Python 是否有像 Mathematica 一样的“巢”function？

Question

Mathematica has a convenient Nest function for repeatedly evaluating f(f(f(f(...f(n))))) . Not something you need every day, but occasionally useful. Here is a trivial implementation:

def nest(f, expr, n):
    assert n >= 0
    if n == 0:
        return expr
    else:
        return f(nest(f, expr, n - 1))

>>> nest(lambda x: (1 + x) ** 2, 1, 3)
676

Is there a Pythonic way to do this?

Answer 1

Perhaps if you like these sorts of things, you can look into functools.reduce :

from functools import reduce

def nest(f, expr, n):
    return reduce(lambda x, _: f(x), range(n), expr)

>>> nest(lambda x: (1 + x) ** 2, 1, 3)
676

Answer 2

Talking Pythonic, which is a bit of an opinion-based concept, I'd say a simple iterative implementation is more readable ( Readability being a core Python principle ) than reduce -based or recursive approaches:

def nest(f, expr, n):
    for _ in range(n):
        expr = f(expr)
    return expr

Certainly, I will not have to stare down what's happening here as much as in the other cases. If it is, however, brevity you're after, you can go with a conditional expression:

def nest(f, expr, n):
    return f(nest(f, expr, n - 1)) if n else expr