# Zip lists in Python

## Question:

I am trying to learn how to “zip” lists. To this end, I have a program, where at a particular point, I do the following:

``````x1, x2, x3 = stuff.calculations(withdataa)
``````

This gives me three lists, `x1`, `x2`, and `x3`, each of, say, size 20.

Now, I do:

``````zipall = zip(x1, x2, x3)
``````

However, when I do:

``````print "len of zipall %s" % len(zipall)
``````

I get 20, which is not what I expected. I expected three. I think I am doing something fundamentally wrong.

When you `zip()` together three lists containing 20 elements each, the result has twenty elements. Each element is a three-tuple.

See for yourself:

``````In : a = b = c = range(20)

In : zip(a, b, c)
Out:
[(0, 0, 0),
(1, 1, 1),
...
(17, 17, 17),
(18, 18, 18),
(19, 19, 19)]
``````

To find out how many elements each tuple contains, you could examine the length of the first element:

``````In : result = zip(a, b, c)

In : len(result)
Out: 3
``````

Of course, this won’t work if the lists were empty to start with.

`zip` creates a new list, filled with tuples containing elements from the iterable arguments:

``````>>> zip ([1,2],[3,4])
[(1,3), (2,4)]
``````

I expect what you try to so is create a tuple where each element is a list.

`zip` takes a bunch of lists likes

``````a: a1 a2 a3 a4 a5 a6 a7...
b: b1 b2 b3 b4 b5 b6 b7...
c: c1 c2 c3 c4 c5 c6 c7...
``````

and “zips” them into one list whose entries are 3-tuples `(ai, bi, ci)`. Imagine drawing a zipper horizontally from left to right.

Basically the zip function works on lists, tuples and dictionaries in Python.
If you are using IPython then just type zip? And check what zip() is about.

If you are not using IPython then just install it: “pip install ipython”

For lists

``````a = ['a', 'b', 'c']
b = ['p', 'q', 'r']
zip(a, b)
``````

The output is `[('a', 'p'), ('b', 'q'), ('c', 'r')`

For dictionary:

``````c = {'gaurav':'waghs', 'nilesh':'kashid', 'ramesh':'sawant', 'anu':'raje'}
d = {'amit':'wagh', 'swapnil':'dalavi', 'anish':'mane', 'raghu':'rokda'}
zip(c, d)
``````

The output is:

``````[('gaurav', 'amit'),
('nilesh', 'swapnil'),
('ramesh', 'anish'),
('anu', 'raghu')]
``````

In Python 2.7 this might have worked fine:

``````>>> a = b = c = range(20)
>>> zip(a, b, c)
``````

But in Python 3.4 it should be (otherwise, the result will be something like `<zip object at 0x00000256124E7DC8>`):

``````>>> a = b = c = range(20)
>>> list(zip(a, b, c))
``````

For the completeness’s sake.

When zipped lists’ lengths are not equal.
The result list’s length will become the shortest one without any error occurred

``````>>> a = 
>>> b = ["2", 3]
>>> zip(a,b)
[(1, '2')]
``````

Example

``````numbers = [1,2,3]
letters = 'abcd'

zip(numbers, letters)
# [(1, 'a'), (2, 'b'), (3, 'c')]
``````

## Input

Zero or more iterables  (ex. list, string, tuple, dictionary)

## Output (list)

1st tuple = (element_1 of numbers, element_1 of letters)

2nd tuple = (e_2 numbers, e_2 letters)

n-th tuple = (e_n numbers, e_n letters)

1. List of n tuples: n is the length of the shortest argument (input)
• len(numbers) == 3 < len(letters) == 4 → short= 3 → return 3 tuples
2. Length each tuple = # of args (tuple takes an element from each arg)
• args = (numbers,letters); len(args) == 2 → tuple with 2 elements
3. `i`th tuple = (element_i arg1, element_i arg2…, element_i arg`n`)

## Edge Cases

1) Empty String: len(str)= 0 = no tuples

2) Single String: len(str) == 2 tuples with len(args) == 1 element(s)

``````zip()
# []
zip('')
# []
zip('hi')
# [('h',), ('i',)]
``````

## Zip in Action!

1. Build a dictionary  out of two lists

``````keys = ["drink","band","food"]
values = ["La Croix", "Daft Punk", "Sushi"]

my_favorite = dict( zip(keys, values) )

my_favorite["drink"]
# 'La Croix'

my_faves = dict()
for i in range(len(keys)):
my_faves[keys[i]] = values[i]
``````
• `zip` is an elegant, clear, & concise solution

2. Print columns in a table

“*”  is called “unpacking”: `f(*[arg1,arg2,arg3]) == f(arg1, arg2, arg3)`

``````student_grades = [
[   'Morty'  ,  1   ,  "B"  ],
[   'Rick'   ,  4   ,  "A"  ],
[   'Jerry'  ,  3   ,  "M"  ],
[  'Kramer'  ,  0   ,  "F"  ],
]

print row_1
# ['Morty', 1, 'B']

names = columns
print names
# ('Morty', 'Rick', 'Jerry', 'Kramer')
``````

## Extra Credit: Unzipping

`zip(*args)` is called “unzipping” because it has the inverse effect of `zip`

``````numbers = (1,2,3)
letters = ('a','b','c')

zipped = zip(numbers, letters)
print zipped
# [(1, 'a'), (2, 'b'), (3, 'c')]

unzipped = zip(*zipped)
print unzipped
# [(1, 2, 3), ('a', 'b', 'c')]
``````
• `unzipped`: tuple_1 = e1 of each zipped tuple. tuple_2 = e2 of each `zipped`

## Footnotes

1. An object capable of returning its members one at a time (ex. list [1,2,3], string ‘I like codin’, tuple (1,2,3), dictionary {‘a’:1, ‘b’:2})
2. {key1:value1, key2:value2…}
3. “Unpacking” (*)

`*` Code:

``````# foo - function, returns sum of two arguments
def foo(x,y):
return x + y
print foo(3,4)
# 7

numbers = [1,2]
print foo(numbers)
# TypeError: foo() takes exactly 2 arguments (1 given)

print foo(*numbers)
# 3
``````

`*` took `numbers` (1 arg) and “unpacked” its’ 2 elements into 2 args

I don’t think `zip` returns a list. `zip` returns a generator. You have got to do `list(zip(a, b))` to get a list of tuples.

``````x = [1, 2, 3]
y = [4, 5, 6]
zipped = zip(x, y)
list(zipped)
``````

In Python 3 `zip` returns an iterator instead and needs to be passed to a list function to get the zipped tuples:

``````x = [1, 2, 3]; y = ['a','b','c']
z = zip(x, y)
z = list(z)
print(z)
>>> [(1, 'a'), (2, 'b'), (3, 'c')]
``````

Then to `unzip` them back just conjugate the zipped iterator:

``````x_back, y_back = zip(*z)
print(x_back); print(y_back)
>>> (1, 2, 3)
>>> ('a', 'b', 'c')
``````

If the original form of list is needed instead of tuples:

``````x_back, y_back = zip(*z)
print(list(x_back)); print(list(y_back))
>>> [1,2,3]
>>> ['a','b','c']
``````

It’s worth adding here as it is such a highly ranking question on zip. `zip` is great, idiomatic Python – but it doesn’t scale very well at all for large lists.

``````books = ['AAAAAAA', 'BAAAAAAA', ... , 'ZZZZZZZ']
words = [345, 567, ... , 672]

for book, word in zip(books, words):
print('{}: {}'.format(book, word))
``````

Use `izip`. For modern processing, it stores it in L1 Cache memory and is far more performant for larger lists. Use it as simply as adding an `i`:

``````for book, word in izip(books, words):
print('{}: {}'.format(book, word))
``````
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