On Wed, 12 Jan 2005, Barnaby Scott wrote:
> I was wondering how you can get an instance of a class to change itself
> into something else (given certain circumstances), but doing so from
> within a method. So:
>
> class Damson:
> def __str__(self):
> return 'damson'
>
> def dry(self):
> self = Prune()
>
> class Prune:
> def __str__(self):
> return 'prune'
>
> weapon = Damson()
> weapon.dry()
> print weapon
Hi Scott,
The issue with that, as you know, is that it conflicts with the way local
variables work in functions. For example:
###
>>> def strip_and_print(x):
... x = x.strip()
... print x
...
>>> message = " hello "
>>> strip_and_print(message)
hello
>>> message
' hello '
###
Our reassignment to 'x' in strip_and_print has no effect on "message"'s
binding to " hello ". That's how we're able to use local variables as
black boxes.
For the same reasons, the reassignment to 'self in:
> class Damson:
> def __str__(self):
> return 'damson'
>
> def dry(self):
> self = Prune()
is limited in scope to the dry() method.
But can you do what you're trying to do with object composition? That is,
would something like this work for you?
###
class Damson:
def __init__(self):
self.state = DamsonState()
def __str__(self):
return str(self.state)
def dry(self):
self.state = PruneState()
class DamsonState:
def __str__(self):
return "damson"
class PruneState:
def __str__(self):
return 'prune'
###
This structuring allows us to switch the way that str() applies to Damson.
The OOP Design Pattern folks call this the "State" pattern:
http://c2.com/cgi/wiki?StatePattern
If you have any questions, please feel free to ask!
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