Ruby has a nice, but very dangerous feature called open classes. That means you can extend any class definition if you want it.

This example shows it

#!/usr/bin/ruby
class Foo
    def do_foo
        return "foo"
    end
end

aFoo = Foo.new()

puts aFoo.do_foo()

class Foo
    def do_bar
        return "bar"
    end
end

puts aFoo.do_bar()

That means you dynamically extend the definition of Foo class, so the last line prints “bar”.

Python behavior is different, because it does not allows this extending. Python just redefines a class Foo, so new instance of Foo will have a do_bar method only. But this not affected an existing ones, like Ruby does.

class Foo:

    def do_foo(self):
        return "foo"

aFoo1 = Foo()

class Bar(Foo):

    def do_nothing(self):
        pass

print issubclass(Bar, Foo)
print isinstance(aFoo1, Foo)

class Foo:

    def do_bar(self):
        return "bar"

aFoo2 = Foo()
aBar  = Bar()

print issubclass(Bar, Foo)
print isinstance(aFoo1, Foo)
print isinstance(aFoo2, Foo)

print dir(aFoo1)
print dir(aFoo2)
print dir(aBar)

But what about method injection? My solution is based on ideas of Recipe 81732: Dynamically added methods to a class.

import types

def funcToMethod(func, instance, method_name=None):
    cls = instance.__class__ if type(instance) != types.TypeType else instance
    setattr(cls, \
            method_name or func.__name__, \
            types.MethodType(func, instance, cls))

And that’s all. The funcToMethod bounds func to instances’s class and allows under method_name (default name is a same as a function one). So lets do some testing.

class Foo(object):

    def list(self):

        return [meth for meth in dir(self) if meth[:3] == 'do_' and type(getattr(self, meth)) == types.MethodType]

    def do_bar(self):

        return "bar"

def do_foo(inst):

    return "foo"

class Bar(Foo):

    def do_nothing(self):
        pass

aBar = Bar()

aFoo1 = Foo()
print aFoo1.list()
# calling it with class instead of instance is also possible and is equivalent
#funcToMethod(do_foo, Foo, "do_foo")
funcToMethod(do_foo, aFoo1, "do_foo")

aFoo2 = Foo()
print aFoo1.list()
print aFoo2.list()

print aFoo1.do_foo()
print aFoo2.do_foo()

print aBar.list()
print aBar.do_foo()

When you run the code, you will see that funcToMethod adds a new method to Foo class and this changes both existing and new instances as Ruby does too. And subclassing is not a problem, they are be affected too.