remove-positionalonly.md

remove-positionalonly and remove-keywordsonly

The ",*," keywordonly syntax in function declarations feels natural to many python developers but it did not exist before python3. It is similar to the ",/," positionalonly syntax but that one came even later in python 3.8. The reason has been that native-code implementations do not like their arguments to be rearranged - and it was felt necessary to inform the type checker about it.

When stripping typehints for python2, both syntax elements get removed.

It gets a bit more complicated when a python3 typehints *.pyi file is generated. By default they are compatible with python 3.6, so the keywordonly hint can stay, but the positionalonly hint must be dropped. A similar case arises when downgrading the main source code to just below python 3.8 - they keywordonly hint can stay but the positionalonly hint must be dropped.

    # original
    class B:
       b: int
       c: str
       def __add__(self, y: list[str], /, a: int = 0, *, b: int = 0) -> list[str]:
           return [self.c] + y

    # transformed
    class B:
        def __add__(self, y, a=0, b=0):
            return [self.c] + y

    # generated pyi
    from typing import List
    class B:
        b: int
        c: str
        def __add__(self, y: List[str], a: int=0, *, b: int=0) -> List[str]:
            pass