第624页 | Learning Python | 阅读 ‧ 电子书库

Methods

Because you already know about functions, you also know about methods in classes. Methods are just function objects created by def statements nested in a class statement’s body. From an abstract perspective, methods provide behavior for instance objects to inherit. From a programming perspective, methods work in exactly the same way as simple functions, with one crucial exception: a method’s first argument always receives the instance object that is the implied subject of the method call.

In other words, Python automatically maps instance method calls to class method functions as follows. Method calls made through an instance, like this:

instance.method(args...)

are automatically translated to class method function calls of this form:

class.method(instance, args...)

where the class is determined by locating the method name using Python’s inheritance search procedure. In fact, both call forms are valid in Python.

Besides the normal inheritance of method attribute names, the special first argument is the only real magic behind method calls. In a class method, the first argument is usually called self by convention (technically, only its position is significant, not its name). This argument provides methods with a hook back to the instance that is the subject of the call—because classes generate many instance objects, they need to use this argument to manage data that varies per instance.

C++ programmers may recognize Python’s self argument as being similar to C++’s this pointer. In Python, though, self is always explicit in your code: methods must always go through self to fetch or change attributes of the instance being processed by the current method call. This explicit nature of self is by design—the presence of this name makes it obvious that you are using instance attribute names in your script, not names in the local or global scope.