Class-based decorator and repr() conservation

Question

I was trying to have my class-based decorator keeping the repr() behavior of the original wrapped function (to match the way the functools.wraps decorator works on functions). I am using python 3.3.

First I tried functools:

import functools

class ClassBasedDecorator():
    def __init__(self, fn):
        self.fn = fn
        functools.update_wrapper(self, fn)
    def __call__(self, *args, **kwargs):
        self.fn(*args, **kwargs)

@ClassBasedDecorator
def wrapped(text):
    pass

But when I call repr() on the decorated function, I get:

>>> repr(wrapped)
'<__main__.ClassBasedDecorator object at 0x2d8860b6850>'

Very well, so I tried to customize the __repr__ method of my decorator, which is supposed to be called by repr().

Using functools again:

class ClassBasedDecorator():
    def __init__(self, fn):
        self.fn = fn
        functools.update_wrapper(
            self, fn,
            assigned=functools.WRAPPER_ASSIGNMENTS + ('__repr__',)
        )
    def __call__(self, *args, **kwargs):
        self.fn(*args, **kwargs)

Doesn't change the output, but something interesting happens:

>>> repr(wrapped)
'<__main__.ClassBasedDecorator object at 0x2d8860b69d0>'
>>> wrapped.__repr__()
'<function wrapped at 0x2d8860a9710>'

Explicitly setting the __repr__ method of the decorator instance has the same effect.

After a little more tests I deduced repr(instance) actually calls instance.__class__.__repr__(instance). Thus the overriden __repr__ method of the instance is never called.

---

So here are my questions:

• Why does repr(instance) call the instance.__class__.__repr__(instance) instead of instance.__repr__()? Or have I missed something else?
• How would you fully reproduce what functools.wraps does with function-based decorators to class-based decorators (including altering the result of repr() calls on the decorated function)?

Answer 1

Special methods are always looked up on the type of the instance (here the class object), not on the instance. Otherwise a __repr__ on a class would be used when you tried to print the representation of the class itself; type(class).__repr__(class) would use the correct magic method, while class.__repr__() would raise an exception because self was not provided.

Implement your own __repr__ hooks:

class ClassBasedDecorator():
    def __init__(self, fn):
        self.fn = fn
        functools.update_wrapper(self, fn)
    def __call__(self, *args, **kwargs):
        self.fn(*args, **kwargs)
    def __repr__(self):
        return repr(self.fn)

e.g. still copy over the __module__, __name__ and __doc__ attributes, and copy over the attributes from the function __dict__, but make any special methods a proxy.