Single line to multi line change by black causes different type hint scope

[dveeden]

Describe the bug A clear and concise description of what the bug is.

When reformatting a single line into a multi line statement the scope of type hints changes.
This is with the default --safe

To Reproduce Steps to reproduce the behavior:

1. Take this file '...'

import requests


class Foo:
    def __init__(self) -> None:
        self.session = requests.Session()

    def bar(self) -> None:
        response = self.session.request(method="GET", url="http://httpbin.org/status/200", params={},)  # type: ignore
        print(response.status_code)


foo = Foo()
foo.bar()

2. Run Black on it with these arguments '....'
   Without arguments

3. See error

Expected behavior A clear and concise description of what you expected to happen.

I would expect black to take the scope of type hints in consideration and either:

• Move/change type hints to have them keep the same scope
• Decide to not touch these lines
• Give a warning/error about this not being safe

Environment (please complete the following information):

• Version: 20.8b1
• OS and Python version: [e.g. Linux/Python 3.7.4rc1]
  Linux/Python 3.8.5

Does this bug also happen on master? To answer this, you have two options:

Yes

Additional context Add any other context about the problem here.

Current behaviour:

-        response = self.session.request(method="GET", url="http://httpbin.org/status/200", params={},)  # type: ignore
+        response = self.session.request(
+            method="GET",
+            url="http://httpbin.org/status/200",
+            params={},
+        )  # type: ignore

One possible solution (a bit ugly, but keeps the same scope)

-        response = self.session.request(method="GET", url="http://httpbin.org/status/200", params={},)  # type: ignore
+        response = self.session.request(   # type: ignore
+            method="GET",   # type: ignore
+            url="http://httpbin.org/status/200",   # type: ignore
+            params={},   # type: ignore
+        )  # type: ignore

[JelleZijlstra]

I'm going to say that we shouldn't change this behavior:

• We already have special handling for type comments that usually avoids moving them around
• Putting a type ignore comment on every line, as suggested above, would be ugly and would likely require manual work from the user anyway (e.g., with the --warn-unused-ignores mypy flag).
• Recent releases of mypy scope a # type: ignore to the whole logical line, so the current behavior should already work.

I also just submitted #2272 clarifying this behavior in the FAQ.