a potential pep to extend the syntax of for loops

Lie Ryan lie.1296 at gmail.com
Mon Mar 9 11:51:17 CET 2009


>   Hello, 
> This is an idea about something I'd like to see
> implemented in python.  I understand that's the purpose of
> PEPs, so I'll write it as a PEP, but send it here to receive
> your valuable feedback.
> 
> Abstract
> 
> This is a proposal to increase the richness of for loops,
> only to the extent that it equals that of list and generator
> comprehensions. In the opinion of the proponent, this will
> make the language more uniform and would reduce the excessive
> level of nesting that is required sometimes, without
> introducing special keywords, or breaking backwards
> compatibility.

The PEP suffers from several problems. Some of them related to being 
quite unclear in several edges, others are real problems.

> This PEP is independent of, but related to PEP 3142.
> 
> Rationale
> The syntax of a for loop is restricted to the following:
>>>>    for element in list:
>>>>        instructions
> 
> Nesting for loops and conditionals is possible, of course:
>>>> for x in list1:
>>>>     for y in list2:
>>>>         if condition(x,y):
>>>>             func2(x,y)
> 
> However, for list and generator comprehensions, the syntax is
> more concise:
>>>> list=[func(x,y,..) for x in list1 for y in list2 
> ...        if condition(x,y,...)]
> 
> Loops and comprehensions serve for similar purposes, and
> sometimes what was first written in one way is finally
> changed into the other. Thus it would be convenient to allow
> for more similar syntax in both of them.  This requires that
> the syntax of a for loop allows for nested for and
> conditionals (and possibly while loops (see PEP 3142))
> 
>>>> for x in list1 for y in list2 ... if condition(x,y,...):
>>>>     ''instructions''
> 
> In general, whenever
>>>> [expression1  iterator]
> is a valid list comprehension, then
>>>> iterator:
>>>>     statements
> should be a valid loop.
> 
> As another example, it is sometimes the case that an
> iteration over two lists can be rewritten as a single
> iteration:
>>>> for x in range(10) for y in range(10) if x+y==5:
>>>>     print x,y

What is that supposed to mean? Nested looping? Why is that (confusing 
thing) better than:

from itertools import product

for x, y in product(range(10), range(10)) if x + y == 5:
     print x, y

> is equivalent to
>>>> for x in range(6):
>>>>     print x,5-y

How is that equivalent? The second one is generated using completely 
different logic than the second one (not to mention it is shorter, 
simpler, and faster).

> it is somewhat more conceptual to think of the first
> iteration as a single loop.
> 
> It is actually possible to come very close to the notation
> proposed using the generator comprehensions introduced in
> python 2.5:
>>>> for x,y in ((x,y) for x in range(10) for y in range(10) if x+y==5):
>>>>     print x,y
> but this notation is cumbersome.
> 
> This PEP responds to the philosophy that if a nesting level
> is unused, it should be avoided. For example, in the code:

If a nesting level is unused... shouldn't just the whole thing be 
deleted? I'm unclear on what you mean by "unused nesting level"

>>>> for x in list1:
>>>>     func1(x)
>>>>     for y in list2:
>>>>        if cond(x,y):
>>>>           func(x,y)
>>>>     func3(x)
> 
> the second nesting  level doesn't contain any statements and
> thus, should this PEP be passed, it should be written:

What do you mean by "doesn't contain any statements"? If statement is 
not a statement?

>>>> for x in list1:
>>>>     func1(x)
>>>>     for y in list2 if cond(x,y):
>>>>           func(x,y)
>>>>     func3(x)


+1 on the "if" and (possibly) "while" conditional looping.
-1 on shorthand nested for.



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