Tests parametrized with a generator expression are not collecting as expected in an inherited test class

[ptpan]

The following tests

class TestBase:
  def run_test( a, b ):
    assert 0 <= a + b <= 2

  @pytest.mark.parametrize(
    "a, b",
    product([0, 1], [0, 1])
  )
  def test( s, a, b ):
    s.run_test( a, b )

class TestDerived( TestBase ):
  def run_test( a, b ):
    assert a == 0 or a == 1
    assert b == 0 or b == 1

are collected as

  <Class TestBase>
      <Function test[0-0]>
      <Function test[0-1]>
      <Function test[1-0]>
      <Function test[1-1]>
  <Class TestDerived>
      <Function test[a0-b0]>

where as the desired collection result is

  <Class TestBase>
      <Function test[0-0]>
      <Function test[0-1]>
      <Function test[1-0]>
      <Function test[1-1]>
  <Class TestDerived>
      <Function test[0-0]>
      <Function test[0-1]>
      <Function test[1-0]>
      <Function test[1-1]>

Explicitly converting the generator expression into a list yields the expected result.

Pytest-related packages

pytest             5.0.1
pytest-cov         2.7.1
pytest-forked      1.0.2
pytest-xdist       1.29.0

I can reproduce this problem with Python 3.6.8 on CentOS and Python 3.7.0 on MacOS.

[RonnyPfannschmidt]

Pytest should warn about that, it's always a mistake to reuse a consumable collection