iterating over two sequences at once

Peter Otten __peter__ at web.de
Sun Jul 25 19:44:24 CEST 2004


Dan wrote:

> This is relatively simple, borrowing from my C++/PHP background:
> 
> str = "abcdefg"
> str2 = "abcde"
> 
> i = 0
> for c in str:
>   print c, " ", str2[i]
>   i += 2
> 
> But that is a C++/PHP way of doing things.
> 
>>From what I've seen of python, I almost expected python to allow:
> 
> for c, c2 in str, str2:
> 
> But it didn't. I would be very surprised if there isn't another clever
> way of doing the same though.

>>> for c, d in zip("abc", "xyzwillbeignored"):
...     print c, d
...
a x
b y
c z

or, if i += 2 isn't there by mistake:

>>> for c, d in zip("abc", "xyzwillbeignored"[::2]):
...     print c, d
...
a x
b z
c i
>>>

Using itertools.izip() instead of zip() doesn't create an intermediate list
of tuples which is an advantage when the zip/izip arguments are large
sequences.

> My question to you is, how would you do this? And do you think I should
> propose a PEP for supporting the above for loop syntax? It seems like a
> relatively harmless feature in that it won't break existing code, but
> then I am very new to python.

What you propose already has a meaning and therefore _will_ break existing
code.

>>> for c, d in "ab", "cd":
...     print c, d
...
a b
c d

Peter




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