from typing import List


def mean(nums: List) -> float:
    """
    Find mean of a list of numbers.
    Wiki: https://en.wikipedia.org/wiki/Mean

    >>> mean([3, 6, 9, 12, 15, 18, 21])
    12.0
    >>> mean([5, 10, 15, 20, 25, 30, 35])
    20.0
    >>> mean([1, 2, 3, 4, 5, 6, 7, 8])
    4.5
    >>> mean([])
    Traceback (most recent call last):
        ...
    ValueError: List is empty
    """
    if not nums:
        raise ValueError("List is empty")
    return sum(nums) / len(nums)


if __name__ == "__main__":
    import doctest

    doctest.testmod()