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]

quoted from ch6ex1 of Functional Programming in Python Edn 2 by Steven Lott (Pakt Publisher)
Line 109 (shown here at line 9) had None and was replaced with 0 in gist (line 10).
Interestingly PyCharm plugin or IDE points to the typing problems highlighting the location of issue.

Discussing with author was useful to me in that I get a better sense of how typing usage becomes part of required work for Python development (I come from Scala background so using types is something I value naturally and find hard to do well).

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.