polymorphism w/out signatures?

[Larry Bates]

Duncan,

Thanks for your reply to my post.  As all programmers
I have adopted coding "techniques" that I see in others
code.  The technique I suggested is lifted directly
from ReportLab code.  I don't know if it predates the
isinstance function or not (which may explain why it
is not used).  BTW-Your example that includes basestring
apparently only works on Python  2.3.  Anyway, I
learned something today.

Thanks again,
Larry Bates
Syscon, Inc.

"Duncan Booth" <me at privacy.net> wrote in message
news:Xns94E25EB583B9Bduncanrcpcouk at 127.0.0.1...
> "Larry Bates" <lbates at swamisoft.com> wrote in
> news:lMWdncBA4LRcAgfd4p2dnA at comcast.com:
>
> > I use:
> >
> > class foo:
> >     _stringtype=type('')
> >     _tupletype=type(())
> >     _listtype=type([])
>
> You could just use the builtin names already supplied for these: str,
> tuple, and list have the same values you just assigned to _stringtype,
> _tupletype, and _listtype. Also, you forgot about unicode.
>
> >
> >     def __init__(self, variable):
> >     if type(variable) == _stringtype:
> >         self.variable=variable # check for string type
> >     if type(variable) in (_listtype, tupletype):
> >         self.variable=str(variable) # check for list/tuple
> >     .
>
> If anyone has been subclassing str, list or tuple this won't work. You may
> not subclass builtin types very often, but it doesn't really hurt to use
> isinstance instead. That's why it is better to write:
>
>     if isinstance(variable, (str, unicode)):
>        ...
>
> or:
>
>     if isinstance(variable, basestring):
>        ...
>
> as either of these will catch subclassing.
>