type-hinting-edge-case ch6ex1 FunctionalProgrammingInPythonV2

ch6ex1excerpt.py

def fibi2(n: int) -> int:
    """Fibonacci numbers with iteration and memoization

    >>> fibi2(20)
    6765
    >>> fibi2(1)
    1
    """
    # originally, line 109 had None (replace with Int as 0) f = [0, 1] + [None for _ in range(2, n+1)]
    f = [0, 1] + [0 for _ in range(2, n+1)]
    for i in range(2, n+1):
        f[i] = f[i-1]+f[i-2]
    return f[n]

Interesting that PyCharm does more strict checking than mypy does.

An alternative is this:

f: List[Optional[int]] = [0, 1] + [None for _ in range(2, n+1)]

Which states that you'll have a mixture of int and None in the list.

This has the tiny advantage of not polluting the list with invalid numeric values. It uses None for not-yet-computed.