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

Tue Jan 29 20:16:57 CET 2013

On 29/01/2013 18:53, Terry Reedy wrote:
> On 1/29/2013 10:02 AM, Rob Cliffe wrote:
>>>> 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! :-)
>> The reference manual does spell it out: "In this case, the elements of 
> the new container are those that would be produced by considering each 
> of the for or if clauses a block, nesting from left to right, and 
> evaluating the expression to produce an element each time the 
> innermost block is reached." Perhaps a non-trivial concrete example 
> (say 4 levels deep) would help people understand that better.
>Definitely. +1. Though I think 3 levels is enough.