Python - zip Examples - Dot Net Perls

Zip. With zip we can act upon 2 lists at once. Zip() is a built-in function—we pass it two iterables, like lists, and it enumerates them together.

With zip objects, we can loop over tuples of the elements. But we cannot access elements by indexes or use len. We must loop over our zip objects.

Initial example. Here we have two string lists, items1 and items2. We use zip on the two lists. We use a for-in loop and then display our elements.

Info In the for-loop we unpack each element from zip's return value. So item1 and item2 are from each respective list.

String List

for

items1 = ["blue", "red", "green", "white"] items2 = ["sky", "sunset", "lawn", "pillow"] # Zip the two lists and access pairs together. for item1, item2 in zip(items1, items2): print(item1, "...", item2)

blue ... sky red ... sunset green ... lawn white ... pillow

Unequal length. When we call zip, the function must find the largest common element count. So if one list has 4 elements and the other 3, the zip object will have just 3 tuples in it.

list1 = [10, 20, 30, 40] list2 = [11, 21, 31] # Zip the two lists. result = zip(list1, list2) # The zipped result is only 3 elements long. # ... The fourth element of list1 was discarded. for element1, element2 in result: print(element1, element2)

10 11 20 21 30 31

Zip object. Zip does not return a list. Instead it returns a special, optimized collection called a "zip object." We can loop over the zip object, but cannot access by indexes.

list1 = [0] list2 = [1] # The result is a zip object. result = zip(list1, list2) print(result)

Not subscriptable. A zip object is not a list. We cannot access each element directly with an index. Instead we must loop over the result of zip or convert it.

Detail We cause TypeErrors when we try to use unavailable functions on zip objects. Please see the "convert to list" example.

list1 = [0, 1, 2] list2 = [3, 4, 5] # Zip together the two lists. zipped = zip(list1, list2) # This causes an error. print(zipped[0])

Traceback (most recent call last): File "C:\programs\file.py", line 12, in <module> print(zipped[0]) TypeError: 'zip' object is not subscriptable

Len error. We cannot take the len of a zip object. This will result in a TypeError and a message stating "zip has no length." Please first convert to a list.

len

zipped = zip([1, 2], [3, 4]) # This causes an error. print(len(zipped))

Traceback (most recent call last): File "C:\programs\file.py", line 7, in <module> print(len(zipped)) TypeError: 'zip' has no length

Convert to list. Zip objects are not the most useful things. For many operations, a list is more effective. We can convert a zip object into a list with the list built-in function.

# Zip up these two lists. zipped = zip([1, 2], [3, 4]) # Convert the zip object into a list. result_list = list(zipped) print(result_list)

[(1, 3), (2, 4)]

In a sense, zip() lets us treat 2 single-dimension lists as one list of pairs—a two-dimensional list. More than two lists can be zipped—try passing 3 or more lists to zip.