How to prevent hasattr from retrieving the attribute value itself

Question

I have a class that implements virtual attributes using __getattr__. The attributes can be expensive, e.g. performing a query. Now, I am using a library which checks if my object has the attribute before actually getting it.

As a consequence, a query is executed two times instead of one. Of course it makes sense to actually execute __getattr__ to really know if the attribute exists.

class C(object):
    def __getattr__(self, name):
        print "I was accessed"
        return 'ok'

c = C()
hasattr(c, 'hello')

Is there any way to prevent this?

If Python supported __hasattr__ then I could simply check if the query exists, has opposed to actually run it.

I can create a cache, but it is heavy since a query might have parameters. Of course, the server might cache the queries itself and minimise the problem, but it is still heavy if queries return a lot of data.

Any ideas?

Willi

If I understand the situation correct, you need to cache based on name only, so that could be just a dictionary. BTW, why do you want to make it look like an attribute? – Quadrat

What do you want to happen if the object doesn't support the query? – Menchaca

hasattr is not a light-weight test for an attribute; it calls getattr essentially like try: getattr('name'); except AttributeError: return False; else: return True. – Lion

I don't think there is any way to do what you want, other than by monkey patching hasattr, which would not be recommended. If there is no way to avoid calling hasattr, I believe your only choices are caching the result or allowing the query to run twice. – Sabir

@shashank, if the query is not available (by name), then it returns the exception for attribute not found. – Willi

@bereal: because queries are accessed has attributes of an object. Sorry, too long to explain it all properly here. – Willi

Answer 1

Although initially I was not fond of the idea of monkey patching, being a "bad idea" in general, I came across a very neat solution from 1999!!

http://code.activestate.com/lists/python-list/14972/

def hasattr(o, a, orig_hasattr=hasattr):
    if orig_hasattr(o, "__hasattr__"):
        return o.__hasattr__(a)
    return orig_hasattr(o, a)

__builtins__.hasattr = hasattr

Essentially it creates support for __hasattr__ in Python, which is what I thought originally would be the optimal solution.

Willi

Monkey patching is awesome. Rubyists have long embraced the ugly monkey. So should Pythonistas. Your augmentation of the stock hasattr() builtin to support __hasattr__() special methods is the perfect example of that. Thanks for rejuvenating this elegant hackery! – Penchant

Answer 2

I think this is generally probably a bad pattern, but you can always check an object's underlying __dict__.

In [1]: class A(object):
   ....:     @property
   ....:     def wha(self):
   ....:         print "was accessed"
   ....:

In [2]: A.__dict__
Out[2]:
<dictproxy {'__dict__': <attribute '__dict__' of 'A' objects>,
'__doc__': None,
'__module__': '__main__',
'__weakref__': <attribute '__weakref__' of 'A' objects>,
'wha': <property at 0x10f6b11b0>}>

In [3]: a = A()

In [4]: "wha" in a.__class__.__dict__
Out[4]: True

Spiv

I think OP is dealing with a third-party library that calls hasattr, so this would require forking the library. – Sabir

I think you misunderstand, the class is fine, and I need to use __getattr__(). Plus what @Sabir said. – Willi