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

Tue Jan 29 16:02:40 CET 2013

On 29/01/2013 10:44, Nick Coghlan wrote:
> 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. This then works to arbitrarily deep
> nesting levels. The surrounding syntax (parentheses, brackets, braces,
> and whether or not there is a colon present in the main expression)
> then governs what kind of result you get (generator-iterator, list,
> set, dict).
>> For example in:
>> (x, y, z for x in a if x for y in b if y for z in c if z)
> [x, y, z for x in a if x for y in b if y for z in c if z]
> {x, y, z for x in a if x for y in b if y for z in c if z}
> {x: y, z for x in a if x for y in b if y for z in c if z}
>> The looping semantics of these expressions are all completely defined
> by the equivalent statements:
>> for x in a:
> if x:
> for y in b:
> if y:
> for z in c:
> if z:
>> (modulo a few name lookup quirks if you're playing with class scopes)
>Thanks for spelling this out so clearly. It helps me remember which 
order to place nested "for"s inside a list comprehension! :-)