Why is "aClass.aProperty" not callable?

Question

class A:
  @property
  def p(self): return 2

  def q(self): return 2

a = A()
A.p(a) #>> TypeError: 'property' object is not callable

A.q(a) #>> no error, returns 2

Why is this? I understand if I referred to the property on an instance : a.p would simply return the method return value, but I am trying to start with the property on the class. I would have expected no error above, with both evaluating to 2.

Answer 1

You're digging into the world of descriptors. A.p is a property and properties are descriptors. It's a class that has magic methods (__get__, __set__ ...) which get called when the descriptor is accessed on an instance. The particular method accessed depends on how it's accessed of course. Accessing a descriptor on a class simply returns the descriptor itself and no magic is performed -- In this case, the property descriptor isn't callable so you get an error.

Notice what happens if you call __get__:

class A(object):
    @property
    def p(self):  
        return 2

a = A()
print (A.p.__get__(a)) #2

foo = A.p.__get__(a) is what actually happens under the hood when you do foo = a.p. I think that's pretty spiffy...

Answer 2

Because properties aren't callable:

In [3]: class A(object):
   ...:   @property
   ...:   def p(self): return 2
   ...:

In [4]: A.p
Out[4]: <property at 0x2d919a8>

In [5]: dir(A.p)
Out[5]:
['__class__',
 '__delattr__',
 '__delete__',
 '__doc__',
 '__format__',
 '__get__',
 '__getattribute__',
 '__hash__',
 '__init__',
 '__new__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__set__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__subclasshook__',
 'deleter',
 'fdel',
 'fget',
 'fset',
 'getter',
 'setter']

Note the lack of a __call__ method. This is because properties can wrap up more than one function.

Here's what happens if you try to invoke the property as a method on an instance:

In [6]: a = A()

In [7]: a.p()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
C:\Users\Marcin\<ipython-input-7-16c8de382321> in <module>()
----> 1 a.p()

TypeError: 'int' object is not callable