1.4.4 Multiple return values

It happens quite often that you want to return multiple things as the result of a function, but a function can only have one return value. To work with the single return value, we simply return a tuple, list, or dictionary with the different components you want to return. For instance, returning four coordinates describing the bounding box of a polygon, we can return a tuple with the four coordinates. Python has a useful mechanism to help with unpacking this single return value by allowing us to assign the elements of a tuple (or other sequences like lists) to several variables in a single assignment. Given a tuple (12, 3, 2, 2) assigned to t, instead of writing

t = (12, 3, 2, 2)

top = t[0] 

left = t[1] 

bottom = t[2] 

right = t[3] 

you can write

top, left, bottom, right = t 

and it will have the exact same effect. The following example illustrates how this can be used with a function that returns a tuple of multiple return values. For simplicity, the function computeBoundingBox() in this example only returns a fixed tuple rather than computing the actual tuple values from a polygon given as input parameter.

def computeBoundingBox(): 
     return (12,3,41,32) 


top, left, bottom, right = computeBoundingBox() # assigns the four elements of the returned tuple to individual variables

print(top)    # output: 12

Or as a dictionary, which can also provide some insights into calculated results:

def computeBoundingBox(x): 
     if x==1:
     	return {'top':12, 'left':3,' bottom':41, 'right':32, 'success':True} 
     else:
        return {'top':12, 'left':3, 'bottom':41, 'right':32, 'success':False} 


bbox = computeBoundingBox(1) # returns the result as a dictionary

if bbox.get('success'):  # if the process succeeds
    print(bbox['top'])    # output: 12