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

Benjamin Peterson benjamin at python.org
Mon Aug 23 22:55:23 CEST 2010

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.

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.

-- Regards, Benjamin

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