Neat way to get descriptor object

(c) zaper3095

Interesting question appeared today on my job:
Quoting from StackOverflow

In Python 3

class A(object):
    attr = SomeDescriptor()
    ...
    def somewhere(self):
        # need to check is type of self.attr is SomeDescriptor()
        desc = self.__class__.__dict__[attr_name]
        return isinstance(desc, SomeDescriptor)

Is there better way to do it? I don't like this self.__class__.__dict__ stuff

Shortly, the answer is NO. No other way to get descriptor object (preventing __get__ being invoked) than getting it from __dict__.

However, there are several workarounds ;)

1) Return self in __get__ method if instance is None (it will happen if call getattr on class object like type(self))
Like this:

class SomeDescriptor():
    def __get__(self, inst, instcls):
        if inst is None:
            # instance attribute accessed on class, return self
            return self
        return 4

class A():
    attr = SomeDescriptor()
    def somewhere(self):
        attr_name = 'attr'
        desc  = getattr(type(self), attr_name)
        # desc = self.__class__.__dict__[attr_name]  # b.somewhere() would raise KeyError
        return isinstance(desc, SomeDescriptor)

2) Second solution came from my colleague Maksym Panibratenko
Since our descriptor is callable, and __get__ returns function, we can assign attribute on this function, and check in runtime is function has this attribute with hasattr()

    class SomeDescriptor():
        def __get__(self, inst, instcls):
            def func():
                pass
            func.implemented = True
            return func

    class A():
        attr = SomeDescriptor()
        def somewhere(self):
            attr_name = 'attr'
            desc  = getattr(self, attr_name)
            return hasattr(desc, 'implemented')

http://stackoverflow.com/questions/21629397/neat-way-to-get-descriptor-object
Follow link to see full discussion