When is it better to use zip instead of izip?
Solution 1
When you know you'll want the full list of items constructed (for instance, for passing to a function that would modify that list in-place). Or when you want to force the arguments you're passing to zip()
to be completely evaluated at that specific point.
Solution 2
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.
Solution 3
The itertools library provides "iterators" for common Python functions. From the itertools docs, "Like zip() except that it returns an iterator instead of a list." The I in izip() means "iterator".
Python iterators are a "lazy loaded" sequence that saves memory over regular in-memory list. So, you would use itertools.izip(a, b) when the two inputs a, b are too big to keep in memory at one time.
Look up the Python concepts related to efficient sequential processing:
"generators" & "yield"
"iterators"
"lazy loading"
Solution 4
In 2.x, when you need a list instead of an iterator.
Related videos on Youtube
Comments
-
Neil G over 3 years
When is it better to use
zip
instead ofitertools.izip
?-
Causality almost 8 yearsOne reason in favor of
zip
, too obvious yet still worth pointing out, is thatizip
returns aniterator
which can be traversed only once. i.e. inii = izip(a,b) ; f(ii) ; g(ii)
, here an empty list[]
is passed tog
. -
Charles L. over 5 yearsFYI, Python 3's
zip
function is Python 2'sizip
. In general Python 3 changed most functions to use iterators, like range, filter, the dict functions, etc
-
-
Neil G about 13 yearsCan you give me an example where that might happen?
-
Ignacio Vazquez-Abrams about 13 yearsNot really. Which is why I tend to prefer
itertools.izip()
except where the gains would be purely statistical. -
Don about 13 yearsYou're right. I started with good intentions and then fell into theoretical stuff...
-
Jan Vlcinsky over 10 yearsOne case, when you need a list, is when you plan to access items of the result by index or need to find total length.
lst = zip(lst_a, lst_b)
allowslst[1]
orlen(lst)
. However, forilst = itertools.izip(lst_a, lst_n)
you will fail trying toilst[1]
orlen(ilst)
. -
user1815201 over 10 yearsWould it not be better to use izip in the first case as its faster since it reuses the tuple and there's no real reason not to use izip?
-
ShadowRanger over 8 years@user1815201:
izip
only reuses thetuple
if thetuple
was released before the next iteration begins, so it doesn't gain you anything. That said, any loss is trivial too, so I agree that there is little reason not to useizip
exclusively, wrapping withlist
if you need alist
; you can actually do this the "proper" way by addingfrom future_builtins import zip
to Py2 code, which makes plainzip
intoizip
(preparing for Py3 transition). -
Rahul over 7 yearsNicely explained.