Let's say that I have a list of objects of mixed type, and I only care that they implement a given method. With duck typing, the types of the objects need not be coupled via a common base class (or even interface). For example:

  # Less coupled
  class A:
     def foo(self):
       print "A"
  class B:
     def foo(self):
       print "B"

  for x in (A(), B()):
    x.foo()

vs:

  # More coupled
  class Base:
     def foo(self):
       pass
  class A(Base):
     def foo(self):
       print "A"
  class B(Base):
     def foo(self):
       print "B"

  for x in (A(), B()):
    x.foo()