[Python-Dev] Monkeypatching idioms -- elegant or ugly? (original) (raw)

"Martin v. Löwis" martin at v.loewis.de
Sat Jan 19 19:53:42 CET 2008

In Objective-C it's perfectly common to extend existing classes using 'categories' and I have often found this idiom very useful. What is described here is basically categories for Python. I've implemented something like this before and I would be happy to see this added to the standard library and formalised.

If they are like Smalltalk categories, then they are semantically different. For method categories, you group the methods by "functionality" or "purpose" or "intended audience", as the developer of a class. It is important that the category has a name, and the categories are part of the class' documentation.

For monkey-patching classes, the grouping is "by application". The additional/changed methods are not written by the author of the class, but by the author of the application using the class. The name of the group is irrelevant, and the documentation (if any exists) documents it as an application-specific thing.

In-between are the partial classes of C# and the way C classes get implemented: it is the implementor of the class who fragments the class into separate pieces of code, but not for the sake of the class clients, but for maintainability of the class itself (so that you can spread a large class over several source files).

They use same implementation strategy; for integrating it into the standard library, I think the usage scenarios need to be considered more. I like to see all three scenarios supported; to support categories properly, you need to also have support in the reflection, in pydoc, and perhaps also in the online documentation (if standard library classes get split into categories).

Regards, Martin

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