When is it better to use zip instead of izip?

Question

When is it better to use zip instead of itertools.izip?

Answer 1

zip computes all the list at once, izip computes the elements only when requested.

One important difference is that 'zip' returns an actual list, 'izip' returns an 'izip object', which is not a list and does not support list-specific features (such as indexing):

>>> l1 = [1, 2, 3, 4, 5, 6]
>>> l2 = [2, 3, 4, 5, 6, 7]
>>> z = zip(l1, l2)
>>> iz = izip(l1, l2)
>>> isinstance(zip(l1, l2), list)
True
>>> isinstance(izip(l1, l2), list)
False
>>> z[::2] #Get odd places
[(1, 2), (3, 4), (5, 6)]
>>> iz[::2] #Same with izip
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'itertools.izip' object is unsubscriptable

So, if you need a list (an not a list-like object), just use 'zip'.

Apart from this, 'izip' can be useful for saving memory or cycles.

E.g. the following code may exit after few cycles, so there is no need to compute all items of combined list:

lst_a = ... #list with very large number of items
lst_b = ... #list with very large number of items
#At each cycle, the next couple is provided
for a, b in izip(lst_a, lst_b):
    if a == b:
        break
print a

using zip would have computed all (a, b) couples before entering the cycle.

Moreover, if lst_a and lst_b are very large (e.g. millions of records), zip(a, b) will build a third list with double space.

But if you have small lists, maybe zip is faster.