reduce()

Reduce is a really useful function for performing some computation on a list and returning the result. It applies a rolling computation to sequential pairs of values in a list.

reduce(function_to_apply, list_of_inputs)

For example, if we want to sum all the numbers in a list, we can do it like this:

from functools import reduce
 
my_list = [1, 2, 3, 4, 5]
result = reduce(lambda x, y: x + y, my_list)
print(result) # 15

NOTE 1: reduce() is not a built-in function in Python 3, so we need to import it from the functools module.

The above is equivalent to the following:

my_list = [1, 2, 3, 4, 5]
result = 0
for x in my_list:
    result += x
print(result) # 15

NOTE 2: reduce() can also take an optional third argument which is the initial value of the computation. For example:

 from functools import reduce
 
 my_list = [1, 2, 3, 4, 5]
 result = reduce(lambda x, y: x + y, my_list, 10)
 print(result) # 25