Vassilios Karakoidas Weblog

… and his victory in the battle of agincourt.

Opposing Armies Formation

I always liked python. It is a rather gentle language with many functional and Object-Oriented characteristics. I was always complaining about python’s Object-Oriented support, and i have a principle as a developer: “If you cannot implement something, do not offer it as a feature”.

Python violates this principle, because it allows developers to use multiple inheritance on a class but does not implement it correctly. Consider the following example:

class ParentOne:
	def __init__(self):
		print "Parent One says: Hello my child!"
		self.i = 1
 
	def methodOne(self):
		print self.i
 
class ParentTwo:
	def __init__(self):
		print "Parent Two says: Hello my child"
 
class Child(ParentOne, ParentTwo):
	def __init__(self):
		print "Child Says: hello"

We have three classes, ParentOne, ParentTwo, and Child. The class Child inherits ParentOne and ParentTwo.

Normally each parent class should be initialised when the child class is initialised through a specified initialisation scheme. Python just do not initialise the parent classes. For example, if we create an instance of the class Child

c = Child()

… we will see in our screen the following output:

bkarak$ python example.py
Child Says: hello

As we can see the __init__ method (aka the constructor) for each parent class is never executed. OOppss.

So, if the ParentOne init method declares a field named “i” (instance field) and the child tries to execute the method, we will end up with the following result:

bkarak$ python example.py
Child Says: hello
Traceback (most recent call last):
  File "example.py", line 20, in 
    c.methodOne()
  File "example.py", line 9, in methodOne
    print self.i
AttributeError: Child instance has no attribute 'i'

Ouch. This fact narrows down the usability of multiple inheritance to simple concatenation of methods with static methods. Really nicely done.

My advice is: “Avoid it, because you are not going to enjoy it”.

I do not really know if something is better implemented in Python 3.0. I will check it and write about it in the distant future.