Here's my opinion, having written many thousands of lines of mypy code.

TypedDict is a dictionary whose keys are always string, and values are of the specified type. At runtime, it behaves exactly like a normal dictionary.

you should only use reveal_type to debug your code, and remove it when you’re done debugging.

What this says is “function double takes an argument n which is an int, and the function returns an int.

This tells mypy that nums should be a list of integers (List[int]), and that average returns a float.

from typing import List

def average(nums: List[int]) -> float:

for starters, use mypy --strict filename.py