Ivan

OOPSLA 2004

OOPSLA 2004 was great! It's the first time I've been back since 1996 (I presented a paper there). I'll write up my notes over the next few weeks as and when I get some time - I'll just be writing the delta between my experience and those of my colleagues: Martin Fowler, Steve Freeman, Nat Pryce and maybe others (sorry if I've left you out!).

Closures in Python (part 3)

Just a short addition to part 2 - I was writing some code the other day and came across another closure related problem with Python. I wanted to create some buttons, with event callbacks, dynamically; something like this:

class Foo:
   def __init__(self):
      self.callbacks = [lambda event: self.callback(name) for name in ['bob','bill','jim']]
   def callback(self, name):
      print name
   def run(self):
      for callback in self.callbacks:
         callback("some event")

Now, what do you suppose happens when you run "Foo().run()"?
You get:

jim
jim
jim

This is because there is only one "name" variable (so Nat Pryce tells me). A simple solution is to do:

class Foo:
   def __init__(self):
      self.callbacks = [self.makeCallback(name) for name in ['bob','bill','jim']]
   def makeCallback(self, name):
      return lambda event: self.callback(name)
   def callback(self, name):
      print name
   def run(self):
      for callback in self.callbacks:
         callback("some event")

I'm sure there are many other solutions too. For the curious, in Ruby the equivalent code would be:

class Foo
   def initialize()
      @callbacks = ['bob','bill','jim'].map {|name| Proc.new {|event| callback(name)}}
   end
   def callback(name)
      print name
   end   
   def run()
      @callbacks.each { |callback| callback.call("some event") }
   end
end

Foo.new.run()

which works just fine. However, I still chose Python over Ruby for pragmatic reasons (a bit ironic I know) - the libraries, tools and support are all superior.