Understand the map function in Python

Tyler Tyler (291)
5 minutes

This guide provides an overview of Python map function. Python 2 and Python 3 have subtle differences in implementation, but they are generally used in the same way.


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The purpose of the map function is to apply the same procedure to every item in an iterable data structure. Iterable data structures can include lists, generators, strings, etc. In a very basic example, map can iterate over every item in a list and apply a function to each item. Then it will return the new list. Assume you have a function that takes an integer and increments it by 1:

def incr_by_one(num):
    return num + 1
You also have a list of integers representing something very important:
important_list = [1, 2, 4, 6, 10, 12, 16, 18]
If you need to increment every item in this list by 1 you can do this with map:
map(incr_by_one, important_list)
You can see that map takes a minimum of two arguments. The first is the function name, and the second is the list. Map will iterate through every item of important_list and pass the element to incr_by_one. Then it will append the output of the incr_by_one function to a new list. Once it finishes iterating it will return a new list leaving the original unaffected. So this map function will return:
[2, 3, 5, 7, 11, 13, 17, 19]

The map function accepts multiple iterables. These can be passed in as optional arguments. If multiple iterables are passed in, an element from each is passed in as an argument to the designated function. Assume we have the following lists:

l1 = [1, 2, 3, 4, 5]
l2 = [11, 12, 13, 14, 15]
l3 = [21, 22, 23, 24, 25]
With map we can create a new list that contains the addition of elements at each index. So in our resulting list at index 0, we will have the sum of l1[0], l2[0], and l3[0]. This will continue for each element in the list. In order for this to work, the function we pass in must take the same number of arguments as the number of iterables you pass in. So if you pass in three lists, your function must take three arguments. Let's create our function:
def add_three_numbers(num1, num2, num3):
    return num1 + num2 + num3
Now that we have our function we can use map to generate our desired list:
map(add_three_numbers, l1, l2, l3)
This will produce:
[33, 36, 39, 42, 45]
You may have noticed that each of our lists has exactly five elements. So the map function worked very nicely. Its important to note how map handles iterables of different sizes. This is where Python 2 and Python 3 are different. Python 2 will iterate through the longest iterable and pass None in as the argument of the short iterables. So if one list has a length of seven and another list has a length of five, map will iterate through seven times but pass in None for the indices of the shorter list that exceed its length. If this is a possibility, your function must be prepared to handle arguments of NoneType (ours would throw a TypeError). Python 3 is a much cleaner implementation. It will only iterate through the length of the shortest iterable. So if you have a list of seven and a list of five, it will only iterate through five times. Read more about Python 2 map Read more about Python 3 map

Tyler Tyler (291)
5 minutes

Continuous integration is pretty trendy, but despite that fact, it's also pretty useful. This guide will show you how to set up your first project in CircleCI.