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(wi         


        

Answer 1

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.

Answer 2

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)

Answer 3

Source: My Blog Post (better formatting)

Example

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

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

Input

Zero or more iterables [1] (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. ith tuple = (element_i arg1, element_i arg2…, element_i argn)

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 [2] 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

"*" [3] 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"  ],
]

row_1 = student_grades[0]
print row_1
# ['Morty', 1, 'B']

columns = zip(*student_grades)
names = columns[0]
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

Answer 4

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