What is the usage of third argument objtype in Python descriptor's __get__ [duplicate]

Question

I know that in Python we have to supply __get__ function when implementing a descriptor. The interface is like:

def __get__(self, obj, objtype=None):
    pass

My question is:

Why we have to supply objtype arg? What is objtype used for?

I did not see some examples about the usage of this arg.

Answer 1

It gives users an option to do something with the class that was used to call the descriptor.

In normal cases when the descriptor is called through the instance we can get the object type by calling type(ins).

But when it is called through the class ins will be Noneand we won't be able to access the class object if the third argument was not present.

---

Take functions in Python for example, each function is an instance of types.FunctionType and has a __get__ method that can be used to make that function a bound or unbound method.

>>> from types import FunctionType
>>> class A(object):
    pass
...
>>> def func(self):
    print self
...
>>> ins = A()
>>> types.FunctionType.__get__(func, ins, A)() # instance passed
<__main__.A object at 0x10f07a150>
>>> types.FunctionType.__get__(func, None, A)  # instance not passed
<unbound method A.func>
>>> types.FunctionType.__get__(func, None, A)()
Traceback (most recent call last):
  File "<ipython-input-211-d02d994cdf6b>", line 1, in <module>
    types.FunctionType.__get__(func, None, A)()
TypeError: unbound method func() must be called with A instance as first argument (got nothing instead)
>>> types.FunctionType.__get__(func, None, A)(A())
<__main__.A object at 0x10df1f6d0>