[FIXED] The type Type[Array] is not generic and not indexable

Issue

This program:

class Array:
    def __init__(self, underlying):
        self.underlying = underlying

    def __class_getitem__(cls, key):
        return Array(key)

    def __getitem__(self, key):
        pass

    def __str__(self):
        return f"Array({self.underlying=})"

def foo(name, kind):
    pass

foo(name="x", kind=Array["int"])

is flagged by mypy:

x.py:17: error: The type "Type[Array]" is not generic and not indexable  [misc]

The __getitem__ is something I added just to see if it would resolve the issue (it did not). I'm not intending to use Array here as a type (in the typing sense), just for cute syntax to pass an array type somewhere else.

What does this mypy error mean and how can I fix it (preferably with something other than # ignore comments)?

Solution

I'm not intending to use Array here as a type (in the typing sense), just for cute syntax to pass an array type somewhere else.

__class_getitem__ is only for typing. See docs:

classmethod object.__class_getitem__(cls, key)

Return an object representing the specialization of a generic class by type arguments found in key.

And more specifically:

Using __class_getitem__() on any class for purposes other than type hinting is discouraged.

So unfortunately, it looks like what you want is not possible. Just use the regular constructor syntax, Array("int").

---

Also, just in case you weren't aware, special methods shouldn't be used in unintended ways:

Any use of __*__ names, in any context, that does not follow explicitly documented use, is subject to breakage without warning.

Answered By - wjandrea