black breaks the type annotation into multiple lines when it's possible to keep it on one line

[passichenko]

Describe the bug

black breaks the type annotation into multiple lines, while existing formatting keep it on one line by using parentheses around expression

To Reproduce:

old formatting:

class Foo:
    def foo(self):
        if True:
            content_ids: Mapping[str, Optional[ContentId]] = (
                self.publisher_content_store.store_config_contents(files)
            )

after black:

class Foo:
    def foo(self):
        if True:
            content_ids: Mapping[
                str, Optional[ContentId]
            ] = self.publisher_content_store.store_config_contents(files)

playground link: https://black.now.sh/?version=stable&state=_Td6WFoAAATm1rRGAgAhARYAAAB0L-Wj4AEZAK5dAD2IimZxl1N_WlXnON2nzNX9HFvHx6cBSdVHWyzJO6O50DK3ha9hxTlR0JwlWYXCXbF4CjYJWeEpyQIXtrrOkKmzjEkT25z6cJ4YzWm7pHh_LmznKdvIU8f2vDLKjoyxv-ZPlxXoOnqhPz8sGlioAQx7_azDc47CPs6gbJw5n5j0LZ-lBGbWsnS8ldMWJXrmleKBm2qB_EmzBNy6IQNsCElvvBmM-J3BIWV55zK6IAAAAAMi_lQVqdYNAAHKAZoCAAARFT-fscRn-wIAAAAABFla

Expected behavior
black keeps existing formatting

Environment:

• Version: both stable and master
• OS and Python version: Linux/Python 3.7.5+

Does this bug also happen on master?
Yes.

[lnking81]

Will it be fixed? Very annoying, actually

[ichard26]

Eventually yes, but we can't really make guarantees for when as no one is being paid to work on this project.

[lnking81]

Sure, I understand. Just wonder if you pay any attention to the issues opened years ago.

[felix-hilden]

Just for housekeeping and eventual implementation, this is likely closely related to #749.