Whitespace in slices when there is an open side

[warsaw]

Black does this, yuck!

-            whitespace = orig_line[:-len(line)]
+            whitespace = orig_line[: -len(line)]

[grantjenks]

I'm 0.25 on this. Sometimes I like to see an additional set of parens for expressions in slices. Like:

            whitespace = orig_line[:(-len(line))]

[grantjenks]

Having looked through a bunch of diffs in the last hour, I've actually come around to Black's formatting here. The added space doesn't hurt.

[warsaw]

I can live with it, but it goes against PEP 8, IMHO:

However, in a slice the colon acts like a binary operator, and should have equal amounts on either side (treating it as the operator with the lowest priority). In an extended slice, both colons must have the same amount of spacing applied. Exception: when a slice parameter is omitted, the space is omitted:

[MatthewScholefield]

Personally, I don't think it makes sense because this rule is inconsistent in that it causes spacing to change purely based on the type of expression (expressions are not treated uniformly).

Ie. Each of the following would be a consistent formatting rule:

• foo[: bar_len] and foo[: len(bar)]
• foo[:bar_len] and foo[:len(bar)].

But not how it does it right now which is:

• foo[:bar_len] and foo[: len(bar)].

[grantjenks]

Addressing my previous comment -- I realize now that the addition of parens is impossible given Black's constraints. Adding parens would modify the AST which Black forbids to prevent changing semantics.

@warsaw having read the PEP 8 section you quote, I don't see what part black violates here.

[warsaw]

I think that's fine. Let's close this one as "won't fix".