Question

class Foo(object):
  pass

foo = Foo()
def bar(self):
  print 'bar'

Foo.bar = bar
foo.bar() #bar

Coming from JavaScript, if a “class” prototype was augmented with a certain attribute. It is known that all instances of that “class” would have that attribute in its prototype chain, hence no modifications has to be done on any of its instances or “sub-classes”.

In that sense, how can a Class-based language like Python achieve Monkey patching?

Answer 1

The real question is, how can it not? In Python, classes are first-class objects in their own right. Attribute access on instances of a class is resolved by looking up attributes on the instance, and then the class, and then the parent classes (in the method resolution order.) These lookups are all done at runtime (as is everything in Python.) If you add an attribute to a class after you create an instance, the instance will still “see” the new attribute, simply because nothing prevents it.

In other words, it works because Python doesn’t cache attributes (unless your code does), because it doesn’t use negative caching or shadowclasses or any of the optimization techniques that would inhibit it (or, when Python implementations do, they take into account the class might change) and because everything is runtime.