FAQ
Hi,

I am wondering how this is evaluated.

a=(x for x in [1,2,3,4])
p=[4,5]

c=[x for x in p if x in list(a)]

c is []

but if I expand a first, like a = list(a)

c is [4]

So it seems that the "if" part don't get expanded ?

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•  at Oct 23, 2005 at 9:35 am ⇧

bonono at gmail.com wrote:
Hi,

I am wondering how this is evaluated.

a=(x for x in [1,2,3,4])
p=[4,5]

c=[x for x in p if x in list(a)]

c is []

but if I expand a first, like a = list(a)

c is [4]

So it seems that the "if" part don't get expanded ?
Well, for every element in p it recalculates list(a). Since the
generator is exhausted after making a list from it, it gives you
nothing afterwards. So after it checks the first element, it's
equivalent to [x for x in p if x in []].
•  at Oct 23, 2005 at 9:38 am ⇧

bonono at gmail.com wrote:
Hi,

I am wondering how this is evaluated.

a=(x for x in [1,2,3,4])
p=[4,5]

c=[x for x in p if x in list(a)]

c is []
No it isn't.

In [1]: a=(x for x in [1,2,3,4])

In [2]: p=[4,5]

In [3]: c=[x for x in p if x in list(a)]

In [4]: c
Out[4]: [4]

I'm willing to bet that you used up the 'a' iterator before you ran that
list comprehension, though.

In [5]: c=[x for x in p if x in list(a)]

In [6]: c
Out[6]: []

Note that "x in list(a)" gets executed on each iteration, but the
iterator is used up on the first time.

In [7]: a=(x for x in [1,2,3,4])

In [8]: p = [4, 5, 2, 3]

In [9]: c=[x for x in p if x in list(a)]

In [10]: c
Out[10]: [4]

--
Robert Kern
rkern at ucsd.edu

"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter
•  at Oct 23, 2005 at 9:48 am ⇧
Ah, no wonder. I test with p=[5,4].

thanks. so basically, I still need to expand it first given this
behaviour.

Robert Kern wrote:
bonono at gmail.com wrote:
Hi,

I am wondering how this is evaluated.

a=(x for x in [1,2,3,4])
p=[4,5]

c=[x for x in p if x in list(a)]

c is []
No it isn't.

In [1]: a=(x for x in [1,2,3,4])

In [2]: p=[4,5]

In [3]: c=[x for x in p if x in list(a)]

In [4]: c
Out[4]: [4]

I'm willing to bet that you used up the 'a' iterator before you ran that
list comprehension, though.

In [5]: c=[x for x in p if x in list(a)]

In [6]: c
Out[6]: []

Note that "x in list(a)" gets executed on each iteration, but the
iterator is used up on the first time.

In [7]: a=(x for x in [1,2,3,4])

In [8]: p = [4, 5, 2, 3]

In [9]: c=[x for x in p if x in list(a)]

In [10]: c
Out[10]: [4]

--
Robert Kern
rkern at ucsd.edu

"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter

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 group python-list categories python posted Oct 23, '05 at 9:25a active Oct 23, '05 at 9:48a posts 4 users 3 website python.org

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