Why don't Python class method decorators receive the method as a bound method?

Question

When I create a decorator for a class method, it always receives the method as of type "function".

However, when I play around with things a bit, I only get back bound methods:

class Test(object):
 def save(self):
 print "Save called"
 def func(self):
 print "Func called"

And then:

>>> type(Test.func)
<type 'instancemethod'>
>>> type(Test().func)
<type 'instancemethod'>

What I would ultimately like to do is create a class method decorator, which also decorates some other method on the same class. How would I go about doing this?

• Marcelo Cantos: Test.func is not a bound method.

Answer 1

This is impossible; you'd have to use a class decorator or metaclass instead. Decorator syntax

class Foo(object):
 @dec
 def bar(self): pass

means

class Foo(object)
 def bar(self): pass
 bar = dec(bar)

where a class definition is processed as: execute the body, then gather the definitions and wrap them in a class object. I.e., decoration is done before the class comes into existence.

Answer 2

It depens on the order of stuff that happens.

If you take a "normal" method. the following happens:

class Test(object):
 def save(self):
 print "Save called"
 def func(self):
 print "Func called"
>>> Test.__dict__["func"]
<function func at 0x00B43E30>
>>> Test.func
<unbound method Test.func>

Should be the same. What happens here? Well, look:

>>> f = Test.__dict__["func"]
>>> m = f.__get__(None, Test)
>>> f, m
(<function func at 0x00B43E30>, <unbound method Test.func>)

The first is the original function object, the second one the method object which is created when doing an actual method call.

Furthermore, if you have an instance:

>>> t = Test()
>>> t.func
<bound method Test.func of <__main__.Test object at 0x00B48AB0>>
>>> f.__get__(t, Test)
<bound method Test.func of <__main__.Test object at 0x00B48AB0>>

So this happens on attribute access.

Now to your question:

The reason this happens is because the original function is present in the class's __dict__. The method object creation happens on access.