'if foo' calls getattr?!?

David Ascher da at ski.org
Mon Aug 30 20:05:59 CEST 1999


On Mon, 30 Aug 1999, Joe Strout wrote:

> [[ This message was both posted and mailed: see
>    the "To," "Cc," and "Newsgroups" headers for details. ]]
> 
> Doing some profiling on Graphite this weekend, we found a very curious
> thing: a gazillion calls to __getattr__ on one of our classes, where we
> expected to find none.  We eventually tracked it down to a seemingly
> innocent test, basically, "if foo: self.myfoo = foo".  The "if foo" was
> invoking foo.__getattr__ (which is very expensive).
> 
> I can think of no reason why it should be this way, but it is.  Below is
> a very simple stand-alone demonstration.  It leads me to the absurd
> idiom of saying "if id(foo) != id(None)" when I really mean "if foo",
> just because using id() avoids the __getattr__ call.
> 
> Can anybody explain why this should be?  Is it a bug in Python?  Is
> there some other way to check whether I have a reference to an object
> that doesn't involve a function call?

Use 'if foo is not None' to do identity checking and not truth testing.

You're getting a getattr call because your class might define its own way
of testing truth -- for more details, see:
http://www.python.org/doc/current/ref/customization.html#l2h-134

--david








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