Class Method Calls

In the prior section, we saw how the attribute reference I2.w in our example class tree was translated to C3.w by the inheritance search procedure in Python. Perhaps just as important to understand as the inheritance of attributes, though, is what happens when we try to call methods (i.e., functions attached to classes as attributes).

If this I2.w reference is a function call, what it really means is “call the C3.w function to process I2.” That is, Python will automatically map the call I2.w() into the call C3.w(I2), passing in the instance as the first argument to the inherited function.

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In fact, whenever we call a function attached to a class in this fashion, an instance of the class is always implied. This implied subject or context is part of the reason we refer to this as an object-oriented model—there is always a subject object when an operation is run. In a more realistic example, we might invoke a method called giveRaise attached as an attribute to an Employee class; such a call has no meaning unless qualified with the employee to whom the raise should be given.

As we’ll see later, Python passes in the implied instance to a special first argument in the method, called self by convention. As we’ll also learn, methods can be called through either an instance (e.g., bob.giveRaise()) or a class (e.g., Employee.giveRaise(bob)), and both forms serve purposes in our scripts. To see how methods receive their subjects, though, we need to move on to some code.