Do not pass on None as scope

Make sure that the default scope always is "function" so as to avoid issues with pytest <= 6, which 'translates' the scope string kwarg into an index from a list. The list does not contain None.

Author: michele-riva

src/pytest_cases/fixture_parametrize_plus.py

@@ -316,7 +316,7 @@ def create(cls,
  i, # type: int
  argvalue, # type: Any
  id, # type: Union[str, Callable]
- scope='function', # type: str
+ scope=None,  # type: str
  hook=None, # type: Callable
  debug=False # type: bool
  ):
@@ -427,7 +427,7 @@ def create(cls,
  to_i, # type: int
  argvalues, # type: Any
  ids, # type: Union[Sequence[str], Callable]
- scope='function', # type: str
+ scope="function", # type: str
  hook=None, # type: Callable
  debug=False # type: bool
  ):
@@ -876,15 +876,17 @@ def _create_params_alt(fh, test_func, union_name, from_i, to_i, hook): # noqa
  return SingleParamAlternative.create(new_fixture_host=fh, test_func=test_func,
  param_union_name=union_name, argnames=argnames, i=i,
  argvalue=marked_argvalues[i], id=_id,
- scope=scope, hook=hook, debug=debug)
+ scope=scope or "function",
+ hook=hook, debug=debug)
  else:
  # If an explicit list of ids was provided, slice it. Otherwise the provided callable will be used later
  _ids = ids[from_i:to_i] if explicit_ids_to_use else ids
 
  return MultiParamAlternative.create(new_fixture_host=fh, test_func=test_func,
  param_union_name=union_name, argnames=argnames, from_i=from_i,
  to_i=to_i, argvalues=marked_argvalues[from_i:to_i], ids=_ids,
- scope=scope, hook=hook, debug=debug)
+ scope=scope or "function",
+ hook=hook, debug=debug)
 
 
  def _create_fixture_ref_alt(union_name, test_func, i): # noqa