Python map

last modified July 6, 2020

Python map tutorial presents the Python built-in map() function.

Python map() function

Python map() built-in function applies the given function on every item of iterable(s) and returns an iterator object.

map(function, iterable, ...)

It is possible to pass more than one iterable to the map() function. The function must take as many parameters as there are iterables.

Python map example

The following example uses Python map() on a list of integers.

python_map.py

#!/usr/bin/python3

def square(x):

    return x * x

nums = [1, 2, 3, 4, 5]

nums_squared = map(square, nums)

for num in nums_squared:

    print(num)

We define a list of integers and apply the square() function on each element of the list with map().

def square(x):

    return x * x

The square() function squares its parameter.

nums = [1, 2, 3, 4, 5]

We define a list of integers.

nums_squared = map(square, nums)

The map() function applies the square() function on each element of the nums list.

for num in nums_squared:

    print(num)

We loop over the returned iterable and print the elements.

$ ./python_map.py
1
4
9
16
25

This is the output.

Python map equivalent

The following example shows a custom equivalent to Python 3 map() function.

mymap_fun.py

#!/usr/bin/python3

def square(x):

    return x * x

def mymap(func, iterable):

    for i in iterable:
        yield func(i)

nums = [1, 2, 3, 4, 5]

nums_squared = mymap(square, nums)

for num in nums_squared:

    print(num)

The mymap() does the same thing as Python 3 map().

Python map with lambda

The next example creates an anonymous function inside map() with lambda operator.

python_map_lambda.py

#!/usr/bin/python3

nums = [1, 2, 3, 4, 5]

nums_squared = map(lambda x: x*x, nums)

for num in nums_squared:
    print(num)

The code example squares the elements of a list with map() and anonymous function created with lambda.

Python map with multiple iterables

We have mentioned earlier that we can pass multiple iterables into map().

python_map_iterables.py

#!/usr/bin/python3

def multiply(x, y):

    return x * y

nums1 = [1, 2, 3, 4, 5]
nums2 = [6, 7, 8, 9, 10]

mult = map(multiply, nums1, nums2)

for num in mult:
    print(num)

In the code example have two iterables holding integers. The values from both iterables are multiplied.

def multiply(x, y):

    return x * y

The function must take two parameters since there are two iterables passed to map().

$ ./python_map_iterables.py
6
14
24
36
50

This is the output.

Python map multiple functions

In the next example, we show how to use multiple functions in Python map().

python_map_multiple_funcs.py

#!/usr/bin/python3

def add(x):
    return x + x

def square(x):
    return x * x

nums = [1, 2, 3, 4, 5]

for i in nums:

    vals = list(map(lambda x: x(i), (add, square)))

    print(vals)

In the example, we apply add() and square() functions on the list of integer values.

for i in nums:

    vals = list(map(lambda x: x(i), (add, square)))

    print(vals)

We go through the elements in the for loop. In each cycle, we create a list of two values, which are computed by applying the add() and square() functions on the value.

$ ./python_map_multiple_funcs.py
[2, 1]
[4, 4]
[6, 9]
[8, 16]
[10, 25]

The first value is formed by addition, the second one by multiplication.

Python list comprehension

The functionality of Python map() can be achieved with Python list comprehensions as well.

python_list_comprehension.py

#!/usr/bin/python3

def square(x):

    return x * x

nums = [1, 2, 3, 4, 5]

nums_squared = [square(num) for num in nums]

for num in nums_squared:

    print(num)

The example creates a list of squared values from a list of integers with Python list comprehension.

In this tutorial, we have worked with the Python map() function.

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