Which idiom is better? try: ... except Attr..Err or hasattr?

[Laurence Tratt]

Peter Funk wrote:

> Look at the following coding alternatives often seen in generic python code:
> 
> class A:
>     def load(self):
>         # ... do something e.g. load a file
>         try:
>             self.update()
>         except AttributeError:
>             pass
> 
> class B:
>     def load(self):
>         # ... do something e.g. load a file
>         if hasattr(self, 'update'):
>             self.update()
> 
> Which coding idiom is better?  The second form is one line shorter than
> the first, but it has the disadvantage, that the method (or attribute) must
> be named twice.  Both idioms are used in the standard libary.

As a general rule, I would always use the second form: your intent - that
you're not sure if update exists - is much clearer.

However, if update nearly always exists, the second form is slower because
it has to perform the lookup for update twice; if update rarely exists then
the overhead in raising the exception causes the first form to be much
slower. In practice this is rarely important unless you have a long loop,
and need to squeeze every last ounce out of the interpreter.

To give you a quick idea, I tried a little script, and the rough figures
suggest that with a small dictionary, if update exists the first form is
about x2 faster than the second; if update doesn't exist the second form is
about x4 - x6 faster. So the difference isn't that much, unless you make the
same call a lot of times.


Laurie