The min and max functions can interact with our classes in relatively simple ways. Similarly, the sort method of a list can also interact with our new class definitions. In all three cases, a keyword parameter of key can be used to control which attributes are used for determining minimum, maximum or sort order.

The key parameter must be given a function, and that function is evaluated on each item that is being compared. Here's a quick example.

class Boat( object ):
    def __init__( self, name, loa ):
        self.name= name
        self.loa= loa

def byName( aBoat ):
    return aBoat.name

def byLOA( aBoat ):
    return aBoat.loa

fleet = [ Boat("KaDiMa", 18 ), Boat( "Emomo", 21 ), Boat("Leprechaun", 30 ) ]

first= min( fleet, key=byName )
print "Alphabetically First:", first
longest= max( fleet, key=byLOA )
print "Longest:", longest

As min, max or sort traverse the sequence doing comparisons among the objects, they evaluate the key function we provided. In this example, the provided function simply selects an attribute. Clearly the functions could do calculations or other operations on the objects.