why does this unpacking work

johnzenger at gmail.com johnzenger at gmail.com
Fri Oct 20 21:36:43 CEST 2006


It's just sequence unpacking.  Did you know that this works?:

pair = ("California","San Francisco")
state, city = pair
print city
# 'San Francisco'
print state
# 'California'

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'))
>  >>> t
> (('hello', 'goodbye'), ('more', 'less'), ('something', 'nothing'),
> ('good', 'bad'))
>  >>> for x in t:
> 	print x
>
>
> ('hello', 'goodbye')
> ('more', 'less')
> ('something', 'nothing')
> ('good', 'bad')
>  >>> for x,y in t:
> 	print x,y
>
>
> 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.
>
> 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
>
> But if so, why is this? It doesn't seem like very intuitive behavior.
> 
> Thanks.




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