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

Mark H Harris harrismh777 at gmail.com
Tue Mar 25 00:44:37 CET 2014

On 3/24/14 6:01 PM, Chris Angelico wrote:

> Easy fix. Use the "explicit capture" notation:

> adders[n] = lambda a, n=n: a+n

> And there you are, out of your difficulty at once!

Yes, yes, yes,  and example:

 >>>> adders= list(range(4))
 >>>> for n in adders:
 >	adders[n] = lambda a, n=n: a+n
 >>>> adders[1](3)
 >>>> adders[2](3)

    But, and this is the big (WHY?) is that necessary? In other words, 
its not about experts knowing how to make this work, its about "normal" 
people not understanding in the first place why its a problem, and why 
the various solutions work to fix it; even though "we" all know that 
nothing is 'broken'.

    And in reference to Marko's post, he's right, its not just python, 
but the issue is not where else is capture and scope a problem for 
confusion, the issue is whether python's lambda provides a significant 
opportunity for confusion that in the larger scheme of things (no pun 
intended) is not warranted.


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