sum() function in Python (original) (raw)

Last Updated : 27 Nov, 2025

The sum() function in Python is used to add up numbers from any iterable such as a list, tuple, set, or dictionary values. It provides a clean and efficient way to calculate totals without writing loops manually.

**Example:

**Input: [1, 5, 2]
**Output: 8

Syntax

sum(iterable, start)

**Parameters:

• **iterable: A sequence containing numbers (list, tuple, set, dictionary values).
• **start(optional): A value added to the final sum. Default is 0.

Examples

**Example 1: This example shows how to use sum() on a list to calculate the total of its elements.

Python `

nums = [5, 10, 15] res = sum(nums) print(res)

`

**Example 2: This example demonstrates using the optional start parameter to add a base value to the final sum.

Python `

nums = [4, 6, 10] res = sum(nums, 5) print(res)

`

**Example 3: This example shows how the sum() function works with different iterable types like dictionary values, sets, and tuples.

Python `

dictionary

d = {'a': 10, 'b': 20, 'c': 30} print(sum(d.values()))

set

s = {1, 2, 3, 4, 5} print(sum(s))

tuple

t = (1, 2, 3, 4, 5) print(sum(t))

`

**Explanation:

• sum(d.values()): accesses dictionary values (10, 20, 30) and adds them.
• sum(s): totals the set elements 1+2+3+4+5.
• sum(t): totals the tuple elements 1+2+3+4+5.

Summing Values Using For Loop

A for loop can be used to add elements of a list one by one. This method shows how summation works step-by-step and is useful when you need full control over how each value is processed during the calculation.

Python `

nums = [10, 20, 30, 40, 50] res = 0

for n in nums: res += n

print(res)

`

Handling Errors When Using sum()

The sum() function requires numeric values. If the iterable contains incompatible types such as strings, Python raises a TypeError. This happens because addition cannot be performed between an integer and a non-numeric value.

Python `

arr = ["a"]

print(sum(arr)) print(sum(arr, 10))

`

Output

ERROR!
Traceback (most recent call last):
File "<main.py>", line 3, in
TypeError: unsupported operand type(s) for +: 'int' and 'str'

**Explanation:

• sum(arr) attempts to add "a" to 0, which is invalid.
• sum(arr, 10) attempts to add "a" to 10, causing the same error.

Calculating an Average

A common use of sum() is computing averages. We first calculate the total using sum() and then divide it by the number of elements.

Python `

nums = [1, 2, 3, 4, 5, 1, 4, 5] avg = sum(nums) / len(nums) print(avg)

`