Sum Function In Python

In this tutorial, I will teach you about the built-in sum function in python. So let’s start by discussing its definition.

Sum Function Definition :

Python built-in sum() function takes an iterable as an argument and returns the sum of all elements inside it. it also accepts a ‘start’ argument and adds it to the sum total.

Syntax :

sum(iterable,start = 0)

Sum Function In Python Examples :

Now let’s look at some examples :

Apply Sum To List

In the following code example, we will create a list of integers and apply sum function to it.

# Apply sum to list 
nums = [1,2,3,4,6,7]


Save and run the above code you should see the following output.

sum function in python code example 1 output

One point to note is that iterable of sum function should only contain integers otherwise it will throw an error. Let’s look at the example.

print(sum(['a',2,3,4,5,6])) # It will Throw Error
print(sum(['1','2','3'])) # It will Also Throw Error

Apply Sum To Tuple

Let’s apply sum function in python to the builtin tuple.

nums = (1,2,3,4,5)


The behavior of sum is the same to both tuple and list.

Output :


Apply Sum To Dictionary

The behavior of the sum function is slightly different to the dictionary data structure. In the case of a dictionary, it returns the sum of all keys. Let’s look at the example.

d = {1:"w",


Safe and run the above code. you should see all keys of the dictionary summed up and printed on the console.


Apply Sum To Strings

Unfortunately, We can’t apply sum to the strings. Instead, we can sum the strings using the join method. here is the example.

# sum(['a','b']) : Error

#Instead use join

Output :


‘Start’ Argument In Sum

As discussed in the Introduction, the sum() function also accepts a ‘start’ keyword argument with a default value of 0. It simply adds that the start value to the sum total. Let’s look at an example.

nums = [1,2]

# Without Start

# with start 
print(sum(nums,start = 10))

Output :


Explanation :

  • In the first case (without start argument) it simply sums the list i.e 1+2 = 3 and prints on the console.
  • In second case (with start = 10). along with a total of 3., it also adds the value of start argument. i.e 3 + 10 = 13

That’s wrap of the tutorial!

Also read about map function in python

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