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?

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.