How to return a value from a recursive Python function?

问题

This question seems a bit specific, and for that I'm sorry, but it has me stumped. I'm writing myself a password generator, one that takes a string (aka the URL of a website) and processes it into a secure password that can't be backtracked based on the name of the website.

In part of the code, I created a recursive function that looks like this:

def get_number(n = 0, nums = ''):
    for i in range(0, len(url)):
        #both n and nums are changed
    if len(nums) < num_length:
        get_number(n, nums)
    else:
        print(nums)
        return(nums)

...

print(get_number())

I would expect 'nums' to output twice, since I print it in the else block and print the return later on. But, if it does go through a recursive loop, 'nums' is printed from the else block and the function returns 'None'. If if len(nums) < num_length is false the first time, then it returns the proper value.

Why would it return 'None', if I verified that the object it is returning is not in fact 'None' the line before?

I'm a little new to Python, do they handle recursions differently?

Thank you for your time

Edit:

Problem fixed. Forgot a return statement on the recursive call. Thanks :D

回答1:

I think you're missing a return before the nested get_number. So, it's executing and returning, but you aren't doing anything with the recursed value.

def get_number(n = 0, nums = ''):
    for i in range(0, len(url)):
        #both n and nums are changed
    if len(nums) < num_length:
        return get_number(n, nums)

    print(nums)
    return nums

来源：https://stackoverflow.com/questions/38882826/how-to-return-a-value-from-a-recursive-python-function

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recursion

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