Implement assignment expressions (a.k.a. named expressions a.k.a. walrus operator)

[gvanrossum]

Python 3.8 (currently in beta) supports a new expression operator, :=, dubbed the "walrus operator" (PEP 572). Mypy should support this. Parsing is not a problem -- when we use Python 3.8 to run mypy, we use the stdlib ast module which knows how to parse it. But mypy always treats this as a blocking error, saying "assignment expressions are not yet supported".

It would be nice if we supported this.

[msullivan]

There was somebody who expressed interest in implementing this but I'm not sure if anything came of it. Was it @JelleZijlstra?

[JelleZijlstra]

#6899 came out of it :) It's still waiting for review

[Bidek56]

PR #6899 was merged but I am not sure if work correctly.

mypy with Python 3.8 with the example code:

import re

advertisement = '10% discount'
discount = 0.0
if (mo := re.search(r'(\d+)% discount', advertisement)):
    discount = float(mo.group(1)) / 100.0

results in: error: Item "None" of "Optional[Match[str]]" has no attribute "group"
Any ideas would be appreciated. Thanks

[JelleZijlstra]

This is already covered in #7316. Closing this issue as there is general support for assignment expressions; I opened a series of more specific issues for smaller remaining problems.