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I'm reading Bruce Tate's latest book, Beyond Java. In it, he argues that Java has become overgrown, unwieldy and vulnerable for replacement in many applications. Prime candidates to replace it are the dynamic languages, particularly Ruby.
As a staunch Python advocate I read his description of Ruby with interest. Most Ruby features that he thinks are cool are available in Python in some form. Some are considered wizard-level tricks in Python instead of the mainstream practices they seem to be in Ruby.
For example, in Ruby you can easily add to a class definition. You just declare the class again and extend the definition. This works even for built-in classes. The Ruby approach is conceptually very simple--it reuses the class definition syntax. In Python you can add methods to a class after it is defined by adding attributes to the class. Python's approach is fairly obscure - getting it right can take a few tries.
Ruby allows mixins--class fragments that can be added to a class definition to extend it. Python can do the same with multiple inheritance, at the time a class is defined, or by appending to __bases__, which might be considered a hack.
Ruby has support for creating aliases of methods and replacing them, and this is considered a good thing. In Python this is called monkeypatching and is generally frowned on.
So there is not that much difference in capability. In Ruby some of these things are easier, and I don't discount that. But the main difference seems to be philosophical or cultural. In Python classes are thought of as fairly static--once you create it, it's done. Metaprogramming tricks are used during class creation to get some special effect, or to meet some unusual need.
In Ruby, though, classes are thought of as malleable. A class definition is just a starting point for the full functionality of the class. It's an interesting way of looking at it.
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