Reduce

The last higher-order function we are going to discuss here is reduce(…). In Python 3, it needs to be imported from the module functools. Its purpose is to combine (or “reduce”) all elements from a list into a single value by using an aggregation function taking two parameters that is used to combine the first and the second element, then the result with the third element, and so on until all elements from the list have been incorporated. The standard example for this is to sum up all values from a list of numbers. reduce(…) takes three parameters: (1) the aggregation function, (2) the list, and (3) an accumulator parameter. To understand this third parameter, think about how you would solve the task of summing up the numbers in a list with a for-loop. You would use a temporary variable initialized to zero and then add each number from that list to that variable which in the end would contain the final result. If you instead would want to compute the product of all numbers, you would do the same but initialize that variable to 1 and use multiplication instead of addition. The third parameter of reduce(…) is the value used to initialize this temporary variable. That should make it easy to understand the arguments used in the following two examples:

import operator
from functools import reduce

result = reduce(operator.add, [234,3,3], 0) # sum
print(result)

Output
240

import operator
from functools import reduce

result = reduce(operator.mul, [234,3,3], 1) # product
print(result)

Output
2106

Other things reduce(…) can be used for are computing the minimum or maximum value of a list of numbers or testing whether or not any or all values from a list of booleans are True. We will see some of these use cases in the practice exercises of this lesson. Examples of the higher-order functions discussed in this section will occasionally appear in the examples and walkthrough code of the remaining lessons.

Lesson content developed by Jan Wallgrun and James O’Brien