Mypy typing errors when upgrading to Twisted 21.2.0

user_5525099 0
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In trying to upgrade the Twisted package from 20.3.0 to 21.2.0.
After the upgrade, when running Mypy I'm getting the following errors when accessing the reactor (from twisted.internet import reactor):

error: Module has no attribute "run"  [attr-defined]  
error: Module has no attribute "running"  [attr-defined]  
error: Module has no attribute "running"  [attr-defined]  
error: Module has no attribute "callLater"  [attr-defined]  
error: Module has no attribute "callInThread"  [attr-defined]  
error: Module has no attribute "callFromThread"  [attr-defined]  
error: Module has no attribute "spawnProcess"  [attr-defined]  
error: Module has no attribute "spawnProcess"  [attr-defined]

When running Mypy with reveal_type(reactor):

Revealed type is '_importlib_modulespec.ModuleType'

I'm trying to understand what is the best way to handle this situation without just ignoring the errors so I can have the type checking when using the reactor.
Is there a better way to import the reactor? Is there a way to mark the type of the reactor for Mypy? should I cast it to a type every time before using it?

python-typing mypy twisted
2 Answers

This seems to be an assigned bug in Twisted (https://twistedmatrix.com/trac/ticket/9909) and the proposed fix sounds like a breaking change.

One of the comments on their Trac describes casting it, as you suggest, as a workaround:

from typing import cast

import twisted.internet.reactor
from twisted.internet.interfaces import IReactorCore

reactor = cast(IReactorCore, twisted.internet.reactor)
reactor.run()
user_16124710

It took a ridiculous amount of searching but I finally found the solution in the Twisted source code itself. Add the following comment to the end of your line:

# type: ignore[attr-defined]

for example:

reactor.callFromThread(reactor.stop)  # type: ignore[attr-defined]

That will "fix" it.

user_10177870
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