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__slots__ Union type should not include None. #1853
Description
Consider the following function:
def all_slots(type: Type) -> Set[str]:
'''
Subclasses of slots classes may define their own slots,
which hold just the additions to the parent class.
Therefore we need to iterate over the inheritance chain to get all slot names.
We use __mro__ here, and hope for the best regarding multiple inheritance.
'''
slots: Set[str] = set()
for t in type.__mro__:
try: s = t.__slots__
except AttributeError: break
else:
if isinstance(s, str):
slots.add(s) # single slot.
else:
slots.update(cast(Iterable[str], s)) # mypy bug: __slots__ cannot be None.
return slots
Without the cast, using latest mypy/typeshed, I get:
Argument 1 to "update" of "set" has incompatible type "Optional[Iterable[str]]"; expected "Iterable[str]"
Mypy thinks that the type of __slots__ is Union[str, Iterable[str], None] (it eliminates the str case due to the check above).
However, I don't think __slots__ can legally be None:
class Z: __slots__ = None
...
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: 'NoneType' object is not iterable
Activity
asottile commentedon Feb 26, 2018
I think what the annotation meant by
Optionalis it's sometimes not present -- though I don't know of a way to express that in mypy.For context, here's where this was added: #780
gvanrossum commentedon Feb 26, 2018
Looks like you're right -- please submit a fix to remove the
Optional[]. There's no way in typeshed (or mypy) to express "this may or may not exist" and mypy does not help detecting cases where that may be an issue, butOptional[]is definitely not the right way to spell that.gwk commentedon Feb 27, 2018
gvanrossum commentedon Feb 27, 2018
remove Optional from type of __slots__
remove Optional from type of __slots__ (#2128)
remove Optional from type of __slots__ (python#2128)