Velocity Reviews > execution order in list/generator expression

# execution order in list/generator expression

bonono@gmail.com
Guest
Posts: n/a

 10-23-2005
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 ?

Devan L
Guest
Posts: n/a

 10-23-2005
http://www.velocityreviews.com/forums/(E-Mail Removed) 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 []].

Robert Kern
Guest
Posts: n/a

 10-23-2005
(E-Mail Removed) 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
(E-Mail Removed)

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

bonono@gmail.com
Guest
Posts: n/a

 10-23-2005
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:
> (E-Mail Removed) 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
> (E-Mail Removed)
>
> "In the fields of hell where the grass grows high
> Are the graves of dreams allowed to die."
> -- Richard Harter