Callback protocols - when do we need the double underscore prefix?

Question

Callback protocols and :py:data:~typing.Callable types can be used interchangeably. Keyword argument names in :py:meth:__call__ <object.__call__> methods must be identical, unless a double underscore prefix is used. For example:
typing_extensions import Protocol

T = TypeVar('T')

class Copy(Protocol):
    def __call__(self, __origin: T) -> T: ...

copy_a: Callable[[T], T]    copy_b: Copy

copy_a = copy_b  # OK    copy_b = copy_a  # Also OK    ```

However, this example also works if we remove the double underscore prefix from before __origin. E.g.

$ cat t.py 
from typing import Callable, TypeVar
from typing_extensions import Protocol

T = TypeVar('T')

class Copy(Protocol):
    def __call__(self, origin: T) -> T: ...

copy_a: Callable[[T], T]
copy_b: Copy

copy_a = copy_b  # OK
copy_b = copy_a  # Also OK
$ mypy t.py 
Success: no issues found in 1 source file

So, this example isn't clear to me. When do we need the double underscore prefix?

You only need them if an attribute should be subject to name mangling or the language definition defines a special name with double underscores and you want to use that. — Klaus D., Feb 05 '21 at 18:06
Check ``reveal_type(copy_a)`` – MyPy infers it to type ``Copy`` since that is what is assigned first. — MisterMiyagi, Feb 06 '21 at 18:03

MisterMiyagi · Accepted Answer · 2021-02-06T18:07:33.820

A named parameter can be used in placed of an anonymous parameter, so the Protocol works as a value for the Callable. The first assignment copy_a = copy_b silently promotes copy_a to a Copy, which is then valid to assign to copy_b: Copy.

...

copy_a = copy_b  # copy_a is a Copy now!
copy_b = copy_a  # assign copy_a: Copy to copy_b: Copy
reveal_type(copy_a)  # Revealed type is 'aaa_testbed.Copy'

Swapping the assignment means copy_b = copy_a happens while copy_a is still the anonymous type. This triggers the expected error:

...

copy_b = copy_a  # Incompatible types in assignment (expression has type "Callable[[T], T]", variable has type "Copy")
copy_a = copy_b

Callback protocols - when do we need the double underscore prefix?

1 Answers1