http://www.daniel-lemire.com/blog/archives/2005/12/21/metaclass-programming-in-python/

Regarding the Javascript example, wow, 60,000 lines of Javascript. I wouldn’t wish that on my worst enemy.

“I find myself wishing for build-time tools, a compiler to catch typos in
variable names, to catch syntax errors earlier rather than runtime
checking.”

This argument can be applied to any scripting language. I think Javascript is a particularly horrible language and I’d tend to lay the blame there as opposed to compile time rather than runtime enforcement of syntax, etc.

Think it needs careful definition of different things you can do – adding a method to a class at runtime is not the same as adding a method to an object (an instance of the class). That “new” approach seems to address the latter.

The former is fairly easy e.g.

class Foo:

def __init__(self):
self.x = “x = 1″
self.y = “y = 2″

def showx(self):
print self.x

def showy(self):
print self.y

if __name__ == ‘__main__’:

# Attach at runtime…
Foo.showy = showy

f2 = Foo()
f2.showx()
f2.showy()

Now all instances of Foo, created _after_ attaching showy, have this method available. The downside there is the definition of showy has “self” embedded – it can’t also just be a function but perhaps that’s a rare requirement.

“Why am I losing faith in inheritence?” the desire to avoid rigid code, gain more flexibility and (perhaps) save time?

“Is this idea of adding methods to existing classes/instances actually a bad idea?” I guess three possible issues (but not show stoppers) might be;

– it’s potentially expensive performance wise – if you’re doing this a lot, you might have missed the chance save some cycles by hardcoding, allowing python to compile upfront

– it’s harder to analyse the source code automatically and understand it’s semantics, without actually executing it

– it’s (subjective) easier for teams of developers to working with concrete / static class definitions. I find this discussion relevant: http://groups.yahoo.com/group/wdf-dom/message/4136&threaded=1 – although that’s Javascript, think it applies

“Is injection of methods into a class equivalent to inheriting from it and extending it?” – does the question matter? Think inheritance is Python is more of a notionally supported feature than something rock solid, as in Java. You have multiple inheritance. You have explicit self rather than implicit “this”. It’s more like – what’s the easiest way to get re-use?