Carl Banks, 16.07.2010 01:14:
Around these parts, we consider the main use of most Design Patterns to be to work around limitations of other languages. Visitor Pattern is probably the worst example of it.In Python it's completely unnecessary (at least in its boilerplate- heavy incarnation as used in C++), and the fact that Python isn't strongly typed, as you put it, is exactly the reason why. Say you have a bunch of unrelated types that define a calculate method, you have a variable x that could be any of these types. Here's how you would do that in Python: x.calculate() Bam, that's it. Visitor Pattern in Python. You don't have to create a bunch of homemade dispatching boilerplate like you do in C++.
Well, you can do that in every OO language. It's not what the visitor pattern is there for, though.
The code I referenced is from the Cython compiler, and we use it to "do stuff" with the AST. The visitor pattern is actually a pretty common way to bind code in a single place that does a certain thing to different parts of a data structure. Without it, if you kept that code *inside* of the data structure, you'd have to spill the type specific parts all over your code.
Stefan -- http://mail.python.org/mailman/listinfo/python-list
