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Re: writable iterators?

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  Re: writable iterators? Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2011-06-22 21:50 -0700

#8262 — Re: writable iterators?

On Wed, 22 Jun 2011 15:28:23 -0400, Neal Becker <ndbecker2@gmail.com>
declaimed the following in gmane.comp.python.general:

> for e in sequence:
>   do something that reads e
>   e = blah # will do nothing
> 
> I believe this is not a limitation on the for loop, but a limitation on the 
> python iterator concept.  Is this correct?

	Neither... It is a misunderstanding of what "=" assignment DOES.

	e = blah

translates into "disconnect the NAME 'e' from whatever object it
currently refers to; connect the NAME 'e' to the same object that the
name 'blah" is connected to"

	"for e in sequence" connects the name "e" to then next object in the
iterable. As long as your "writable" is a MUTABLE object, you CAN mutate
it...

>>> lol = [	[1, 2, 3],
... 	[4, 5, 6],
... 	[7, 8, 9]	]
>>> for e in lol:
... 	e[1] = sum(e)
... 	
>>> lol
[[1, 6, 3], [4, 15, 6], [7, 24, 9]]
>>> 

>>> for e in lol:
... 	e.reverse()
... 	
>>> lol
[[3, 6, 1], [6, 15, 4], [9, 24, 7]]
>>> 
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/

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