Properties, Methods, Events
aleaxit at yahoo.com
Tue May 8 13:38:33 CEST 2001
"John Flynn" <transpicio at yahoo.com.au> wrote in message
news:XwJJ6.2678$ZJ.100694 at ozemail.com.au...
> I quite like the simple Properties-Methods-Events paradigm used by Delphi
> (and recently C#).
And Borland C++ and Microsoft Visual C++ as a non-standard extension, FWIW.
It's quite popular on PCs:-).
> My question is: has anyone emulated this in Python, eg. by overriding
> __setattr__ ?
Sure, it IS pretty trivial after all. Here's a typical example:
def __setattr__(self, name, value):
setter = getattr(self, 'set_'+name, None)
if setter is None: self.__dict__[name]=value
def __getattr__(self, name):
if name.startswith('get_'): raise AttributeError
getter = getattr(self, 'get_'+name, None)
if getter is None: raise AttributeError
else: return getter()
def __init__(self, initSpam=None):
self.__spam = initSpam
def set_spam(self, value):
self.__spam = value
wp = WithProps(23)
wp.spam += 100
Any class desiring client code to use it with "property"
syntax just needs to inherit from mixin class Props (and
ensure its __setattr__ and __getattr__ special methods
are called -- it's automatic if it doesn't override them,
if it does it must be sure to delegate to them at need,
as usual) and implement methods named get_XXX and set_XXX
and with the obvious signature for any XXX property it
wants to expose. That's it -- the getters and setters
will now automatically be called when appropriate, upon
client-code use of attribute-syntax.
> I'm going to try this out for fun, but I'd be interested to hear of
> views on how it is best done in Python (or why it shouldn't be done at
I think it's best done in a mixin (why rewrite, or copy
and paste, when you can reuse!). And I think it's most
often better than have client code calling get_foo and
set_bar all over the place! You can freely expose your
attributes to client code, AND switch to getter and setter
methods, *transparently to client code*, if and when you
find out a given attribute needs to be 'wrapped' upon
access and/or setting. "Do the simplest thing that could
possibly work", &c. I believe this possibility is in fact
what makes Python-inappropriate the common Java/C++ idiom
of exposing any attribute you DO need to expose through
getter and setter methods *ALWAYS*, "just in case".
Hmmm, wonder if this deserves a Cookbook recipe...
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