[Python-ideas] while conditional in list comprehension ??

[Steven D'Aprano]

On 29/01/13 21:44, Nick Coghlan wrote:
> On Tue, Jan 29, 2013 at 11:30 AM, Steven D'Aprano<steve at pearwood.info>  wrote:
>> Why would it translate that way? That would be a silly decision to make.
>> Python can decide on the semantics of a while clause in a comprehension in
>> whatever way makes the most sense, not necessarily according to some
>> mechanical, nonsensical translation.
>
> Terry is correct: comprehensions are deliberately designed to have the
> exact same looping semantics as the equivalent statements flattened
> out into a single line, with the innermost expression lifted out of
> the loop body and placed in front.


You have inadvertently supported the point I am trying to make: what is
*deliberately designed* by people one way can be deliberately designed
another way instead. List comps have the form, and limitations, they
have because of people's decisions. People could decide differently.

A while clause in a comprehension can map to the same statement form as
currently used. Just because the parser sees "while" inside a
comprehension doesn't mean that the underlying implementation has to
literally insert a while loop inside a for-loop. Terry is right about
one thing: that would lead to an entirely pointless infinite loop.

Where Terry gets it wrong is to suppose that the only *conceivable* way
to handle syntax that looks like [x for x in seq while condition] is to
insert a while loop inside a for loop. But "while" is just a convenient
keyword that looks good, is readable, and has a natural interpretation
as executable pseudo-code. We could invent a new keyword if we wished,
say "jabberwock", and treat "jabberwock cond" inside a comprehension as
equivalent to "if cond else break":

    (x, y for x in a jabberwock x for y in b jabberwock y)

     for x in a:
         if x:
             for y in b:
                 if y:
                     yield (x, y)
                 else: break
         else: break


If you, as a core developer, tell me that in practice this would be
exceedingly hard for the CPython implementation to do, I can only trust
your opinion since I am not qualified to argue.

But since you've already allowed that permitting "if cond else break"
in comprehensions would be possible, I find it rather difficult to
believe that spelling it "jabberwock cond" is not.



> The only remotely plausible proposal I've seen in this thread is the
> "else break" on the filter conditions,

Which just begs for confusion and misunderstanding. Just wait until people
start asking why they can't write "else some_expression", and we have to
explain that inside a comprehension, the only thing allowed to follow "else"
is "break".




-- 
Steven