[Python-Dev] Avoid formatting an error message on attribute error (original) (raw)

Steven D'Aprano steve at pearwood.info
Thu Nov 7 01:20:31 CET 2013

On Wed, Nov 06, 2013 at 11:32:33PM +0100, Victor Stinner wrote:

Hi,

I'm trying to avoid unnecessary temporary Unicode strings when possible (see issue #19512). While working on this, I saw that Python likes preparing an user friendly message to explain why getting an attribute failed. The problem is that in most cases, the caller doesn't care of the message: the exception is simply deleted. For example, hasattr() deletes immediatly the AttributeError.

My initial instinct here was to say that sounded like premature optimization, but to my surprise the overhead of generating the error message is actually significant -- at least from pure Python 3.3 code.

That is, the time it takes to generate the message:

"'%s' object has no attribute %r" % (obj.class.name, attrname)

is about the same as the time to raise and catch an exception:

try: raise AttributeError except AttributeError: pass

(measured with timeit).

Given that, I wonder whether it would be worth coming up with a more general solution to the question of lazily generating error messages rather than changing AttributeError specifically. Just thinking aloud -- perhaps this should go to python-ideas? -- maybe something like this:

class _LazyMsg: def init(self, template, obj, attrname): self.template = template self.typename = type(obj).name self.attrname = attrname def str(self): return self.template % (self.typename, self.attrname)

Then:

py> err = AttributeError( ... _LazyMsg("'%s' object has no attribute '%s'", None, "spam")) py> raise err Traceback (most recent call last): File "", line 1, in AttributeError: 'NoneType' object has no attribute 'spam'

Giving the same effect from C code, I leave as an exercise for the reader :-)

It would be nice to only format the message on demand. The AttributeError would keep a reference to the type.

Since only the type name is used, why keep a reference to the type instead of just type.name?

AttributeError.args would be (type, attr) instead of (message,). ImportError was also modified to add a new "name "attribute".

I don't like changing the signature of AttributeError. I've got code that raises AttributeError explicitly.

-- Steven

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