How to Calculate the Sum of List Elements in Python

Iterates over a list with a for loop, accumulates each number into a total variable, and returns the sum of all elements.

Easy Python 3.9+ Aug 9, 2026 Lists & loops 14 views 0 copies

sum list loop iteration

Python code

11 lines
Python 3.9+
Run it 
def sum_list_elements(numbers):
    """Return the sum of all elements in a list."""
    total = 0
    for num in numbers:
        total += num
    return total

if __name__ == "__main__":
    sample_list = [1, 2, 3, 4, 5]
    result = sum_list_elements(sample_list)
    print(f"The sum of {sample_list} is {result}")

Output

stdout 
The sum of [1, 2, 3, 4, 5] is 15

How it works

The function initializes total to 0 and then loops through each num in the list, adding it to total with += (shorthand for total = total + num). This works for any iterable, not just lists, and handles numeric types like integers and floats. After the loop, the accumulated value is returned. The if __name__ == "__main__" guard ensures the demo only runs when the script is executed directly, not when imported.

Common mistakes

  • Forgetting to initialize `total` to 0 before the loop, causing a NameError.
  • Using `total + num` without reassigning it, so the sum never updates.
  • Assuming non-numeric elements will work; strings cause a TypeError.
  • Not returning the total but printing inside the function, reducing reusability.

Variations

  1. Use the built-in `sum()` function: `sum(numbers)` for a concise one-liner.
  2. Use `functools.reduce` with `operator.add` for a functional style.

Real-world use cases

  • Calculating the total revenue from a list of order amounts in an e-commerce application.
  • Summing user scores from a leaderboard list to compute average performance.
  • Aggregating sensor readings to get total energy consumption over time.

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