Re: Bug in list comprehensions?

2006-06-07 Thread Sion Arrowsmith 
Fredrik Lundh  <[EMAIL PROTECTED]> wrote:
>Iain King wrote:
>> I'm guessing I'm the one confused here... but I'm confused!  What's 
>> going on?
>the clauses nest from left to right, not from right to left, so "[x for 
>x in y for y in beta]" is equivalent to
>
>  out = []
>  for x in y:
>  for y in beta:
>  out.append(x)

And a list comprehension doesn't get a namespace to itself (cf.
generator comprehensions) so "leaks" its variables. Exactly as
above. So the y being iterated over in "for x in y" is the y
from the previous inner iteration ("for y in beta").

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Re: Bug in list comprehensions?

2006-06-07 Thread Duncan Booth 
Iain King wrote:

 [x for x in y for y in beta]
> ['C', 'C', 'C']
 [y for y in beta]
> [['one', 'two', 'three'], ['one', 'two', 'three'], ['one', 'two',
> 'three']]
 [x for x in y for y in beta]
> ['one', 'one', 'one', 'two', 'two', 'two', 'three', 'three', 'three']
> 
> Shoudn't both lines '[x for x in y for y in beta]' produce the same
> list?

[x for x in y for y in beta] is a shorthand for:

tmp = []
for x in y:
for y in beta:
   tmp.append(x)

So x iterates over whatever y is before the loop starts, and y iterates 
over beta (but that doesn't affect what x is iterating over).

The important thing is to remember that the order of 'for' and 'if' 
statements is the same as though you had written the for loop out in full.
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Re: Bug in list comprehensions?

2006-06-07 Thread Fredrik Lundh 
Iain King wrote:

> I'm guessing I'm the one confused here... but I'm confused!  What's 
> going on?

reading the documentation may help:

 /.../ the elements of the new list 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 a list element each
 time the innermost block is reached.

the clauses nest from left to right, not from right to left, so "[x for 
x in y for y in beta]" is equivalent to

  out = []
  for x in y:
  for y in beta:
  out.append(x)



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Bug in list comprehensions?

2006-06-07 Thread Iain King 
I was playing with list comprehensions, to try and work out how doubled
up versions work (like this one from another thread: [i for i in
range(9) for j in range(i)]).  I think I've figured that out, but I
found something strange along the way:

>>> alpha = ["one", "two", "three"]
>>> beta = ["A", "B", "C"]
>>> [x for x in alpha for y in beta]
['one', 'one', 'one', 'two', 'two', 'two', 'three', 'three', 'three']
>>> [x for x in y for y in beta]
['C', 'C', 'C']
>>> beta = [alpha, alpha, alpha]
>>> beta
[['one', 'two', 'three'], ['one', 'two', 'three'], ['one', 'two',
'three']]
>>> [x for x in y for y in beta]
['C', 'C', 'C']
>>> [y for y in beta]
[['one', 'two', 'three'], ['one', 'two', 'three'], ['one', 'two',
'three']]
>>> [x for x in y for y in beta]
['one', 'one', 'one', 'two', 'two', 'two', 'three', 'three', 'three']

Shoudn't both lines '[x for x in y for y in beta]' produce the same
list?
I'm guessing I'm the one confused here... but I'm confused!  What's
going on?

Iain

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