from typing import List


def prime_factors(number: int) -> List[int]:
    """Calculate the prime factors of a number.

    Calculate the prime factors of a given natural number. Note that 1 is
    not a prime number, so it will not be included.

    Args:
        number: input natural number

    Returns:
        factors: list of prime factors
    """
    factors = []
    i = 2
    while number > 1:
        if number % i == 0:
            number = number // i
            factors.append(i)
            continue
        i += 1

    return factors