Python: Essential Reference, Third Edition

Special Methods

All the built-in data types implement a collection of special object methods. The names of special methods are always preceded and followed by double underscores (__). These methods are automatically triggered by the interpreter as a program executes. For example, the operation x + y is mapped to an internal method, x.__add__(y), and an indexing operation, x[k], is mapped to x.__getitem__(k). The behavior of each data type depends entirely on the set of special methods that it implements.

User-defined classes can define new objects that behave like the built-in types simply by supplying an appropriate subset of the special methods described in this section. In addition, built-in types such as lists and dictionaries can be specialized ...