Question

I want to make a wrapper class that behaves in exactly the same way as the wrapped object (with a few specific exceptions). The problem I’m having at the moment is with built-in functions. How could I redirect built-in functions to the wrapped object?

class Wrapper:
    def __init__(self, wrapped):
        object.__setattr__(self, '_wrapped', wrapped)
    def __getattr__(self, name):
        return getattr(object.__getattribute__(self, '_wrapped'), name)

class Foo:
    def __init__(self, val):
        self.val = val
    def __abs__(self):
        return abs(self.val)

foo = Wrapper(Foo(-1))
print(foo.val) # Okay
print(abs(foo)) # TypeError: bad operand type for abs(): 'Wrapper'

Answer 1

You can dynamically create a new class that is subclass of both Wrapper and Foo, so you’ll have all the properties needed:

class Wrapper:
    def __new__(self, wrapped):
        cls = type(wrapped)
        new_type = type(cls.__name__ + '_wrapped', (Wrapper, cls), {})
        return object.__new__(new_type)

    def __init__(self, wrapped):
        self._wrapped = wrapped
    def __getattr__(self, name):
        return getattr(self._wrapped, name)

So now you can do:

>>> foo = Wrapper(Foo(-1))
>>> abs(foo)
1
>>> type(foo)
<class '__main__.Foo_wrapped'>

PS:

• You don’t need object.__getattr__ (or __setattr__) in the __init__ and __getattr__ functions to get and set this attribute.
• You may want to cache this operation to avoid creating a new class at every new object.