[Python-Dev] 'hasattr' is broken by design (original) (raw)

Michael Foord fuzzyman at voidspace.org.uk
Mon Aug 23 23:02:42 CEST 2010

On 23/08/2010 23:55, Benjamin Peterson wrote:

2010/8/23 Raymond Hettinger<raymond.hettinger at gmail.com>:

On Aug 23, 2010, at 1:13 PM, Benjamin Peterson wrote:

2010/8/23 Michael Foord<fuzzyman at 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.

Michael

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.

-- http://www.ironpythoninaction.com/

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