Mention dict and set comps in library reference #67865

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1. frankmillman commented on Mar 16, 2015

   

   frankmillmanmannequin

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   This is from the documentation at Section 4.6.4. 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)
   """

   Comprehensions are mentioned as a constructor.

   This is from the documentation at Section 4.10. Mapping Types

   """
   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.
   """

   There is no mention of dictionary comprehensions.

   For consistency, I believe that the documentation for Dicts and Sets should mention comprehensions.
2. frankmillman

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   docspython

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   docsDocumentation in the Doc dirDocumentation in the Doc dir

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4. bitdancer commented on Mar 16, 2015

   

   bitdancer

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   Sounds reasonable. Dict and set comprehensions were added later than list comprehensions, and we probably just didn't notice this needed updating.

   Mark's patch, however, is incorrect. Mark: the dict/set literal notation is a different thing from a comprehension.
5. BreamoreBoy commented on Mar 17, 2015

   

   BreamoreBoymannequin

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   That was embarrassing, hopefully this is rather better.
6. frankmillman commented on Mar 18, 2015

   

   frankmillmanmannequin

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   Lists and tuples are described like this -

   class list([iterable]) 
   Lists may be constructed in several ways:
   [...]
   
   class tuple([iterable]) 
   Tuples may be constructed in a number of ways:
   [...]

   I think a similar approach to Dicts and Sets could make sense -

   class dict([**kwarg])
   Dicts may be constructed in a number of ways:

   • Using a pair of braces to denote the empty dict: {}
   • Placing a comma-separated list of key: value pairs within braces: {'jack': 4098, 'sjoerd': 4127} or {4098: 'jack', 4127: 'sjoerd'}
   • Using a dict comprehension: {k: v for k, v in iterable}
   • Using the dict() built-in: dict() or dict(**kwarg) or dict(mapping, **kwarg) or dict(iterable, **kwarg)

   Add a new example -

   f = {k: v for k, v in [('one', 1), ('two', 2), ('three', 3)]}
   
   
   class set([iterable])
   class frozenset([iterable])
   Sets may be constructed in a number of ways:

   • Non-empty sets (not frozensets) can be created by placing a comma-separated list of elements within braces, for example: {'jack', 'sjoerd'}
   • Non-empty sets (not frozensets) can be created by using a set comprehension: {x for x in iterable}
   • Using the set() or frozenset() built-in

   The 'bullet-point' construction is not really necessary for Sets, but it would make it consistent with the others.

   A related point (I can raise a separate Issue if preferred) -

   For me, the power of comprehensions lies in their 'filtering' ability. This is not mentioned in any of the above examples, so newcomers may wonder why they should use them.

   We don't want to make the examples too complicated. Maybe just add 'if ...' to the example, and provide a cross-reference to Section 6.2.4 in the Language Reference (Displays for lists, sets and dictionaries).
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8. vadmium commented on Jan 3, 2016

   

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   The distiction about non-empty sets is a bit misleading. You can create an empty set via comprehension:

   >>> {x for x in ()}
   set()
9. MojoVampire commented on Jan 5, 2016

   

   MojoVampiremannequin

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   Heck, with the addition of additional unpacking generalizations in 3.5, you can make an empty set even without a comprehension: {*()}

   Not really recommending the "one-eyed monkey operator", but the addition of unpacking generalizations undoes several of the limits on literals that previously existed.
10. csabella commented on Feb 24, 2019

    

    csabella

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    Assigning to @Mariatta for the CPython mentored sprint.
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    docspython

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13. furkanonder commented on May 10, 2020

    

    furkanondermannequin

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    PR has been sent.
14. ezio-melotti

    transferred this issue fromon Apr 10, 2022
15. StanFromIreland

    closed this as a duplicateof Update Index for set, dict, and generator 'comprehensions' #74167 on Aug 20, 2026