why does this unpacking work

Gabriel Genellina gagsl-py at yahoo.com.ar
Fri Oct 20 22:18:17 CEST 2006


At Friday 20/10/2006 16:14, John Salerno wrote:

>I'm a little confused, but I'm sure this is something trivial. I'm
>confused about why this works:
>
>  >>> t = (('hello', 'goodbye'),
>       ('more', 'less'),
>       ('something', 'nothing'),
>       ('good', 'bad'))
>I understand that t returns a single tuple that contains other tuples.
>Then 'for x in t' returns the nested tuples themselves.
>
>But what I don't understand is why you can use 'for x,y in t' when t
>really only returns one thing. I see that this works, but I can't quite
>conceptualize how. I thought 'for x,y in t' would only work if t
>returned a two-tuple, which it doesn't.

You can think of

for x in t:
   whatever

as meaning "for each element contained in t, name it x and do whatever"

The other concept involved is unpacking:
 >>> w = (1,2,3)
 >>> x,y,z = w
 >>> x
1

When you say "for x,y in t:" there is an implicit unpacking, it means 
"for each element contained in t, unpack it into x and y and do whatever"

>What seems to be happening is that 'for x,y in t' is acting like:
>
>for x in t:
>      for y,z in x:
>          #then it does it correctly

No, it acts like:

for w in t:
   x,y = w
   ...


-- 
Gabriel Genellina
Softlab SRL 

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