execution order in list/generator expression

[Robert Kern]

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