How to robustly check a Python property exists?

Question

Given the following class (with a buggy property) then what is the best foolproof way of checking that the bar property exists?

class Foo(object):
    @property
    def bar(self):
        raise AttributeError('unforeseen attribute error!')

Both hasattr and getattr fail and only dir works:

foo = Foo()

print hasattr(foo, 'bar')
# False

try:
    getattr(foo, 'bar')
    print True
except AttributeError as e:
    print False
# False    

print 'bar' in dir(foo)
# True

The best all round solution I can think of is:

def robust_hasattr(obj, attr):
    return hasattr(obj, attr) or attr in dir(obj)

Is there a better way?

Answer 1

If you have a buggy property, fix the bug. If raising AttributeError is a bug, then make the property not do that. Raising that exception is the way to signal that you should not be using that attribute.

Using dir() can be a work-around, but it is not foolproof, as dir() is a debugging aid that can both omit information and can be overridden by the object.__dir__ hook (giving your code another vector to introduce bugs). Then there is the possibility of a buggy object.__getattr__ hook, a buggy object.__getattribute__ hook, or even descriptors on the metaclass, all of which would not be detectable by using dir().

Since you are specifically looking for a property, look for the same attribute on the class of your object:

hasattr(foo, 'bar') or isinstance(getattr(type(foo), 'bar', None), property)

For your specific case, the above returns True:

>>> class Foo(object):
...     @property
...     def bar(self):
...         raise AttributeError('unforeseen attribute error!')
...
>>> foo = Foo()
>>> hasattr(foo, 'bar') or isinstance(getattr(type(foo), 'bar', None), property)
True

because there indeed is such a property object on the class.

Answer 2

By the rules of Python, the bar attribute does not exist. An object is considered to have an attribute if an attempt to access the attribute doesn't raise an exception.

If you want to use a different notion of whether an attribute exists, you can implement that notion yourself. For example, to check for the existence of an entry corresponding to bar in the instance dict or one of the class dicts:

for obj in (foo,) + type(foo).__mro__:
    if 'bar' in obj.__dict__:
        print "It's there."
        break
else:
    print "Didn't find it."