Inheritance

In (Object Oriented) programming terms, "Inheritance" is used to define a type of data (an "object") which behaves like another type of data with some additions and differences.

For example, we can define a type of data that's a piece of clothing. We can then define a pair of shoes which is like a piece of clothing, except that it has two extra pieces of information involved - a length and a width. Languages that are known as Object Oriented languages provide facilities to make such definitions easy without repeating code.

In the following piece of code (in Python), we define a piece of clothing (it's known as a base class) and then we extend it into a new class (called a subclass) which has the same code and properties as a piece of clothing, with the exception of changes that we list.

The tshirt and tights classes are also subclasses of clothing and we can make up a list of pieces of clothing (you'll see it done in the test code at the bottom) which we can then loop through reporting colour and size. As we loop through in this way, the getcolour method is INHERITED from the clothes base class in each case (i.e. it is common code) and the getsize method uses what we call POLYMORPHISM as it uses different code to return the size for different types of clothing.

class clothing:
  def __init__(self,colour,weight):
    self.colour=colour
    self.weight=weight
  def getcolour(self):
    return self.colour
  def getsize(self):
    return "n/a"
class shoes(clothing):
  def __init__(self,colour,weight,len,wid):
    clothing.__init__(self,colour,weight)
    self.length = len
    self.width = wid
  def getsize(self):
    r = "Length "+str(self.length)+" and width "+self.width
    return r
class tshirt(clothing):
  def __init__(self,colour,weight,size):
    clothing.__init__(self,colour,weight)
    self.size = size
  def getsize(self):
    return self.size
class tights(clothing):
  pass

if __name__ == "__main__":
  wearing = []
  wearing.append(tshirt("blue",1.0,"XL"))
  wearing.append(tights("pink",0.1))
  wearing.append(shoes("brown",2.0,11,"D"))
  for haveon in wearing:
    print "I am wearing something",haveon.getcolour(),
    print " size ",haveon.getsize()

Let's run that ...

[localhost:~/ipy] graham% python dresser
I am wearing something blue size XL
I am wearing something pink size n/a
I am wearing something brown size Length 11 and width D
[localhost:~/ipy] graham%

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