[Tutor] Would somebody kindly...

[Dave Angel]

"Martin A. Brown" <martin at linux-ip.net> Wrote in message:
> 
> Hi there Clayton,
> 
>> values = [ ('a', 1), ('b', 2), ('a', 5), ('c', 7)]
>> key = 'a'
>> pair=[]      # -- this assignment is unnecessary
>> x=[pair for pair in values if key == pair[0]]
>> print(x)
>>
>> I get [('a', 1), ('a', 5)]
> 
> I also get that result.  Good.
> 
>> So, what does that first pair do? I see and have used the first comprehension.
> 
> I'll point out that somebody (Cameron Simpson, specifically) already 
> gave you a hint that might have helped, but maybe you missed it as 
> you were trying to understand list comprehensions.
> 
> Let's play a little game....I'm going to assume that the variables 
> values and key are initialized as you have initialized them above.
> 
> 
> Game #1:  Recognize that the name of the variable in the list
>    comprehension is ephemeral.
> 
>    >>> [frobnitz for frobnitz in values if key == frobnitz[0]]
>    [('a', 1), ('a', 5)]
> 
> Yes, I guess that's obvious now.  So, this is why people often use 
> 'x' in these situations.
> 
>    >>> [x for x in values if key == x[0]]
>    [('a', 1), ('a', 5)]
> 
> The variable will probably contain the final element of the input 
> sequence after the list comprehension terminates.

NO, NO, NO.  The OP is using Python 3.4, and has consistently
 shown results accordingly.  x does NOT exist after the list
 comprehension.  That was a flaw in python 2.x which has been
 fixed.





-- 
DaveA