Python recursion with list returns None [duplicate

问题:

This question already has an answer here:

• Why does my function return None? 4 answers

def foo(a):
    a.append(1)
    if len(a) > 10:
        print a
        return a
    else:
        foo(a)

Why this recursive function returns None (see transcript below)? I can\'t quite understand what I am doing wrong.

In [263]: x = []

In [264]: y = foo(x)
[1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1]

In [265]: print y
None

回答1:

You don\'t return anything in the else clause:

    else:
        return foo(a)