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

**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!

# RESULT: 6

**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] print(sum(a))

# 6

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

:

Argument | Description |
---|---|

`iterable` |
Sum 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`

iterates over all elements**for y in a**for x in y`y`

in the nested list`a`

. - The part
`x for y in a`

iterates over all elements**for x in y**`y`

in the inner list`y`

. - The part

stores the inner element in the iterable.**x**for y in a for x in y

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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