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Sum a list of numbers in Python

David Y.

The Problem

In Python, how do I add all the numbers in a list to get a total?

The Solution

Python provides a built-in sum function, which returns the sum of all items in an iterable (such as a list). For example:

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

total = sum(numbers)
print(total) # Will print 15 (1 + 2 + 3 + 4 + 5)

sum is an example of a fold (also called reduce), which is a type of higher-order functions that produces a single result from a sequence of items. It does this by applying a function (in this case, addition) to the first two items (1 + 2), getting a result (3) and then applying the same function to that result and the next item (3 + 4). This process continues until the list is consumed, and then the final result is returned.

sum takes an optional start parameter, which provides a first value to use in the first addition. For example:

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

total = sum(numbers, 6)
print(total) # Will print 21 (6 + 1 + 2 + 3 + 4 + 5)

We can write our own folds using the reduce function from Python’s built-in functools module. For example, here’s how we might find the product of all numbers in a list:

from functools import reduce

numbers = [1, 2, 3, 4, 5]
product = reduce(lambda x, y: x * y, numbers)

print(product)  # Will print 120 (1 * 2 * 3 * 4 * 5)

We supply reduce with two arguments: a function which takes two arguments and a list. It will then apply this function to the contents of the list in the manner described above, ultimately producing a single result. We’ve used an anonymous lambda function here, but reduce will also work with a standard function, as below:

from functools import reduce

def multiply(x, y):
    return x * y

numbers = [1, 2, 3, 4, 5]
product = reduce(multiply, numbers)

print(product)  # Will print 120 (1 * 2 * 3 * 4 * 5)

reduce supports an optional initializer parameter, which works the same as the start parameter in sum.

from functools import reduce

numbers = [1, 2, 3, 4, 5]
product = reduce(lambda x, y: x * y, numbers, 6)

print(product)  # Will print 720 (6 * 1 * 2 * 3 * 4 * 5)

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