Sum and average of a list in Python

The average (mean) of a list is its total divided by the number of items. Python has sum() for the total and len() for the count.

The program

marks = [72, 85, 90, 64, 88]

total = sum(marks)
average = total / len(marks)

print("Marks:", marks)
print("Sum:", total)
print("Average:", average)
print(f"Average to 1 decimal place: {average:.1f}")

Marks: [72, 85, 90, 64, 88]
Sum: 399
Average: 79.8
Average to 1 decimal place: 79.8

How it works

• sum(marks) adds every number in the list. len(marks) counts the items, here 5.
• / always gives a float, so the average is 79.8, not a rounded whole number.
• :.1f inside the f-string prints the number with exactly one digit after the decimal point. It changes only how the number is shown, not the value stored in average.

An empty list

If the list is empty, len() is 0 and the division raises a ZeroDivisionError. A small function can check for that first:

def average(numbers):
    if not numbers:
        return None
    return sum(numbers) / len(numbers)

print(average([10, 20, 30, 40]))
print(average([]))

25.0
None

if not numbers: is true for an empty list. The standard library also has statistics.mean(), which raises a StatisticsError with a clear message for an empty list.