Determine if given class attribute is a property or not, Python object

Question

It's all in the title. Here is the following example:

class A(object):     my_var = 5      def my_method(self, drink='beer'):         return 'I like %s' % drink      @property     def my_property(self):         return 'I do not drink coffee' 

I instantiate an A object and I want to know the type of each attribute and if it is a callable. For this I'm using dir().

obj = A()  for attr in dir(obj):     print 'Type: %s' % type(obj)     print 'Is callable: %s' % callable(attr) 

I have to know also if an attribute is a property. I'm sure that there is a way to know this. All suggestions will be appreciated.

Answer 1

You need to look at the class (this is the case for descriptors in general), which for objects you can find via the __class__ attribute or by using the type function:

>>> obj.__class__.my_property <property object at 0xb74bd16c> 

or by

>>> type(obj).my_property <property object at 0xb720b93c> 

These result in the same "property object" as if you were to directly check the attribute of the class (implying you know the class' name in your code instead of checking it dynamically like you probably should rather do):

>>> A.my_property <property object at 0xb7312345> 

So to test if a specific attribute of an object is a property, this would be one solution:

>>> isinstance(type(obj).my_property, property) True 

Answer 2

I once asked a similar question. The trouble you'll run into, of course, is that you can't access the property through the instance to determine its type without calling the getter, which gets you the type of whatever the getter returns. So you have to access the property through its class rather than through the instance.

property is already a type, so you can just compare directly to that. (I originally had some superfluous code here that got the property type out of a class that had a property. I thought this was necessary due to a typo when I was testing things.)

obj_type = type(obj)  for attr in dir(obj):     if isinstance(getattr(type(obj), attr, None), property):         print attr, "is a property" 

Don't worry about having an instance attribute with the same name. It's ignored in attribute lookup if there's a data descriptor of the same name on the class (property is a data descriptor).

Of course, any class can be a data descriptor, not just property, so in theory you really want to check for __get__() and/or __set__() and/or __delete__() attributes on the type. But the problem with that approach is that all functions and methods are themselves descriptors and therefore would be caught by that check. It quickly becomes silly to try to find all the exceptions.