Bug in list comprehensions?

[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:
[color=blue][color=green][color=darkred]
>>> alpha = ["one", "two", "three"]
>>> beta = ["A", "B", "C"]
>>> [x for x in alpha for y in beta][/color][/color][/color]
['one', 'one', 'one', 'two', 'two', 'two', 'three', 'three', 'three'][color=blue][color=green][color=darkred]
>>> [x for x in y for y in beta][/color][/color][/color]
['C', 'C', 'C'][color=blue][color=green][color=darkred]
>>> beta = [alpha, alpha, alpha]
>>> beta[/color][/color][/color]
[['one', 'two', 'three'], ['one', 'two', 'three'], ['one', 'two',
'three']][color=blue][color=green][color=darkred]
>>> [x for x in y for y in beta][/color][/color][/color]
['C', 'C', 'C'][color=blue][color=green][color=darkred]
>>> [y for y in beta][/color][/color][/color]
[['one', 'two', 'three'], ['one', 'two', 'three'], ['one', 'two',
'three']][color=blue][color=green][color=darkred]
>>> [x for x in y for y in beta][/color][/color][/color]
['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

[Duncan Booth]

Re: Bug in list comprehensions?

Iain King wrote:
[color=blue][color=green][color=darkred]
>>>> [x for x in y for y in beta][/color][/color]
> ['C', 'C', 'C'][color=green][color=darkred]
>>>> [y for y in beta][/color][/color]
> [['one', 'two', 'three'], ['one', 'two', 'three'], ['one', 'two',
> 'three']][color=green][color=darkred]
>>>> [x for x in y for y in beta][/color][/color]
> ['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?[/color]

[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.

[Fredrik Lundh]

Re: Bug in list comprehensions?

Iain King wrote:
[color=blue]
> I'm guessing I'm the one confused here... but I'm confused! What's
> going on?[/color]

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)

</F>

[Sion Arrowsmith]

Re: Bug in list comprehensions?

Fredrik Lundh <fredrik@python ware.com> wrote:[color=blue]
>Iain King wrote:[color=green]
>> I'm guessing I'm the one confused here... but I'm confused! What's
>> going on?[/color]
>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)[/color]

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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