style question - hasattr

[Miles]

On Tue, Apr 8, 2008 at 10:21 PM, ian <ian.team.python at saltmob.com> wrote:
>  Ok, so what about 'hasattr'  ??
>     hasattr(myObject,'property')
>  seems equivalent to
>     'property' in dir(myObject)
>
>  I would suggest that using the 'in' is cleaner in this case also. Is
>  there a performance penalty here? Or is there reason why the two are
>  not actually the same?

>>> class HasAll(object):
...     def __getattr__(self, name): pass
...
>>> hasattr(HasAll(), 'spam')
True
>>> 'spam' in dir(HasAll())
False

>From the docs:  "Because dir() is supplied primarily as a convenience
for use at an interactive prompt, it tries to supply an interesting
set of names more than it tries to supply a rigorously or consistently
defined set of names, and its detailed behavior may change across
releases.  ... [hasattr] is implemented by calling getattr(object,
name) and seeing whether it raises an exception or not."
http://docs.python.org/lib/built-in-funcs.html

> Which style is preferred??

Don't test for the existence of the attribute if you're going to get
it when it exists; just go ahead and get it.

try:
    x = myObject.property
except AttributeError:
    x = None

- Miles