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What is the correct type hint for x = []?
The type checker in my PyCharm editor flagged this as an error:
labelframes: List[ttk.LabelFrame] = []
'Optional' is not an option as in:
labelframes: List[Optional[ttk.LabelFrame]] = []
because the documentation for typing.Optional states that this is equivalent to:
labelframes: List[Union[ttk.LabelFrame, None]] = []
and [None] is not [].
I ought to mention that PyCharm doesn't like this either:
labelframes: List[Union[ttk.LabelFrame, None]] = [None]
Whatever type hint I try. PyCharm flags it as an error with, "Expected to return my type hint here, got no return," so I tried:
labelframes: Optional[List[ttk.LabelFrame, None]] = []
That didn't work.
I am aware that PEP 526 has numerous examples which follow the pattern:
x: List[str] = []
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Python's type hints provide you with optional static typing to leverage the best of both static and dynamic typing. Besides the str type, you can use other built-in types such as int , float , bool , and bytes for type hintings. To check the syntax for type hints, you need to use a static type checker tool.
Empty lists are considered False in Python, hence the bool() function would return False if the list was passed as an argument.
In a type hint, if we specify a type (class), then we mark the variable as containing an instance of that type. To specify that a variable instead contains a type, we need to use type[Cls] (or the old syntax typing. Type ).
Union type; Union[X, Y] is equivalent to X | Y and means either X or Y. To define a union, use e.g. Union[int, str] or the shorthand int | str . Using that shorthand is recommended.
Looking at PEP 484, specifically the section on type comments (the precursor to variable annotations) this does indeed seem like a bug with PyCharm's checker.
Quoting from the PEP:
In non-stub code, there is a similar special case:
from typing import IO stream = None # type: IO[str]Type checkers should not complain about this (despite the value
Nonenot matching the given type), nor should they change the inferred type toOptional[...](despite the rule that does this for annotated arguments with a default value of None ). The assumption here is that other code will ensure that the variable is given a value of the proper type, and all uses can assume that the variable has the given type.
So, in a similar fashion, type-checkers (as mypy currently does), should not complain about the fact that the initializing value you provide doesn't strictly match the annotation.
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