CODEWITHSUNDEEP
pythongetattrhasattr
koriander
koriander

Reputation: 3258

How to prevent hasattr from retrieving the attribute value itself

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?

Upvotes: 9

Views: 2862

Answers (2)

koriander
koriander

Reputation: 3258

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.

Upvotes: 6

jwilner
jwilner

Reputation: 6606

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

Upvotes: 0

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