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Decorators and Metaclasses: Part 1

Because the staticmethod call technique described in the prior section initially seemed obscure to some users, a feature was eventually added to make the operation simpler. Function decorators provide a way to specify special operation modes for functions, by wrapping them in an extra layer of logic implemented as another function.

Function decorators turn out to be general tools: they are useful for adding many types of logic to functions besides the static method use case. For instance, they may be used to augment functions with code that logs calls made to them, checks the types of passed arguments during debugging, and so on. In some ways, function decorators are similar to the delegation design pattern we explored in Chapter 30, but they are designed to augment a specific function or method call, not an entire object interface.

Python provides some built-in function decorators for operations such as marking static methods, but programmers can also code arbitrary decorators of their own. Although they are not strictly tied to classes, user-defined function decorators often are coded as classes to save the original functions, along with other data, as state information. There’s also a more recent related extension available in Python 2.6 and 3.0: class decorators are directly tied to the class model, and their roles overlap with metaclasses.