More accurate message types for email stubs

[max-muoto]

Fix long-standing email.parser issues detailed in #2417 and #10762

This takes the approach recommended by @andersk and utilizies generic defaults to help us get accurate types accounting for the default arguments here without any overloads.

[andersk]

Policy.message_factory, Policy.__init__(message_factory), Policy.handle_defect, Policy.register_defect should use _M rather than Message.

[max-muoto]

Policy.message_factory, Policy.__init__(message_factory), Policy.handle_defect, Policy.register_defect should use _M rather than Message.

Sounds good, wasn't 100% sure there. Let me fix that.

[max-muoto]

Policy.message_factory, Policy.__init__(message_factory), Policy.handle_defect, Policy.register_defect should use _M rather than Message.

c8e2982

[max-muoto]

See microsoft/pyright#8084 for some context on why we need this.

[andersk]

Oh and EmailPolicy.__init__(message_factory) should return EmailMessage rather than Message.

[max-muoto]

Oh and EmailPolicy.__init__(message_factory) should return EmailMessage rather than Message.

85ed295

[max-muoto]

@srittau It seems like you looked into making Policy generic at some point? Do you maybe want to review here?

[max-muoto]

Sorry to bug, but just wanted to follow-up again @srittau, or if maybe someone else is able to take a look.

[srittau]

LGTM, a few optional suggestions below.

[max-muoto]

LGTM, a few optional suggestions below.

Thanks, accepted all of your suggestions.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

setuptools (https://github.com/pypa/setuptools)
+ setuptools/command/bdist_wheel.py:591: error: Argument "policy" to "Generator" has incompatible type "EmailPolicy"; expected "Optional[Policy[Message[str, str]]]"  [arg-type]

[srittau]

Looking at the new primer hit, I think we need to make EmailMessage generic, too: class EmailMessage(MIMEPart[_HeaderRegistryT, _HeaderRegistryParamT]): ....

Although I have to admit, I'm a bit lost in how all these classes work together.

[max-muoto]

Looking at the new primer hit, I think we need to make EmailMessage generic, too: class EmailMessage(MIMEPart[_HeaderRegistryT, _HeaderRegistryParamT]): ....

Although I have to admit, I'm a bit lost in how all these classes work together.

I'll take another deeper look in the next day or two, since it's a been a while since I worked on this originally now haha.