Added signature information to incompatible signature failure.

[markkohdev]

Currently, if a method is incompatible with its base class's method, it only says "Hey, this subclass method is incompatible with its superclass method" which is not very helpful. This branch makes it so that incompatible methods will actually print out readable signatures, arguments, etc. in a human-readable manner.

In order to accomplish this, we've added a new PrettyTypeStrVisitor which will do actual "pretty-printing" that's safe for human eyes. We've also enabled allowing duplicate messages in output.

The new output structure looks like

main:6: error: Signature of "f" incompatible with supertype "A"
main:6: note:     Method "f" of "A":
main:6: note:       def f(self, x: __main__.A) -> None
main:6: note:     Method "f" of "B":
main:6: note:       def f(self, x: __main__.A, y: __main__.A) -> None

Fixes #3379.

[markkohdev]

I'm trying to get special args like self or cls here, but since they're not in t.arg_names I have to reach through to t.definiton.arg_names and do a comparison. This this the most effective way to do this?

[JukkaL]

It would be nicer to have a multi-line note if you have an overloaded function, and output it like this or something:

...: note: Signature of "f" in superclass:
...: note:     @overload
...: note:     def f(...) -> ...
...: note:     @overload
...: note:     def f(...) -> ...

[markkohdev]

@JukkaL it would probably be difficult/messy to split the output between multiple note outputs. Would it suffice to have one note output multiple lines?

[JukkaL]

It's fine to repeat note multiple times. Some tools might expect the path and the line number to be always present on every line. If there are very many overload items (say, more than 3) we can just show the first 3 and something like (<additional 3 overload variants omitted>).

[JukkaL]

Here's my suggestion for how the messages could look like:

rt.py:6: error: Signature of "f" incompatible with superclass "A"
rt.py:6: note:      Superclass:
rt.py:6: note:          def f(self, x: int) -> None
rt.py:6: note:      Subclass:
rt.py:6: note:          def f(self, x: str, y: List[int]) -> None

Notes about the above idea:

• Superclass: is indented 4 spaces deeper than Signature
• def f(...) is indented 4 spaces deeper than Superclass:
• I'd prefer simple formatting (see format_simple in messages.py) for more readable and less verbose type names. This may require moving the conversion code to messages.py, but it may be a good idea anyway. This would display List[int] instead of builtins.list[builtins.int], for example.

Other things:

• It would be good to include @classmethod and @staticmethod for class and static methods, respectively.

[markkohdev]

Still working on this. I should have some time this week to move the formatting stuff into its own module so that we can print valid types like List[int] as requested above.

[gvanrossum]

Hey Mark, Are you still interested in this? This PR has not received any updates in over three months and your last comment suggested you'd like to get to it "sometimes this week". It's fine if you're no longer interested in this -- if we don't hear from you soon we will just close the PR (rather than it accumulating dust in our attic). If after that you decide you want to pick it up still, please just submit a new PR (but mention this one for context).

[markkohdev]

@gvanrossum Hey Guido, sorry about the radio silence - I got busy adding MyPy to some code bases at work and never got the chance to finish this up. I'll close this PR for now and will try my best to finish this branch in the near future since the issue is still open.