Python recursion returns None

Question

This will be really funny... Given following python codes:

def getBinary(binaryInput, kSize, beginBit):
    if int(binaryInput[beginBit + kSize-1])==1:
        print 'entered!!!'
        shortE = binaryInput[beginBit:kSize+beginBit]
        print 'shortE is now: ', shortE
        print 'kSize is now: ', kSize
        return (shortE,kSize)
    else :
        print 'else entered...'
        kSize -=1
        getBinary(binaryInput, kSize, beginBit)

result = getBinary("{0:b}".format(6), 3, 0)
print result

The output is:

else entered...
entered!!!
shortE is now:  11
kSize is now:  2
None

I mean since shortE is 11 and kSize is 2, why the return value is None?

possible duplicate of [Python recursion with list returns None](http://stackoverflow.com/questions/2599149/python-recursion-with-list-returns-none) — Peter Wood, Feb 18 '15 at 08:28
@Liquid Because it is helpful to everybody. Including, and especially, you. — David Heffernan, Feb 18 '15 at 19:54
@Liquid Because I saw someone ask exactly the same question the other day, and it had been asked 5 years ago. This place is great as a resource for others because of the density of good answers to specific questions. You're watering it down; I'm trying to thicken it. — Peter Wood, Feb 18 '15 at 21:19
Does this answer your question? [Why does my recursive function return None?](https://stackoverflow.com/questions/17778372/why-does-my-recursive-function-return-none) — VLAZ, Nov 03 '22 at 18:02

score 2 · Accepted Answer · answered Feb 18 '15 at 07:56

2

When a function ends without executing a return statement, it returns None. Instead of

getBinary(binaryInput, kSize, beginBit)

you mean

return getBinary(binaryInput, kSize, beginBit)

answered Feb 18 '15 at 07:56

David Heffernan

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But why it does not executing `return`? From the console output, I can tell that the in the second time, the function go into the `if` statement and calculate the value of `shortE` and `keySize`, and then it should go to the `return` statement and return the according value, right? – chrisTina Feb 18 '15 at 19:50
The else is entered on the outer most call, and that's the one that returns and assigns to `result`. – David Heffernan Feb 18 '15 at 19:52

score 1 · Answer 2 · answered Feb 18 '15 at 07:55

The code is missing in the else part:

def getBinary(binaryInput, kSize, beginBit):
    if int(binaryInput[beginBit + kSize-1])==1:
        print 'entered!!!'
        shortE = binaryInput[beginBit:kSize+beginBit]
        print 'shortE is now: ', shortE
        print 'kSize is now: ', kSize
        return (shortE,kSize)
    else :
        print 'else entered...'
        kSize -=1
        return getBinary(binaryInput, kSize, beginBit)
        # ^^^^

Python recursion returns None

2 Answers2

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