Tuple of lists concatenation - function vs comprehension

[Ivan Evstegneev]

Hi Shiyao,

Now I see, that it was kind of dumb question...

>>>>x = ([1, 2], [3, 4], [5, 6])
>>>>L = []
>>>[L.extend(i) for i in x]
[None, None, None]

BUT when I check L itself I'll see this one

>>>L
[1,2,3,5,6]

Ok. Thanks. 

-----Original Message-----
From: Shiyao Ma [mailto:i at introo.me] 
Sent: Sunday, December 7, 2014 17:18
To: Ivan Evstegneev
Cc: python-list at python.org
Subject: Re: Tuple of lists concatenation - function vs comprehension

On 12/07, Ivan Evstegneev wrote:
> Hello,
>
> When I have worked  in Python shell (IDLE) I found this issue:
>
> >>>x = ([1, 2], [3, 4], [5, 6])
> >>>L = []
> >>>for I in x:
> 	L.extend(i)
>
> >>>L
> [1,2,3,4,5,6]
>
> But when I try to make comprehension using above expression, I get this:
>
> >>>x = ([1, 2], [3, 4], [5, 6])
> >>>L = []
> >>> [L.extend(i) for i in x]
> [None, None, None]

Yes. per the doc, list.extend() returns None.

>
> But this works fine within function:
>
> >>> def myfunc():
> 	x = ([1, 2], [3, 4], [5, 6])
> 	L = []
> 	[L.extend(i) for i in x]
> 	print(L)
>
> >>>myfunc()
> [1, 2, 3, 4, 5, 6]

This is also so true, as you are print the var 'L'.

>
> The question is why I'm getting empty list while working with 
> comprehension in interactive console?

You are also getting [None]*3 in comprenhension inside a function.

--
Shiyao Ma
http://introo.me