SolveWithPython

Challenge 7: Sum Numbers in a List

In this challenge, you’ll write a function that calculates the sum of all numbers in a list. The math is simple—but the real goal is handling edge cases and writing code that behaves predictably.

Your Task

Write a function that accepts a list of numbers and returns their total sum.

Function Signature

def sum_numbers(numbers: list) -> int | float:
…

Rules

  1. The input must be a list.
  2. Every element in the list must be a number (int or float).
  3. If the list is empty, return 0.
  4. If the input is invalid, raise a TypeError.

Examples

sum_numbers([1, 2, 3]) → 6
sum_numbers([1.5, 2.5]) → 4.0
sum_numbers([-1, 5, -2]) → 2
sum_numbers([]) → 0

Invalid Input Examples

sum_numbers("123") → TypeError
sum_numbers([1, "2", 3]) → TypeError
sum_numbers(None) → TypeError

Hints (Optional)

  • Start by validating the input before doing any math.
  • You can use a loop or Python’s built-in tools—but make sure your behavior matches the rules.
  • Think about how you want your function to fail when input is wrong.

Python Challenge 7 Solution Sum Numbers In A List

Python Challenge 7 Test Sum Numbers In A List

What These Tests Cover

✔️ Valid integer and float lists
✔️ Empty list behavior
✔️ Negative numbers
✔️ Single-item lists
✔️ Non-list input validation
✔️ Mixed-type list rejection

This keeps the challenge:

  • Predictable
  • Defensive
  • Beginner-friendly but correct

What This Challenge Teaches

  • Iterating over lists
  • Accumulators (total += n)
  • Input validation
  • Defensive programming
  • Writing predictable return values

Bonus Challenges

  • Ignore non-numeric values instead of raising an error
  • Support nested lists
  • Compare your solution with Python’s built-in sum()

Why This Matters

Real-world data is rarely clean. Writing functions that fail clearly or handle edge cases gracefully is just as important as getting the correct result.

Encouring again you to take some extra time to check the test code. I’ll be using this more in the coming challenges.

🔗 View reference solution on GitHub
(After you’ve tried the challenge)

👉 Next Challenge → Remove Duplicates from a List

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