Python 3.8 — PEP 572 compatibility

[smartsammler]

Operating system: Fedora 29 Linux
Python version: 3.8.0a1 (build from Python.org sources)
Black version: 18.9b0
Does also happen on master: yes, localy. on black.now.sh it fails with type object 'FileMode' has no attribute 'from_configuration'

My example for a too long line in a file trying the upcoming PEP 572 Assignment Expressions failed with

error: cannot format test.py: Cannot parse: 4:23:     while not (0 < (answer := int(input("Please guess a positive number up to 10: "))) <= 10):
All done! 💥 💔 💥
1 file failed to reformat.

The example is the following:

#!/usr/bin/env python38
# -*- coding: utf-8 -*-

while not (0 < (answer := int(input("Please guess a positive number up to 10: "))) <= 10):
    print("That is not correct input, please retry.")

print(f"Your guess {answer} is not the number I chose :-p")

I don't think that this is an urgent issue, because Python 3.8 got its first alpha not long ago. I just wanted to test if it works and because it does not, and I haven't found an issue yet, I'm filing this one.

Thank you for your work!

[zsol]

Thanks for reporting. We'll definitely add support for := before 3.8 is released ;)

[gvanrossum]

Ping? 3.8 beta 1 is out now and people are testing 3.8 in their staging setup.

[rhettinger]

This is still not supported as 21 July 2019 in beta 2:

$ python3.8 --version
Python 3.8.0b2
$ python3.8 -m black --skip-string-normalization Lib/statistics.py
error: cannot format Lib/statistics.py: Cannot parse: 618:20: def quantiles(dist, /, *, n=4, method='exclusive'):
All done! 💥 💔 💥
1 file failed to reformat.

[zsol]

@rhettinger I've opened #943 for that. I thought I'd implement it fast last night but then I looked at the cpython grammar that implements this and realized this will take a while :)

[asottile]

This seems not fixed properly:

$ pre-commit try-repo https://github.com/psf/black --all-files --show-diff-on-failure
[INFO] Initializing environment for https://github.com/psf/black.
===============================================================================
Using config:
===============================================================================
repos:
-   repo: https://github.com/psf/black
    rev: c9689b80fbb108316b0330fc1ed444ccab419b02
    hooks:
    -   id: black
===============================================================================
[INFO] Installing environment for https://github.com/psf/black.
[INFO] Once installed this environment will be reused.
[INFO] This may take a few minutes...
black....................................................................Failed
hookid: black

Files were modified by this hook. Additional output:

reformatted t.py
All done! ✨ 🍰 ✨
1 file reformatted.

pre-commit hook(s) made changes.
If you are seeing this message in CI, reproduce locally with: `pre-commit run --all-files`.
To run `pre-commit` as part of git workflow, use `pre-commit install`.
All changes made by hooks:
diff --git a/t.py b/t.py
index 7aa9164..767c39b 100644
--- a/t.py
+++ b/t.py
@@ -1,2 +1,2 @@
-if (x := 1):
+if (x := 1) :
     print(x)