Python One Line Sum List

Article summary: Here’s a quick visual overview of the contents of this tutorial.

How to sum over all values in a given Python list?
  • Flat List: To sum over a list of numbers in a single line of Python code, use Python’s built-in function sum(list).
  • Nested List: To sum over a list of lists in one line Python, use a generator expression to flatten the list and pass the result into the function: sum(x for y in list for x in y).

Method 1: Sum over a Flat List in One Line

Problem: How to sum over all values in a given Python list?

Example: Given the following list.

a = [1, 2, 3]

You want to calculate the sum of all values in the list—using only a single line of Python code!


Solution: Python’s built-in sum() function helps you to sum over all values in an iterable, such as a Python list.

Summing up a list of numbers appears everywhere in coding. Fortunately, Python provides the built-in sum() function to sum over all elements in a Python list—or any other iterable for that matter. (Official Docs)

Code: Here’s the minimal code example.

a = [1, 2, 3]

# 6

How does it work? The syntax is sum(iterable, start=0):

iterableSum over all elements in the iterable. This can be a list, a tuple, a set, or any other data structure that allows you to iterate over the elements.
Example: sum([1, 2, 3]) returns 1+2+3=6.
start(Optional.) The default start value is 0. If you define another start value, the sum of all values in the iterable will be added to this start value.
Example: sum([1, 2, 3], 9) returns 9+1+2+3=15.

Exercise: Try to modify the sequence so that the sum is 30 in our interactive Python shell:

Method 2: Sum over a Nested List of Lists in One Line

Problem: Given multiple lists in a list of lists. How can you sum over all values in a list of lists such as [[1, 2], [3, 4], [5, 6]] in Python?

Solution: Use a generator expression to flatten the values in the nested list and pass the resulting iterable into the sum() function.

Code: The following code creates a list of lists:

a = [[1, 2], [3, 4], [5, 6]]

To sum over the values in the list of lists, use the following one-liner:

print(sum(x for y in a for x in y))

The output is printed on the shell:

# OUTPUT: 21

But how does it work? The main part of the code is the generator expression x for y in a for x in y that flattens the list.

  • The part x for y in a for x in y iterates over all elements y in the nested list a.
  • The part x for y in a for x in y iterates over all elements y in the inner list y.
  • The part x for y in a for x in y stores the inner element in the iterable.

Here’s a recap on the technique of list comprehension.

To learn more about different ways to sum() elements in a list, check out my detailed blog tutorial:

Related Tutorial: Python sum() List — Ultimate Guide

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