[Tutor] Would somebody kindly...

[Cameron Simpson]

On 28Oct2014 21:33, Clayton Kirkwood <crk at godblessthe.us> wrote:
>!From: Cameron Simpson [mailto:cs at zip.com.au]
>!Let me try a less wordy diagram. You will need to be displaying in a
>!constant width font :-)
>!
>!   [ pair for pair in values if key == pair[0] ]
>!     ^^^^-- the expression that accrues in the resulting list
>!              ^^^^-- the loop variable, taken from the loop source values
>!                      ^^^^^^-- the loop source values
>!                                ^^^^^^^^^^^^^^-- condition for including the
>!                                            expression in the resulting list
>!
>!So that first "pair" could be any expression, it is almost only
>!coincidence that it is the same as the loop variable. It is the same in
>!this case because this is the idiomatic way to select particular values
>!form an existing list.
>!
>!If we'd wanted the new list to contain double the original values we'd
>!write:
>!
>!   [ pair*2 for pair in values if key == pair[0] ]
>
>Ok, I am somewhat confused again. In the original example up above, it
>appears that the pair list or tuple gets overridden. In this one right
>above, once again, the list gets overwritten again, but what is being
>doubled?, each part of the tuple?

First up, this makes a list, not a tuple. (Hence the outermost [] instead of 
(). No, there is not a tuple equivalenti:-)

So, to get things straight:

   - values is a list of pairs (2-tuples)

   - the list comprehension above creates a new list consisting the first 
   expression computed for each pair in "values" matching the condition 
   expression

And I had not noticed that "pair" was a 2-tuple. So "pair*2" is not a sensible 
example. The point is that the first "pair" (or bad example "2*pair") is an 
arbitrary expression, used to compute each value in the new list.

Cheers,
Cameron Simpson <cs at zip.com.au>

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