This post originated from an RSS feed registered with Python Buzz by Ian Bicking. Original Post: Adaptation vs. Magic Methods Feed Title: Ian Bicking Feed URL: http://www.ianbicking.org/feeds/atom.xml Feed Description: Thoughts on Python and Programming. Latest Python Buzz Posts Latest Python Buzz Posts by Ian Bicking Latest Posts From Ian Bicking

There are lots of magic methods in Python. Some of them are especially magic, hooking into the underlying object model -- methods like __getattr__, or its newer cousins like __get__ (used in descriptors), or __getattribute__.

I'll refer to a different set of magic attributes, things like __len__, __nonzero__, __gt__, etc. These are generally ways for functions to allow objects to provide overrides -- len(x) calls x.__len__()__, a > b calls a.__gt__(b), etc. Lots of these functions also perform some other fallbacks -- if __gt__ is not found, then __cmp__ is tried, and finally the comparison is done based on some obscure comparison of class names, or IDs or something. When testing a value for truth, __nonzero__ is called, then if that's not found __len__ (to check if it is zero length) then if that's not found the object is considered "true".

A lot of these methods exist, both inside base Python, the standard library, and extension modules (like ZODB). One could argue that they are a sign of a hackish object model in Python -- Ruby is certainly an instance of a language that does not do this sort of thing. But by using functions (or operators) instead of directly using methods, they do allow naive objects to be handled easily.

It occurred to me -- specifically when thinking about the __iter__ special method -- that many of these are examples of adaptation (I've been using PyProtocols, which is well documented and concisely distributed). In the case of __iter__ we are adapting an object to the iterator interface. __nonzero__ (and friends) adapts to the boolean interface, and so on. v.__class__ could become adapt(v, IType) (since classes are of type type).

In a strange sort of way, this could make things more context-sensitive, like Perl. Unlike Perl the context is explicit (using explicit interfaces, and adapting the object explicitly with adapt()), but like Perl an object could be used and reused in different contexts to provide different kinds of information. Adaptation to an iterator is easy enough to consider -- how would adapation work for __len__? Would we create a ILength interface, to distinguish the unique semantics (but not type!) of length? (Do we need to start annotating our numbers with units to handle this?)

Following this further (too far?) does adaptation start to replace traversal? Instead of out.write('text') do we say adapt(out, IWriter)('text')? We quickly find ourselves using a wildly different object model, and a different notion of object-orientation.