Can I inherit member variables?

[Gabriel Genellina]

At Thursday 21/9/2006 06:52, lm401 at cam.ac.uk wrote:

>class animal:
>   def __init__(self, weight, colour):
>     self.weight = weight
>     self.colour = colour
>
>
>class bird(animal):
>   def __init__(self, wingspan):
>     self.wingspan = wingspan
>     print self.weight, self.colour, self.wingspan
>
>class fish(animal):
>   def __init__(self, length):
>     self.length = length
>     print self.weight, self.colour, self.length
>
>
>So basically I have a base class (animal) that has certain attributes.
>When animal is inherited by a specific instance, further attributes are
>added, but I need to have access to the original attributes (weight,
>colour). When I run my code from within the derived class, self.weight
>and self.colour are not  inherited (although methods are inherited as I
>would have expected).

You have to call the base __init__ too. If a bird is some kind of 
animal, it has a weight and a colour, and you have to provide them too:

class bird(animal):
   def __init__(self, weight, colour, wingspan):
     animal.__init__(self, weight, colour)
     self.wingspan = wingspan
     print self.weight, self.colour, self.wingspan

>It seems from reading the newsgroups that a solution might be to
>declare weight and colour as global variables in the class animal:

You can declare them as class attributes inside animal; this way they 
act like a default value for instance attributes.

class animal:
   weight = 0
   colour = ''
   ...

>I'm not very experienced with OOP techniques, so perhaps what I'm
>trying to do is not sensible. Does Python differ with regard to
>inheritance of member variables from C++ and Java?

They are not called "member variables" but "instance attributes". 
They *are* inherited [1] but you have to set their value somewhere. 
Any object can hold virtually any attribute - this is *not* usually 
determined by the object's class.
Base constructors ("initializer" actually) are *not* invoked 
automatically, so you must call them explicitely.

Read the Python Tutorial, it's easy and will teach you a lot of 
things about the language. You can read it online at 
<http://docs.python.org/tut/tut.html>

[1] kinda... remember that classes don't determine the available 
attributes; class attributes are inherited, and any attribute you set 
on an instance will be accessible by any method of the object, even 
above in the hierarchy.



Gabriel Genellina
Softlab SRL 


	
	
		
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