TYPING: add some type hints to core.generic by simonjayhawkins · Pull Request #27646 · pandas-dev/pandas (original) (raw)

because we're still supporting Python 3.5 we are using a comment-based syntax for variable annotations.

_AXIS_ALIASES = None # type: Dict[str, int]

and to be valid Python syntax need to initialize to None, whereas shortly when we drop Python 3.5 we can use..

_AXIS_ALIASES: Dict[str, int]

and we can declare the variable type without any initialization necessary which will not change runtime behaviour.

see also https://stackoverflow.com/questions/56584140/python-3-5-type-annotated-variable-without-initial-value

so my preference is to add if TYPE_CHECKING: blocks for compatibility with Python 3.6 and simplify the adoption of Pyhton 3.6 variable annotations when we drop Python 3.5.