visitor_goodness.py

# Happily turned this ugly thing:
class Foo(object):
    pass

class Bar(object):
    pass

class User(object):
    def collect(objects):
        foos = [x for x in objects if isinstance(x, Foo)]
        bars = [x for x in objects if x not in foos]

# To this!
class Foo(object):
    def register_yourself(self, registrator):
        registrator.register_foo(self)

class Bar(object):
    def register_yourself(self, registrator):
        registrator.register_bar(self)

class User(object):
    def collect(objects):
        self.registrator.register(objects)
        foos = self.registrator.foos()
        bars = self.registrator.bars()