__getnewargs__ for a subclass incompatible with return type of supertype

[eli-schwartz]

Here is an example commit with the issue: eli-schwartz/meson@4d204bb

mypy.ini: https://github.com/mesonbuild/meson/blob/27626124d0bdd89f5f276f70a73b4713aa3aac3d/.mypy.ini

$ mypy --version
mypy 0.961 (compiled: yes)

$ mypy mesonbuild/interpreter/primitives/string.py
mesonbuild/interpreter/primitives/string.py:211:5: error: Return type "Tuple[str, str]" of "__getnewargs__" incompatible with return type "Tuple[str]" in supertype "str"

I subclassed str and treat it specially with its own incompatible __new__() to enforce a specific creation pattern. After discovering I need to pickle it as well, I added a __getnewargs__() which is also incompatible. mypy is happy with my incompatible new, but not with the getnewargs that needs to match the argument contract of new.

[erictraut]

Could you provide a minimal self-contained example that demonstrates the problem?

[eli-schwartz]

If I just extract that class on its lonesome, and add in a primitive version of the join_paths() function from elsewhere in the project that makes this whole thing useful:

# foo.py
import os
import typing as T

class OptionString(str):
    optname: str

    def __new__(cls, value: str, name: str) -> 'OptionString':
        obj = str.__new__(cls, value)
        obj.optname = name
        return obj

    def __getnewargs__(self) -> T.Tuple[str, str]:
        return (str(self), self.optname)


def join_paths(s1: str, s2: str) -> str:
    val = os.path.join(s1, s2)

    if isinstance(s1, OptionString):
        name = os.path.join(s1.optname, s2)
        return OptionString(val, name)
    else:
        return val

Expected usage:

>>> foo.OptionString('/usr', '{prefix}')
'/usr'
>>> s = foo.OptionString('/usr', '{prefix}')
>>> print(s)
/usr
>>> s + '/lib'
'/usr/lib'
>>> type(s + '/lib')
<class 'str'>
>>> type(s)
<class 'foo.OptionString'>
>>> foo.join_paths(s, 'lib')
'/usr/lib'
>>> type(foo.join_paths(s, 'lib'))
<class 'foo.OptionString'>
>>> foo.join_paths(s, 'lib').optname
'{prefix}/lib'