On 23/08/2010 23:55, Benjamin Peterson wrote:
2010/8/23 Raymond Hettingerraymond.email@example.com:
On Aug 23, 2010, at 1:13 PM, Benjamin Peterson wrote:
2010/8/23 Michael Foordfuzzyman@voidspace.org.uk:
To me hasattr *looks* like a passive introspection function, and the fact that it can trigger arbitrary code execution is unfortunate - especially because a full workaround is pretty arcane.
hasattr(x, "y") doesn't look any more passive to me the x.y.
One says "does this object have attribute y" the other fetches attribute y. I'm amazed they don't look different to you. Given Python's object model there is no reason that the first *should* fetch the attribute to determine that it is present, *except* for the dynamic attribute creation of __getattr__ and __getattribute__.
For properties there is *no reason* why code should be executed merely in order to discover if the attribute exists or not.
I'm in both camps though. As we *are* triggering code execution I don't think we should mask exceptions. I just wish we weren't triggering code execution unnecessarily.
Well said. The surprise to me in the OP's example was that the property() was executed. Regular methods aren't run by hasattr() so it's hard to remember that when writing code using hasattr().
Hard to remember compared to what?
That is especially unfortunate because someone turning a regular attribute into a property may be doing so long after client code has been written (IIRC, that was a key use case for properties). IOW, the user of the hasattr() may have had no way of knowing that an exception could ever be raised (because it is perfectly safe with regular attributes and methods).
Better to raise an exception into unexpecting code that to have subtly different lookup rules between getattr and hasattr.