How to correctly "stub" objclass in a Python class?

Question

I have Python class looking somewhat like this:

class some_class:
    def __getattr__(self, name):
        # Do something with "name" (by passing it to a server)

Sometimes, I am working with ptpython (an interactive Python shell) for debugging. ptpython inspects instances of the class and tries to access the __objclass__ attribute, which does not exist. In __getattr__, I could simply check if name != "__objclass__" before working with name, but I'd like to know whether there is a better way by either correctly implementing or somehow stubbing __objclass__.

The Python documentation does not say very much about it, or at least I do not understand what I have to do:

The attribute __objclass__ is interpreted by the inspect module as specifying the class where this object was defined (setting this appropriately can assist in runtime introspection of dynamic class attributes). For callables, it may indicate that an instance of the given type (or a subclass) is expected or required as the first positional argument (for example, CPython sets this attribute for unbound methods that are implemented in C).

I don't see why you need to stub it. If you just raise an AttributeError in your `__getattr__`, it'll work the same way as it would in a class that has no `__getattr__` (i.e. it would work _correctly_). What reason do you have to manually set this attribute's value? — Aran-Fey, Mar 21 '18 at 12:11
@Aran-Fey Hmm I did not think of just raising the right error. Thanks, this makes sense. — s-m-e, Mar 21 '18 at 12:16
@Aran-Fey you should repost your comment as an answer obviously. — bruno desthuilliers, Mar 21 '18 at 12:34
@brunodesthuilliers Done, thanks. It actually didn't occur to me that that might be worthy of an answer. — Aran-Fey, Mar 21 '18 at 12:50
@s-m-e: if Aran-Fey's answer solves your problem then you may want to accept it ;) — bruno desthuilliers, Mar 21 '18 at 13:55

score 4 · Accepted Answer · answered Mar 21 '18 at 12:49

You want to avoid interfering with this attribute. There is no reason to do any kind of stubbing manually - you want to get out of the way and let it do what it usually does. If it behaves like attributes usually do, everything will work correctly.

The correct implementation is therefore to special-case the __objclass__ attribute in your __getattr__ function and throw an AttributeError.

class some_class:
    def __getattr__(self, name):
        if name == "__objclass__":
            raise AttributeError

        # Do something with "name" (by passing it to a server)

This way it will behave the same way as it would in a class that has no __getattr__: The attribute is considered non-existant by default, until it's assigned to. The __getattr__ method won't be called if the attribute already exists, so it can be used without any issues:

>>> obj = some_class()
>>> hasattr(obj, '__objclass__')
False
>>> obj.__objclass__ = some_class
>>> obj.__objclass__
<class '__main__.some_class'>

How to correctly "stub" __objclass__ in a Python class?

1 Answers1

How to correctly "stub" objclass in a Python class?