[issue22316] Add rule about "extraneous whitespace around colon" to "Whitespace In Expressions and Statements" of PEP8

[Guido van Rossum]

Guido van Rossum added the comment:

Does anyone care that there is now one bullet in the "avoid spaces ..." list 
that isn't strictly about avoiding spaces?

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[issue22316] Add rule about "extraneous whitespace around colon" to "Whitespace In Expressions and Statements" of PEP8

[Guido van Rossum]

Guido van Rossum added the comment:

Committed rev e98737176f1d.

If there's more discussion I can add more.

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resolution:  -> fixed
status: open -> closed

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[issue22316] Add rule about "extraneous whitespace around colon" to "Whitespace In Expressions and Statements" of PEP8

[Nick Coghlan]

Nick Coghlan added the comment:

Looks good to me! (and covers several cases that hadn't occurred to me
before)

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[issue22316] Add rule about "extraneous whitespace around colon" to "Whitespace In Expressions and Statements" of PEP8

[Guido van Rossum]

Guido van Rossum added the comment:

Just like for other binary operators (except for the ones mentioned as always 
needing spaces), the spaces around ":" are neither mandatory nor objectionable. 
 I am not making up a new rule, this is how I've always thought -- we just have 
to make it explicit that x[1: n] is wrong.

How about this:

- 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. ::

  Yes::

  ham[1:9], ham[1:9:3], ham[:9:3], ham[1::3], ham[1:9:]
  ham[lower:upper], ham[lower:upper:], ham[lower::step]
  ham[lower+offset : upper+offset]
  ham[: upper_fn(x) : step_fn(x)], ham[:: step_fn(x)]
  ham[lower + offset : upper + offset]

  No::

  ham[lower + offset:upper + offset]
  ham[1: 9], ham[1 :9], ham[1:9 :3]
  ham[lower : : upper]
  ham[ : upper]

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[issue22316] Add rule about "extraneous whitespace around colon" to "Whitespace In Expressions and Statements" of PEP8

[Ezio Melotti]

Ezio Melotti added the comment:

> Yes: ham[lower + offset : upper + offset], ham[lower : upper : 3]

This feels a bit weird to me, perhaps because I seldom have expressions in 
slices and don't feel the need to add further spaces.
For the first case I would definitely not put spaces around the +, and likely 
not even around the :.
For the second case I also wouldn't put spaces around the :.
In general complex expressions that requires additional spaces around the : to 
better separate start, stop, and step should be avoided or, if really 
necessary, additional variables should be used*.
I would be -0.5 if this is kept but with a "Maybe" instead of the "Yes", -0 if 
the spaces around the + are removed.

* note that I'm not too familiar with bumpy/scipy, where I believe complex 
slice expressions might be more common.

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nosy: +ezio.melotti

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[issue22316] Add rule about "extraneous whitespace around colon" to "Whitespace In Expressions and Statements" of PEP8

[Guido van Rossum]

Guido van Rossum added the comment:

Oooh, yes. The colon in a slice should be treated as a binary operator, with 
equal space before and after.

I think we can add this (as a new bullet following the bullet "Immediately 
before a comma, semicolon, or colon"):

- However, the colon in a slice acts like a binary operator, and
  should have equal amounts on either side.  In an extended slice,
  both colons must have the same amount of spacing applied. ::

  Yes: ham[1:9], ham[1:9:3], ham[:9:3], ham[1::3], ham[1:9:]
  Yes: ham[lower + offset : upper + offset], ham[lower : upper : 3]
  No: ham[1: 9], ham[1 :9]


I'm not sure what to recommend for extended slices when one or several of the 
slots are empty; my intuition suggests that there should be no spaces around 
any colons in that case, but I'm not sure what to do if you really have a long 
expression as one slot. Which is better?

ham[lower + 1 :: step]

or

ham[lower + 1 : : step]

similar for other cases, e.g.

ham[lower + 1 : upper + 1 :]

vs.

ham[lower + 1 : upper + 1 : ]

To me, *all* of those feel weird.

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nosy: +gvanrossum

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[issue22316] Add rule about "extraneous whitespace around colon" to "Whitespace In Expressions and Statements" of PEP8

[Anthony Mayer]

New submission from Anthony Mayer:

After discussion about extraneous whitespace around colons in a list slice not 
being an error on the pep8 checker project (see 
https://github.com/jcrocholl/pep8/issues/321#issuecomment-53649841), ncoghlan 
suggested filing a ticket here to get the issue added to PEP8. The issue being 
that PEP8 doesn't say that

x = [1, 2, 3, 4]
x[1: 3]

is wrong. It should suggest doing

x = [1, 2, 3, 4]
x[1:3]

instead. This rule should probably be added to the "Whitespace In Expressions 
and Statements" section of PEP8 
(http://legacy.python.org/dev/peps/pep-0008/#whitespace-in-expressions-and-statements)

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assignee: docs@python
components: Documentation
messages: 226185
nosy: Guido.van.Rossum, anthonymayer, barry, docs@python, ncoghlan
priority: normal
severity: normal
status: open
title: Add rule about "extraneous whitespace around colon" to "Whitespace In 
Expressions and Statements" of PEP8

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