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

[Robert Brewer]

Mike Meng wrote:
> Solution #1 is concluded from 'Text Processing in Python', section
> 1.1.3, `Pythonic Polymorphisim', around there, here is an example he
> provides:
> 
> def toDOM(xml_src = None):
>     from xml.dom import minidom
>     if hasattr(xml_src, 'documentElement'):
>         return xml_src     # it's already a DOM object
>     elif hasattr(xml_src, 'read'):
>         # it is something that knows how to read data
>         return minidom.parseString(xml_src.read())
>     elif type(xml_src) in (StringType, UnicodeType):
>         # it is a filename of an XML document
>         xml = open(xml_src).read()
>         return minidom.parseString(xml)
>     else:
>         raise ValueError, "Must be initialized with " + \
>             "filename, file-like object, or DOM object"
>
> What's your opinion?

My name isn't Dan, but IMO hasattr works fine here. The second case
('read') is in one sense looking before leaping. The only change I would
make would be to replace "type(xml_src) in (StringType, UnicodeType)"
with "isinstance(xml_src, basestring)" in recent versions of Python
(2.2+ I think?). A die-hard no-type-checks-for-me fanatic would probably
write something like:

def toDOM(xml_src = None):
    from xml.dom import minidom
    if hasattr(xml_src, 'documentElement'):
        return xml_src     # it's already a DOM object
    
    if not hasattr(xml_src, 'read'):
        try:
            # it is a filename of an XML document
            xml_src = open(xml_src)
        except (IOError, OSError):
            raise ValueError, "Must be initialized with " + \
                "filename, file-like object, or DOM object"
    
    return minidom.parseString(xml_src.read())

I don't have a problem with either approach.


Robert Brewer
MIS
Amor Ministries
fumanchu at amor.org