Spurious "undefined name" with "type: ignore" comment for mypy

[gward]

mypy has the ability to selectively ignore type errors with special syntax in a type annotation comment. These comments confuse pyflakes and cause it to emit spurious "undefined name" errors.

Sample code using this mypy feature:

def foo(a: str):
    if a == 37:
        return ""
    return a + ""

foo(37)                         # type: ignore[arg-type]

(See https://mypy.readthedocs.io/en/latest/error_codes.html#silencing-errors-based-on-error-codes for more detail.)

pyflakes reports undefined names in that type annotation comment:

$ pyflakes pyflakes-test.py
pyflakes-test.py:6: undefined name 'ignore'
pyflakes-test.py:6: undefined name 'arg'

If I change the type annotation comment to

foo(37)                         # type: ignore

then pyflakes is happy. But I'm not happy, because that will prevent mypy from finding other errors in this line.

[asottile]

#455 #475 (duplicate) please try against master