conditional for-statement

Rami Chowdhury rami.chowdhury at gmail.com
Tue Aug 25 20:17:49 CEST 2009


> We could as consistenly explain that the syntax
>
> for n in range(10) if n%3==0:
>   body
>
> means
>
> for n in range(10):
>   if n%3==0:
>     body
>
> This syntax has also the benefit of avoiding an extra level of
> indentation (the one for the if) that bears no real meaning on a
> structural level.
>

I'm sorry, I don't see what you mean about avoiding the extra level of  
indentation? I can see a very real structural and logical distinction that  
the if-block makes, and IMO it's a good thing that that needs to be set  
apart.

On Tue, 25 Aug 2009 10:25:27 -0700, seb <sdementen at gmail.com> wrote:

>
> Tx Chris for your reply !
>
> i am still a bit puzzle by the following.
>
> I read in  
> http://en.wikipedia.org/wiki/Python_syntax_and_semantics#Generators
>
> """Python 3.0 unifies all collection types by introducing dict and set
> comprehensions, similar to list comprehensions:
>
>>>> [ n*n for n in range(5) ] # regular list comprehension
> [0, 1, 4, 9, 16]
>>>>
>>>> { n*n for n in range(5) } # set comprehension
> {0, 1, 4, 16, 9}
>>>>
>>>> { n: n*n for n in range(5) } # dict comprehension
> {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
> """
> and we can add to this list the quite similar syntax for generator
> expressions.
>
> On all these loop constructs, one can consistenly add filtering on a
> condition by adding an "if ..." after the "for ... in ..." part (and
> it looks to me difficult to argue, for instance, that we should not
> allow filtering for dict comprehesion because we could get the same
> result by some other construct)
>
> If the "if ..." part after the "for ... in ..." is so much used in all
> these list/dict/set comprehensions and in the generator expressions,
> it makes sense to me to have it also for the "for as a statement"
> syntax :
> [ n*n for n in range(10) if n%3 == 0]
> { n*n for n in range(10) if n%3 == 0}
> { n: n*n for n in range(10) if n%3 == 0}
> ( n*n for n in range(10) if n%3 == 0)
> for n in range(10) if n%3 == 0:
>   print n*n
>
> In fact, we often see the list comprehension [ n*n for n in range(10)
> if n%3 == 0] explained as being equivalent to
> l = []
> for n in range(10):
>   if n%3 == 0:
>     l.append(n)
>
> We could as consistenly explain that the syntax
>
> for n in range(10) if n%3==0:
>   body
>
> means
>
> for n in range(10):
>   if n%3==0:
>     body
>
> This syntax has also the benefit of avoiding an extra level of
> indentation (the one for the if) that bears no real meaning on a
> structural level.
>
> Maybe a PEP could do the job...
>
> Sébastien



-- 
Rami Chowdhury
"Never attribute to malice that which can be attributed to stupidity" --  
Hanlon's Razor
408-597-7068 (US) / 07875-841-046 (UK) / 0189-245544 (BD)



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