Colon inside a range gets a colon added to it

[WhyNotHugo]

The colon before inside a range gets a space added to it:

input:

def a(code: str, pos: int):
    return code[pos: pos + 10]

output

def a(code: str, pos: int):
    return code[pos : pos + 10]
    #              ^ Note this space

This seems to be the only scenario in which black adds a space before a colon, so I'm assuming it's a bug. Flake8 immediately complains since that's, well, wrong.

This doesn't seem to happen if + 10 is not there.

Environment:

$ black --version
black, version 19.10b0
$ python --version
Python 3.8.2
$ uname -a
Linux aphrodite 5.6.12-arch1-1 #1 SMP PREEMPT Sun, 10 May 2020 10:43:42 +0000 x86_64 GNU/Linux

Does this bug also happen on master?:
Yes

[hugovk]

This is documented in the README:

https://github.com/psf/black/blob/master/README.md#slices

[WhyNotHugo]

But it would seem that this is not enforced consistently, eg:

def a(code: str, pos: int):
    x = code[pos : pos + 10]  # Spaces here
    y = code[pos:pos]  # No spaces here

[zsol]

We should probably expand that paragraph a bit in the docs. PEP8 recommends no spaces around : for "simple" expressions, and extra space for "complex expressions". Black treats anything more than variable names as "complex".

[WhyNotHugo]

Ah, this makes sense. I guess I'm just too used to flake8's syntax.

I think it would be good to mention, in "installation and usage", a basic configuration for flake8 that aligns with black. Would you guys accept a PR that describes this?

[WhyNotHugo]

(I realise the individual settings are mentioned in different places, but I'm talking about a single-block copy-pasteable thing :) )

[zsol]

FWIW here's the relevant code that implements this behavior:

black/src/black/__init__.py

Lines 1411 to 1413 in 45c98cf

	leaf.prefix += whitespace(
	leaf, complex_subscript=self.is_complex_subscript(leaf)
	)

I would turn off flake8 warnings related to subscripts (in fact, related to all code formatting). #1371 added an example flake8 config. If you think that needs tuning, feel free to open a PR :)

BTW this is a duplicate of #889, #940, #1343, and possibly more. Thanks @hugovk for replying to all of them :)

[WhyNotHugo]

Oh, it has! Guess I have to read the updated README! Thanks, sorry for the noise!