GNU Info

(python2.1-tut.info)Inheritance

---

Next: Private Variables Prev: Random Remarks Up: Classes

---

Enter node , (file) or (file)node

Inheritance
===========

Of course, a language feature would not be worthy of the name "class"
without supporting inheritance.  The syntax for a derived class
definition looks as follows:

     class DerivedClassName(BaseClassName):
         <statement-1>
         .
         .
         .
         <statement-N>

The name `BaseClassName' must be defined in a scope containing the
derived class definition.  Instead of a base class name, an expression
is also allowed.  This is useful when the base class is defined in
another module, e.g.,

     class DerivedClassName(modname.BaseClassName):

Execution of a derived class definition proceeds the same as for a base
class.  When the class object is constructed, the base class is
remembered.  This is used for resolving attribute references: if a
requested attribute is not found in the class, it is searched in the
base class.  This rule is applied recursively if the base class itself
is derived from some other class.

There's nothing special about instantiation of derived classes:
`DerivedClassName()' creates a new instance of the class.  Method
references are resolved as follows: the corresponding class attribute
is searched, descending down the chain of base classes if necessary,
and the method reference is valid if this yields a function object.

Derived classes may override methods of their base classes.  Because
methods have no special privileges when calling other methods of the
same object, a method of a base class that calls another method defined
in the same base class, may in fact end up calling a method of a
derived class that overrides it.  (For C++ programmers: all methods in
Python are effectively `virtual'.)

An overriding method in a derived class may in fact want to extend
rather than simply replace the base class method of the same name.
There is a simple way to call the base class method directly: just call
`BaseClassName.methodname(self, arguments)'.  This is occasionally
useful to clients as well.  (Note that this only works if the base
class is defined or imported directly in the global scope.)

Multiple Inheritance