Python function returns None (all trivial solutions checked and they do not work)

Question

Now, I have written a binary search for Python (version 2.7). Sometimes, it works just fine, but at other times it returns None although the searched value is in the array. I have tried every trivial way of fixing this: I have checked whether the variable the function returns is defined, whether the branch of the workflow in which the return statement is located is executed. And: the variable is defined, the branch is executed.

Here is the code:

def binarySearch( array, desiderata, iMin, iMax ):
    # Returns the index of the first instance of what we search
    print 'min'
    print iMin
    print 'max'
    print iMax
    
    # If our search array is empty
    if ( iMin > iMax ):
        return None
    
    midP = (iMin + iMax)/2
    curre = tapeNr( array[midP][TAPE_NUMBER] )
    final = tapeNr( desiderata )
    print 'curre'
    print curre
    print 'final'
    print final
    print 'midP'
    print midP
    
    if ( curre < final ):
        # print midP
        print "t: " + array[midP][TAPE_NUMBER] + ", d: " + desiderata
        binarySearch( array, desiderata, midP + 1, iMax )
    
    else:
        if ( curre > final ):
            # print midP
            print "t: " + array[midP][TAPE_NUMBER] + ", d: " + desiderata
            binarySearch( array, desiderata, iMin, midP - 1 )
    
        else: 
            print 'hooray'
            # Now, find the first occurence of the value I need
            i = midP
            while ( array[i][TAPE_NUMBER] == desiderata ):
                i -= 1
                print i
            print (i + 1)
            return (i + 1)

There are a lot of 'print' statements because of my debugging. The last of them, 'print (i+1)', actually prints (!) the index value of the thing I need, but the function still returns None.

Do you have a clue about the source of the problem?

score 5 · Accepted Answer · answered Jul 16 '13 at 14:48

You ignore the return value of recursive calls:

binarySearch( array, desiderata, midP + 1, iMax )

and

binarySearch( array, desiderata, iMin, midP - 1 )

So when curre < final is True:

if ( curre < final ):
    # print midP
    print "t: " + array[midP][TAPE_NUMBER] + ", d: " + desiderata
    binarySearch( array, desiderata, midP + 1, iMax )

you call binarySearch() after which your function ends. Without an explicit return that means your function return value is set to None instead.

Add return statements to those lines:

return binarySearch( array, desiderata, midP + 1, iMax )

# ...

return binarySearch( array, desiderata, iMin, midP - 1 )

Python function returns None (all trivial solutions checked and they do not work)

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