Explanation of this Python language feature? [x for x in x for x in x] (to flatten a nested list)

Rustom Mody rustompmody at gmail.com
Sat Mar 22 03:06:06 CET 2014


On Saturday, March 22, 2014 3:00:10 AM UTC+5:30, Ian wrote:
> On Fri, Mar 21, 2014 at 3:09 PM, Rustom Mody  wrote:
> > A 'for' introduces a scope:

> This is false.


And 

On Saturday, March 22, 2014 3:04:48 AM UTC+5:30, Gregory Ewing wrote:

> > A 'for' introduces a scope:

> No, it doesn't!


Ha -- funny that *I* missed that one. Thanks both Ian and Gregory

In fact one of my grumbles against python is that list comprehension's
are a poor imitation of haskell's comprehensions.

And then I promptly forgot about it!

Actually there are two leakages in python 2, one of which is corrected
in python 3.

One: a comprehension variable leaks outside after the comprehension
This is corrected in python3.  
Two: A comprehension variable is not bound but reassigned across the
comprehension. This problem remains in python3 and causes weird behavior when
lambdas are put in a comprehension

>>> fl = [lambda y : x+y for x in [1,2,3]]
>>> [fl[i](2) for i in [0,1,2]]
[5, 5, 5]

The same in haskell:

Prelude> let fl = [\ y -> x + y | x <- [1,2,3]]
Prelude> [(fl!!i) 0 | i<- [0,1,2]]
[1,2,3]



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