize(
e/lib/python3.8/site-packages/_pytest/python.py:1100: in parametrize
    newcallspec.setmulti2(
e/lib/python3.8/site-packages/_pytest/python.py:932: in setmulti2
    self._checkargnotcontained(arg)
e/lib/python3.8/site-packages/_pytest/python.py:909: in _checkargnotcontained
    raise ValueError(f"duplicate {arg!r}")
E   ValueError: duplicate 'foo'

environment:

(e) dispater.local:~/Desktop$ pip list
Package    Version
---------- -------
attrs      20.3.0
iniconfig  1.1.1
packaging  20.9
pip        20.3.3
pluggy     0.13.1
py         1.10.0
pyparsing  2.4.7
pytest     6.2.2
setuptools 51.1.0
toml       0.10.2
wheel      0.36.2
WARNING: You are using pip version 20.3.3; however, version 21.0.1 is available.

[RonnyPfannschmidt]

I wonder if this is about the hook order

The Parametrize mark is implemented using that hook

[rjboczar]

Gotcha; I tried this out to force a specific fixture to always be indirect = True in parametrize calls. It worked, but I bet with a more complicated setup something would break. I poked through the code a bit and there were definitely some scoping things you’d also have to fix manually.