##
## Exercise:
##
## Define a function that takes a list of numbers and returns the
## smallest one. If the list is empty, the function should return
## None.
##

def find_min (l):

    if l == []:
        return None

    else:
        smallest = l[0]

        for element in l:
            if element < smallest:
                smallest = element
            
    return smallest

## To test:
print "[]", find_min([])
print "[1, 2, 3, 4, 5]", find_min([1, 2, 3, 4, 5])
print "[10, 12, 31, 4, 50]", find_min([10, 12, 31, 4, 50])