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Date: Wed, 08 May 2013 23:54:24 -0700
From: Gary Herron <gary.herron@islandtraining.com>
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Xref: csiph.com comp.lang.python:45013

On 05/08/2013 11:36 PM, RAHUL RAJ wrote:
> Checkout the following code:
>
> sample2 = [x+y for x in range(1,10) for y in range(1,10) if x!=y]
> output=[]

> output=[x for x in sample2 if x not in output]
This statement is not doing what you expect.  It is not building a list 
in the variable named output, it is building a list (anonymously) then 
binding it to the variable output once it's built.  Therefore output is 
[] for the whole list building operation.

The later operation works, because your *are* building the list in place 
as you go.

>
> the output I get is
> 3 4 5 6 7 8 9 10 3 5 6 7 8 9 10 11 4 5 7 8 9 10 11 12 5 6 7 9 10 11 12 13 6 7 8 9 11 12 13 14 7 8 9 10 11 13 14 15 8 9 10 11 12 13 15 16 9 10 11 12 13 14 15 17 10 11 12 13 14 15 16 17
>
> which contains duplicate values.
>
>
>
>
> But if I do like this:
>
> sample2 = [x+y for x in range(1,10) for y in range(1,10) if x!=y]
> output=[]
> for x in sample2:
>    if x not in output:
>       output.append(x)
>
>
> the value of 'output' I get like this:
> 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
>
> I know that both the programs have the same functionality, but why do I have different outputs?
>
> Please help!