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Subclasses of mailbox.Mailbox not recognized as such #14935
Description
Consider the following code:
from abc import ABC, abstractmethod
import mailbox
class MailboxFactory(ABC):
@abstractmethod
def open(self) -> mailbox.Mailbox: ...
class MboxFactory(MailboxFactory):
def __init__(self, path: str) -> None:
self.path = path
def open(self) -> mailbox.mbox:
return mailbox.mbox(self.path)I'm 90% sure this should be acceptable, but running 1.18.2 on this code produces the error:
mailbox02.py:14: error: Return type "mbox" of "open" incompatible with return type "Mailbox[Message[str, str]]" in supertype "MailboxFactory" [override]
Found 1 error in 1 file (checked 1 source file)
Similar errors happen for other Mailbox subclasses as well.
Activity
srittau commentedon Oct 30, 2025
The problem is that
Mailboxis generic over the message type (_MessageT), which is invariant and defaults toMessage[str, str]. Easy workaround is to usemailbox.Mailbox[Any], although we should probably change the default forMailboxin typeshed for cases like this.srittau commentedon Oct 30, 2025
In fact, we can make the type variable covariant in typeshed, which should fix this problem.
Various improvements for the mailbox stubs
Various improvements for the mailbox stubs (python#14950)