A good example of this would be the file like objects used in Python. In various modules, you can use file like objects for different input/ouput operations.

For instance, take the pickle module: http://docs.python.org/library/pickle.html . It provides object de/serialization. It requires a file object for loading or dumping object. If you were to use an old tape component where there is no built in mechanism to access it, you could create a Tapefile class simply by specifying the few methods required by pickle. It could look like this:

class Tapefile():
“A tape file”
def __init__(self,params):
#various initialization for tape access
pass
def read(len):
#use special methods to access tape
pass
def readline():
#use special methods to read a whole line on tape
pass
def write(value):
#use special methods to write on tape
pass

You could than pass a Tapefile object to the pickle module and it would simply work. The pickle module do not care if it has a file object or another object pretending to act like a file. As long as it has the right methods, it will work.