[Python-Dev] Python Language Reference has no mention of list comÃprehensions

[Andrew Barnert]

> On Dec 3, 2015, at 08:15, MRAB <python at mrabarnett.plus.com> wrote:
> 
>>> On 2015-12-03 15:09, Random832 wrote:
>>> On 2015-12-03, Laura Creighton <lac at openend.se> wrote:
>>> Who came up with the word 'display' and what does it have going for
>>> it that I have missed?  Right now I think its chief virtue is that
>>> it is a meaningless noun.  (But not meaningless enough, as I
>>> associate displays with output, not construction).
>> 
>> In a recent discussion it seemed like people mainly use it
>> because they don't like using "literal" for things other than
>> single token constants.  In most other languages' contexts the
>> equivalent thing would be called a literal.
> "Literals" also tend to be constants, or be constructed out of
> constants.

I've seen people saying that before, but I don't know where they get that. It's certainly not the way, say, C++ or JavaScript use the term. But I don't see any point in arguing about it if people just accept that "literal" is too broad a term to capture any useful intuition here. 

> A list comprehension can contain functions, etc.

A non-comprehension display can include function calls, lambdas, or any other kind of expression, just as easily as a comprehension can. Is [1, x, f(y), lambda z: w+z] a literal? If so, why isn't [i*x for i in y] a literal?

The problem is that we need a word that distinguishes the former; trying to press "literal" into service to help the distinction doesn't help.

At some point, Python distinguished between displays and comprehensions; I'm assuming someone realized there's no principled sense in which a comprehension isn't also a display, and now we're stuck with no word again.

>>> I think that
>>> 
>>>    6.2.4 Constructing lists, sets and dictionaries
>>> 
>>> would be a much more useful title, and
>>> 
>>>    6.2.4 Constructing lists, sets and dictionaries -- explicitly or through the use of comprehensions
>> 
>> I don't like the idea of calling it "explicit construction".
>> Explicit construction to me means the actual use of a call to the
>> constructor function.

Agreed.

The obvious mathematical terms are "extension" and "intention", but I get the feeling nobody would go for that.

Ultimately, the best we have is "displays that aren't comprehensions" or "constructions that aren't comprehensions".

Which means that something like "list, set, and dictionary displays (including comprehensions)" is about as good as you can make it without inventing a new term. There's nothing to contrast comprehensions with.