polymorphism (was Re: Type checking in python?)

[Oivvio Polite]

Hi all.

I'm building a class with a uniform interface and I'm trying to come to
terms with polymorphism in python.

For instance I want the constructor to take either a filename, a file object
or a database connection object.
In C++ this would be done using three different constructors. (I'm not quite
sure about this, cause I don't know C++ :-)
But how should one do it in python.

My first idea was something like this:

class myclass:
    def __init__:(self, arg):
        if type(arg) == types.StringType:
            #check if that's a filename and do the right initialization.
        elif type(arg) == types.FileType:
            #do the initialization in an other way
        # and so on

Looking at resent posts I found a thread on type checking that contained
this unsettling post:

<paul at prescod.net> wrote:
> Let me also point out that in many cases, this style of programming
> would be frowned upon by serious Python programmers. For instance, if
> you check that something is a string, your code will complain when it is
> handed a Unicode string, even though it would probably work fine. If you
> check that it is an open file, then your code probably will complain
> about stringIO file-like objects, even though it would probably work
> fine. If you check that it is an integer, your code will complain about
> integers, even though ti would probably work fine.

What are your experiences? Does the type(arg) checking really become
unmanageable as Paul's post implies? If so what are the alternative routes?

oivvio