Python "implements <interface>" equivalent?

[Jeff McNeil]

I don't know how "clean" it is, but there are a few situations in which I do
something like this:

getattr(obj, "method", default_method)(*original_method_args)

The default_method is a base implementation or a simple error handler. For
example, when a client hits one of our XMLRPC servers and passes a method
name that's not defined, I've got a default catch-all that logs that and
handles the error accordingly.

-Jeff

On 10/4/07, Grant Edwards <grante at visi.com> wrote:
>
> On 2007-10-04, Bruno Desthuilliers <
> bruno.42.desthuilliers at wtf.websiteburo.oops.com> wrote:
>
> > Yes, and it's even simpler : just pass your object. If it effectively
> > implements the desired interface, everything will work fine !-)
> [...]
>
> >> What I'd like to do is create a feature detection system for
> >> my work -- specifically, a general class / interface called
> >> "Feature" and then subclasses that implement functions like
> >> isFeaturePresent() in all of their different and unique ways.
> >> I'd love to hear how I can do this in Python.
> >
> > I'm not sure about what you exactly want to do, but FWIW, checking if an
> > object has a given attribute is quite simple:
> >
> > if has_attr(obj, 'attribute_name'):
> >    print "Hurray"
> > else:
> >    print "D'oh"
> >
> > Note that in Python, methods are attributes too - only they
> > are callable.
>
> On a slight tangent....
>
> The "Pythonic" way to handle things like that is often just
> to call the method you want to call.  If the object doesn't
> have that method, then you catch the exception and do whatever
> it is you do in the case where the object doesn't have the
> feature in question.
>
> The tricky bit is only catching the AttributeError exception
> generated by the attempt to access the non-existant method, and
> not catching AttributeError exceptions generated by bugs in the
> method when it does exist.
>
> Once you've added code to make sure you only catch exceptions
> you care about, it's simpler to just call has_attr
>
> the obvious method
>
>    try:
>        myobj.feature1()
>    except AttributeError:
>        print "object doesn't implement feature1"
>
> isn't correct, since an unhandled AttributeError generated by
> the feature1 method will print "object doesn't implement
> feature1".  Attempting to isolate the attributeError we care
> about looks like this:
>
>    try:
>        m = myobj.feature1
>    except AttributeError:
>        print "object doesn't implement feature1"
>    else:
>        m()
>
> That's just too messy compared with the has_attr method that
> goes like this:
>
>    if has_attr(myobj,'feature1'):
>        myobj.feature1()
>    else:
>        print "object doesn't implement feature1"
>
> However, I don't like that alot because you've got to supply
> the method name twice: once as a string and once as the method
> name.
>
> What's the cleanest way to call a method that might not be
> there?
>
> --
> Grant Edwards                   grante             Yow! My haircut is
> totally
>                                   at               traditional!
>                                visi.com
> --
> http://mail.python.org/mailman/listinfo/python-list
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.python.org/pipermail/python-list/attachments/20071004/3fd453f8/attachment.html>