Is there a consensus on how to check a polymorphic instance?

[Dan Sommers]

On 23 Nov 2004 19:58:31 -0800,
"Mike Meng" <meng.yan at gmail.com> wrote:

First off, I have only been following this thread on and off, so if I'm
repeating things, I apologize.

> It seems to me the virtue of dynamic langauge is, if it looks like a
> cat, it's a cat ...

Google for "duck typing."

> ... But the problem is still there: how do you know what it looks like
> before you treat it as a cat? isinstance(), , as Steve state, is too
> rigid.

You don't.

Python also takes a "we're all adults here" philosophy.  If your API
specifies "cat," and I pass your function an "InterfaceAdapter," it's
*my* problem.

If you're truly paranoid, make sure that your arguments at least have
the methods you're going to use:

    # write 'foo' to the file-like object x; don't bother if x is not
    # sufficiently file-like

    def f( x ):
        try: x.write
        except AttributeError:
            pass
        else:
            x.write( 'foo' )

    # this works, too, but doesn't scale well (i.e., if 'foo' is some
    # other non-trivial (maybe non-trusted) operation that can raise
    # AttributeError):

    def f( x ):
        try: x.write( 'foo' )
        except AttributeError:
            pass

Regards,
Dan

-- 
Dan Sommers
<http://www.tombstonezero.net/dan/>
Never play leapfrog with a unicorn.