Better message for invalid types like List(int)

[JukkaL]

The error message "invalid type comment or annotation" isn't very helpful if a user writes List(int) instead of List[int] (or Optional(int)). This seems to be a common error.

Example:

from typing import List

def f():
    # type: () -> List(int)   # invalid type comment or annotation
    return [1]

A better message could be something like this:

program.py:3: error: Syntax error in type annotation
program.py:3:note: Suggestion: Use List[...] instead of List(...)

[paultgibbons]

Another example that's not immediately obvious is missing parens around input eg

def f(x):
    # type: List[int] -> Tuple[List[int], Optional[str]]
    ...

throws message error: syntax error in type comment,
but would be nicer if it was more like

error: syntax error in type comment
Suggestion: Expected: `(` but found `L` at column 13

or

error: syntax error in type comment
Suggestion: Wrap input in parens

or something else to highlight the issue

[gvanrossum]

Here's one that took a while to figure out:

def update_state(conf,               # type: SourceConf
                 new_state,          # type: SourceState
                 tid,                # type: int
                 vid,                # type: int
                 update_ts=None,     # type: Optional[float],
                 ):         # type: (...) -> SourceConf

This gave error: Invalid tuple literal type on the def line.

spoiler alert

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There's an extra comma after Optional[float].

[emmatyping]

@gvanrossum Well, #3871 should help a bit in your case. I'd fix it but I'm currently occupied with my PEP.

[ilevkivskyi]

It looks like #4323 fixed only part of the problems mentioned here, I am reopening this, just not to lose track.