"casting" Python objects

[Chris Liechti]

"DJW" <dwelch91 at home.nospam.com> wrote in
news:achcu0$j62$1 at news.vcd.hp.com: 

> I know there is no such thing in Python as C-style casting, but I
> don't understand what idiom is supposed to be used instead.
> 
> Here's the situation: I'm using jabber.py (in a worker thread) to
> recieve and send messages for my application. When messages arrive
> within jabber.py, a message is created of type jabber.Message and
> placed in a Queue by my connection object.
> In the main thread, I have a sub-class of jabber.Message, call it
> "FooMsg" (there are actually going to be multiple derived types of
> jabber.Message). My main application thread has a loop that pulls
> messages off the Queue and calls the appropriate handler inside of the
> message. However, the type of the messages are of type jabber.Message,
> not my FooMsg that contains the handlers:
> 
> 
> class FooMsg( jabber.Message):
> 
>     def Handler( self ):
>         # call approriate handler for message
>         ...
> 
>  while 1:
>     msg = msg_queue.get()  # returns type jabber.Message

you're thinking in a strongly typed manner - but python is dynamicly typed.
the variable msg is not typed. it can hold any object, including subclasses 
of your Message class - no cast needed.
thats why some people speak of 'bind an object to a name' rather than 
'assign a value to a variable'.

>     msg.Handler() # ERROR: Handler() is only in type FooMsg!!!

simple solution: if you want to "Handle" all messages, why not put that 
method in all classes, i.e. in the base class?

or simple too: try to call it and see what happens:

try:
    	msg.Handler()
except AttributeError:
    	pass

or use hasattr:

if hasattr(msg, "Handler"):
    	msg.Handler()


chris
-- 
Chris <cliechti at gmx.net>