[Python-Dev] Evil isinstance() (original) (raw)

Patrick K. O'Brien pobrien@orbtech.com
Mon, 1 Apr 2002 16:56:37 -0600

[Guido van Rossum]

> This doesn't seem so much of a problem to me to warrant rejecting > perfectly good numberlike and stringlike classes' instances, by > isinstance'ing away.

Agreed, and I think the solution ought to be sought in asking a question involving hasattr().

Having recently removed most calls to hasattr() from PyCrust's introspection routines, I'd be curious to get more of your opinion on this issue. Specifically, I removed calls to hasattr() because xmlrpclib always returns true for hasattr(), rendering it effectively meaningless.

import xmlrpclib p = xmlrpclib.ServerProxy("http://betty.userland.com", verbose=1) hasattr(p, 'bogus') 1 p.bogus [big error message snipped]

I discussed this with Mr. Lundh and ultimately worked around this by removing calls to hasattr(), but I'm still left feeling a bit uncomfortable by this whole situation. Perhaps I was being naive, but I thought I could rely on hasattr(). According to Mr. Lundh:

"Having an attribute doesn't mean that you carry out any possible operation on this attribute. In this case, the server proxy will return a method proxy for any possible attribute (hasattr and getattr always succeeds). You won't get an error until you attempt to call the method proxy..."

This bit of advice seems to fall toward the opposite end of the spectrum from the advice to look to hasattr() for a solution. So I figured I would open this can of worms. I hope this isn't too off-topic.

Patrick K. O'Brien Orbtech