Function returns None after recursion

Question

I have written a function to determine the height of a display, given a width and a format. The function operates recursively, if it cannot find a match for the given width and

Answer 1

You are not actually returning the recursive call result:

elif height == width:
    getDisplayDimensions(float(width)-1,FormatX,FormatY)

Add return there:

elif height == width:
    return getDisplayDimensions(float(width)-1,FormatX,FormatY)

Without the return the outer call just ends and returns the default None instead.

Demo:

>>> def getDisplayDimensions(width,FormatX,FormatY):
...     Format = float(FormatX)/FormatY
...     if FormatX < FormatY:
...         return "illegal format."
...     for height in range(1,int(width)+1):
...         if float(width)/height == float(Format):
...             return width,height
...             break
...         elif height == width:
...             return getDisplayDimensions(float(width)-1,FormatX,FormatY)
... 
>>> print getDisplayDimensions(801,16,9)
(800.0, 450)