Headers.__delitem__ signature incompatible with ImmutableHeadersMixin.__delitem__

[ambv]

Mypy recently introduced stricter checks for function signature compatibility (python/mypy#2521). Since werkzeug is included in the typeshed library with typing stubs, it found an issue with the signatures of __delitem__() in datastructure.py. Consider:

class Headers(Mapping):
  def __delitem__(self, key, _index_operation=True): ...

vs.

class ImmutableHeadersMixin(object):
  def __delitem__(self, key): ...

class EnvironHeaders(ImmutableHeadersMixin, Headers): ...

Basically, it's invalid to replace __delitem__() accepting an optional kwarg with an implementation that doesn't.

[untitaker]

@ambv Am I correct that changing the signature of ImmutableHeadersMixin.__delitem__ to accept arbitrary args would work?

[ambv]

Yes, that would work. __delitem__() in the immutable mixin is supposed to be a no-op so it's reasonable for it to accept (and ignore) arbitrary arguments.

To rephrase the problem for future reference:

• When you're using Headers directly, you can pass _index_operation=True to __delitem__(). Remember, the "user" of this functionality might be the class itself.
• EnvironHeaders is a subclass of Headers so it should accept _index_operation=True, too, but it doesn't because ImmutableHeadersMixin comes in first in the MRO.

[srittau]

It sounds to me that the correct fix would be to rename Headers.__delitem__ and add a new __delitem__ implementation that just calls the renamed method with default arguments.

[davidism]

Going with the simple solution by just adding **kwargs to ImmutableHeadersMixin.__delitem__.