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

[Chris Angelico]

On Fri, Dec 4, 2015 at 12:25 PM, Steven D'Aprano <steve at pearwood.info> wrote:
> I don't see any good reason for maintaining that there's just one
> syntax, "display", which comes in two forms: a comma-separated set of
> values, or a for-loop. The only thing they have in common (syntax-wise)
> is that they both use [ ] as delimiters. They look different, they
> behave differently, and only one matches what the list actually displays
> as. Why use one term for what is clearly two distinct (if related)
> syntaxes?

You come across something syntactic that begins by opening a square
bracket, and you know that its semantics are: "construct a new list".
That's what's common here.

What goes *inside* those brackets can be one of two things:

1) A (possibly empty) comma-separated sequence of expressions

2) One or more nested 'for' loops, possibly guarded by 'if's, and a
single expression

So we have two subforms of the same basic syntax. The first one
corresponds better to the output format, in the same way that a string
literal might correspond to its repr under specific circumstances.
Neither is a literal. Neither is a call to a constructor function
(contrast "list()" or "list.__new__(list)", which do call a
constructor). So what is this shared syntax? Whatever word is used,
it's going to be a bit wrong. I'd be happy with either "constructor"
or "display", myself.

ChrisA