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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Dict comprehensions - improvement to docs?
Date: Mon, 16 Mar 2015 04:26:46 -0400
References: <me5pgk$en5$1@ger.gmane.org> <CALwzidnTUifj_L=DSH_8s+z0L44pxVvdpG1+pfz1Tzm=ECTLsw@mail.gmail.com> <me5u6p$jmr$1@ger.gmane.org>
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On 3/16/2015 2:45 AM, Frank Millman wrote:
>
> "Ian Kelly" <ian.g.kelly@gmail.com> wrote in message
> news:CALwzidnTUifj_L=DSH_8s+z0L44pxVvdpG1+pfz1Tzm=ECTLsw@mail.gmail.com...
>> On Sun, Mar 15, 2015 at 11:25 PM, Frank Millman <frank@chagford.com>
>> wrote:
>>> Hi all
>>>
>>> I like dict comprehensions, but I don't use them very often, so when I do
>>> I
>>> need to look up the format.
>>
> [...]
>>
>> Dict comprehensions aren't covered in the library reference because
>> they're a language feature, not a part of the standard library. The
>> syntax is defined at:
>>
>> https://docs.python.org/3/reference/expressions.html#dictionary-displays
>>
>
> This is what the library reference says about lists -
>
> """
> Lists may be constructed in several ways:
>
> Using a pair of square brackets to denote the empty list: []
> Using square brackets, separating items with commas: [a], [a, b, c]
> Using a list comprehension: [x for x in iterable]
> Using the type constructor: list() or list(iterable)
> """
>
> This is what it says about dictionaries -
>
> """
> Dictionaries can be created by placing a comma-separated list of key: value
> pairs within braces, for example: {'jack': 4098, 'sjoerd': 4127} or {4098:
> 'jack', 4127: 'sjoerd'}, or by the dict constructor.
>
> class dict(**kwarg)
> class dict(mapping, **kwarg)
> class dict(iterable, **kwarg)
>
> Return a new dictionary initialized from an optional positional argument and
> a possibly empty set of keyword arguments.
> """
>
> If you did not know about dict comprehensions, there is nothing to tell you
> that they even exist.
>
> I feel that it should be mentioned.

Given that list comprehensions are mentioned in the library reference, I 
agree that set and dict comprehensions should be also.  Open an issue on 
the tracker pointing to the list entry.



-- 
Terry Jan Reedy