How Can You Implement `getattr`-like Behavior for Modules in Python?

getattr Implementation in Modules

When encountering an attribute lookup failure in a module, there is a desire to create an instance of a class associated with the module and invoke a method with the same name as the failed attribute. However, this is not directly supported in Python due to limitations with xxx methods and restrictions on modifying module attributes.

The two challenges encountered are:

1. xxx methods are only invoked on the class itself, making it difficult to implement attribute substitution for the entire module.
2. Modules are not mutable, prohibiting the direct assignment of new attributes to them using __setattr__.

Solution

To overcome these limitations, a workaround is to create a wrapper instance of a substitution class and store it in sys.modules in place of the original module. This effectively redirects all attribute lookups to the wrapper instance, allowing the implementation of getattr__-like behavior. However, it's important to note that this approach can lead to different behavior compared to using __getattr directly on the module since the wrapper creates a new instance each time.

Guido van Rossum's Hack

Guido van Rossum proposed an alternative approach that leverages the import machinery's allowance for the replacement of modules in sys.modules with instances or classes. In this method, a class with the desired functionality is defined in the module and assigned to sys.modules[__name__] as the final act of the module. This allows for the implementation of getattr and other metaprogramming features on the module itself. However, it's important to ensure that all necessary functionality is encapsulated within the replacement class, as other elements of the module will be lost upon the sys.modules assignment.

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