hasattr on class names

Question

hasattr documentation says that it takes an object and an attribute name and lets you know if that attribute exists on that object.

I have discovered that it seems to work on class names too (i.e. not an instance object).

Something like:

class A:
    def Attr1(self):
        pass

> hasattr(A, 'Attr1')
True
> 

I would like to use this to make some test code easier to write, but don't want to be bitten later in case this is a side effect of the implementation and not really intended.

_{Please don't ask to see the test code to see if I can do something else, as that is not really the question.}

Is there any official python stance on this? I presume the object referred to, in the documentation is talking about an instance object.

I tried googling (and looking at some questions in StackOverflow), but didn't seem to find anything.

Answer 1

The fact that it works on a class in addition to an instance is intentional. In fact, hasattr should work on any object that you pass to it. Basically, it does something like this:

def hasattr(obj, attribute):
    try:
        getattr(obj, attribute)
    except:
        return False
    return True

You can rely on that and not be afraid of getting bit by a change later. Note, there is some discussion on python-dev about hasattr being broken by design. The gist of it is that hasattr catches any exception which can be misleading.

---

Note that, in python classes are themselves instances of type.

>>> class Foo(object):
...   pass
... 
>>> type(Foo)
<type 'type'>

so everything is an instance of something¹.

^{1type is an instance of itself which is something that would be impossible to do in pure python...}