Map

Like our more specialized versions, map(…) takes a function (or method) as the first input parameter and a list as the second parameter. It is the responsibility of the programmer using map(…) to make sure that the function provided as a parameter is able to work with whatever is stored in the provided list. In Python 3, a change to map(…) has been made so that it now returns a special map object rather than a simple list. However, whenever we need the result as a normal list, we can simply apply the list(…) function to the result like this:

l = list(map(…, …))

The three examples below show how we could have performed the conversion to upper-case and first character capitalization, and the rounding task with map(...) instead of using our own higher-order functions:

map(str.upper, ['Building', 'Road', 'Tree'])

map(lambda s: s[:1].upper() + s[1:].lower(), ['Building', 'ROAD', 'tree']) # uses lambda expression for only first character as upper-case

map(round, [12.3, 42.8])

Map is actually more powerful than our own functions from above in that it can take multiple lists as input together with a function that has the same number of input parameters as there are lists. It then applies that function to the first elements from all the lists, then to all second elements, and so on. We can use that to, for instance, create a new list with the sums of corresponding elements from two lists as in the following example. The example code also demonstrates how we can use the different Python operators, like the + for addition, with higher-order functions: The operator module from the standard Python library contains function versions of all the different operators that can be used for this purpose. The one for + is available as operator.add(...).

import operator
map(operator.add, [1,3,4], [4,5,6])

Output
[5, 8, 10]

As a last map example, let’s say you instead want to add a fixed number to each number in a single input list. The easiest way would then again be to use a lambda expression:

number = 11
map(lambda n: n + number, [1,3,4,7])

Output
[12, 14, 15, 18]

Lesson content developed by Jan Wallgrun and James O’Brien