Python sum() Function

Spread the love

The sum() function in Python is a built-in function that adds all the items in an iterable, such as a list, tuple, or set, and returns the total. It is an example of a “reduce” function, which takes an iterable and reduces it to a single cumulative value.


Here’s the basic syntax of the sum() function:

sum(iterable, start)


  • iterable – This is the mandatory parameter, which can be anything that you can iterate over (lists, tuples, dictionaries, sets, etc.). The elements must be numbers, either integers or floats.
  • start – This is an optional parameter. It is the value that is added to the sum of the items of the iterable. The default value is 0. If the start is provided, it effectively serves as the initial value of the sum before adding the elements of the iterable.

Return Value:

sum( ) returns the sum of start and items of the given iterable.

Python Sum Function on a List

numbers = [1, 2, 3, 4, 5]
total = sum(numbers)
print(total)  # Output will be 15

In this example, sum() adds all the numbers in the list [1, 2, 3, 4, 5], resulting in a total of 15.

Using start Parameter

numbers = [1, 2, 3, 4, 5]
total = sum(numbers, 10)
print(total)  # Output will be 25

Here, we have a start value of 10, which is added to the sum of the list items, resulting in 25.

Summing a List of Floats

float_numbers = [1.5, 2.5, 3.5, 4.5, 5.5]
total = sum(float_numbers)
print(total)  # Output will be 17.5

sum() is not limited to integers; it can also be used with floats.

Python Sum Function on a Dictionary

In Python, the sum() function can also be used with dictionaries, but it requires a bit of understanding of how dictionaries work. Dictionaries in Python are a collection of key-value pairs, and when you iterate over a dictionary, you are iterating over its keys by default.

Summing Dictionary Values

To sum all the values in a dictionary, you must explicitly tell the sum() function to iterate over the dictionary’s values. Here’s how you do it:

my_dict = {'a': 1, 'b': 2, 'c': 3}
total = sum(my_dict.values())
print(total)  # Output will be 6

In the above example, my_dict.values() returns an iterable view of the dictionary’s values, and sum() adds them up.

Summing Dictionary Keys

If you try to sum the keys of a dictionary directly without specifying that you want to sum the values, you will get an error if the keys are not numeric. If the keys are numeric, then it will sum the keys, not the values.

my_dict = {1: 'apple', 2: 'banana', 3: 'cherry'}
total_keys = sum(my_dict)
print(total_keys)  # Output will be 6

Here, the keys are integers, so sum(my_dict) will return the sum of the keys.

However, if the keys are strings, as in the first example, trying sum(my_dict) would result in a TypeError because strings cannot be added up in the context of sum().

Summing with a Start Value

Just like with lists, you can provide a start value to the sum() function when summing dictionary values:

my_dict = {'a': 1, 'b': 2, 'c': 3}
total = sum(my_dict.values(), 10)
print(total)  # Output will be 16

In this example, the sum of the dictionary values plus the start value (10) is calculated, resulting in 16.

Important Considerations

  • The sum() function only works with iterables containing numeric data types (integers or floats).
  • When using sum() on a dictionary, it will operate on the keys by default. To sum the values, you must use .values().
  • You cannot use sum() to concatenate dictionary keys or values if they are strings; use a different method like a for-loop or the join() method for strings.

Practical Example with sum( )

Suppose you have a dictionary with product names as keys and their prices as values, and you want to calculate the total price of all products:

products = {'apple': 0.40, 'banana': 0.50, 'cherry': 0.90}
total_price = sum(products.values())
print(f"Total price: ${total_price:.2f}")

This will output the total price of all products in a nice formatted string:

Total price: $1.80

Here, sum(products.values()) sums up the prices of all the items in the products dictionary, and the formatted string prints the total with two decimal places.

Python Sum Function on a Set

The sum() function in Python can be used on any iterable, and that includes sets. A set in Python is a collection of unordered items with no duplicates. Here’s how you can apply the sum() function to a set:

Basic Usage on a Set

my_set = {1, 2, 3, 4, 5}
total = sum(my_set)
print(total)  # Output will be 15

In the example above, sum() adds all the numbers in the set {1, 2, 3, 4, 5}, resulting in a total of 15. Since sets cannot have duplicate elements, each number is only counted once in the sum.

Considerations When Using sum( ) with Sets

  • Sets are unordered, so while you can sum the elements in a set, you cannot rely on the order in which they will be processed.
  • Sets cannot contain mutable items, so all elements in a set must be immutable (and therefore hashable), like numbers, strings, and tuples.
  • The sum() function requires that all elements in the iterable (in this case, the set) are numeric. So, while sets can contain various hashable data types, if you attempt to use sum() on a set with non-numeric types, you will get a TypeError.

Summing with a Start Value

You can also provide an optional start value to sum() when working with sets:

my_set = {1, 2, 3, 4, 5}
total = sum(my_set, 10)
print(total)  # Output will be 25

Here, the start value of 10 is added to the sum of the set items, resulting in a total of 25.

Unique Characteristic of Sets in Summation

A unique feature of using sum() with sets is that you don’t need to worry about duplicate values affecting the sum. For example, if you have a list with duplicates and convert it to a set before summing, the duplicates will be removed:\

my_list = [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]
my_set = set(my_list)
total = sum(my_set)
print(total)  # Output will be 10

Even though the original list had multiple duplicates of the numbers 2, 3, and 4, converting it to a set removes these duplicates, and the sum is the same as that of {1, 2, 3, 4}.

Practical Example

Imagine you have a set of measurements, and you want to calculate their cumulative value:

measurements = {5.5, 3.75, 2.25, 4.0, 3.0}
total_measurement = sum(measurements)
print(f"Total measurement: {total_measurement}")

The sum() function will correctly add up these floating-point numbers, providing the total measurement.

Python Sum Function on a Tuple

The sum() function in Python can be applied to tuples just as it is used with other iterable types. A tuple is an ordered collection of items which can be of mixed data types. However, when using sum(), all elements within the tuple must be numeric.

Basic Usage on a Tuple

Here’s how you can use sum() with a tuple:

my_tuple = (1, 2, 3, 4, 5)
total = sum(my_tuple)
print(total)  # Output will be 15

In this example, sum() adds all the numbers in the tuple (1, 2, 3, 4, 5), resulting in a total of 15.

Summing with a Start Value

You can also use the optional start parameter with tuples:

my_tuple = (1, 2, 3, 4, 5)
total = sum(my_tuple, 10)
print(total)  # Output will be 25

Here, the sum of the tuple’s items plus the start value of 10 gives a result of 25.

Constraints and Considerations

  • As with other iterables, using sum() with a tuple requires that all elements be numeric (integers, floats, etc.).
  • Tuples can contain mixed types, but if you have non-numeric types in a tuple and try to use sum(), Python will raise a TypeError.
  • Tuples are immutable, which means that once a tuple is created, you cannot change its contents. However, this does not affect the sum() operation, which does not modify the tuple.

Why Use a Tuple with sum( )

Tuples, being immutable, can provide a performance benefit when you have a fixed collection of numeric items that you want to process. Since Python can assume that the contents will not change, it can optimize the access to tuple elements.

Practical Example with sum( )

Suppose you have a tuple representing distances (in miles) covered each day of a week, and you want to find the total distance covered in the week:

weekly_distances = (3.2, 5.1, 2.5, 0, 5.5, 4.3, 3.6)
total_distance = sum(weekly_distances)
print(f"Total distance covered in the week: {total_distance} miles")

The sum() function will add up these numbers, giving the total distance covered in the week.

Using sum( ) with Generators

Using the sum() function with generator expressions in Python is a powerful combination that enables efficient summation of large sequences of numbers without the need to store the entire sequence in memory. A generator expression is a high-performance, memory–efficient generalization of list comprehensions and set comprehensions.

Generators and Generator Expressions

Generators are iterators that yield items instead of returning a list. They generate values on the fly and do not store them in memory. When you use a generator expression, Python evaluates each item in the sequence as needed, rather than all at once.

For example, (x*x for x in range(10)) is a generator expression that generates the squares of numbers from 0 to 9, one at a time, as you iterate over it.

Using sum( ) with a Generator Expression

You can pass a generator expression directly to the sum() function:

# Sum of squares from 0 to 9
total = sum(x*x for x in range(10))
print(total)  # Output will be 285

In this code, the generator expression x*x for x in range(10) generates the squares of the numbers from 0 to 9, and sum() adds them up as they are generated. The result is 285.

Advantages of Using Generators with sum( )

  1. Memory Efficiency: Since the values are generated on the fly and not stored in memory, generator expressions are much more memory-efficient than using a list or tuple, especially for large sequences.
  2. Performance: Generator expressions can be faster because they do not require the entire list to be created in memory before the summation starts.
  3. Simplicity: It provides a concise syntax for operations that would otherwise require more complex loops or list comprehensions.

Practical Example

Imagine you want to calculate the sum of the inverse of the squares of the first one million numbers. Using a list comprehension to create the list of numbers would consume a lot of memory. Instead, you can use a generator expression with sum():

# Sum of the inverse of the squares of numbers from 1 to 1,000,000
total_inverse_squares = sum(1/(x*x) for x in range(1, 1000001))

This code calculates the sum without ever storing the one million inverse squares in memory.


sum() is implemented in C and is highly optimized. It is much faster than manually summing up numbers with a loop in Python code.


In certain cases, you might want to use math.fsum() instead of sum() if you need better precision when adding floats, as it helps mitigate loss of precision during addition.


The sum() function is a convenient tool in Python that provides a quick way to add up all elements in an iterable. It is versatile, efficient, and can be combined with other Python features such as generator expressions to perform complex summations concisely and efficiently. When using sum(), it’s essential to remember that it only works with numeric data types and that the optional start parameter can be used to provide a custom initial value for the summation.

Leave a Reply