On 9/20/06, Alan Gauld <[EMAIL PROTECTED]> wrote:
> > This all seems to work okay.
> >
> > I want the assignment operator ('=')
>
> There is no assignment operator in Python, assignment is a
> binding of an object to a name.
as others have mentioned, the assignment operator is used to assign an
object to a name in the current namespace... IOW, you cannot "overload
the assignment operator."
__coerce__() is used for arithmetic operations, so this won't work
either. what you really want to do is to allow (validated) access to
mydie.value (e.g., self.value).
instead of "mydie = 5" -- which is taking the 'mydie' name and
reassigning it to the integer object that has a value of 5 (thus
losing the reference to your instance object, decrementing its
reference count, etc.), you want to allow the user to do something
like:
mydie.value = 5
... BUT, you want that value to be validated before it actually
assigns it to the mydie.value instance attribute. here is where
properties become useful. you can create a getter, setter, and even a
deleter and doc string if you want. here's how you use it... add the
following to your class:
def get_value(self):
return self.__value
# pretty much your set() method
def set_value(self, newval):
assert isinstance(newval, int), 'must be an int!'
assert 1 <= newval <= self.n, 'invalid value!'
self.__value = newval
value = property(get_value, set_value, doc='value of mydie')
-----
in actuality, the value is stored in self.__value, but access is via
self.value. this should give you what you need provided you are happy
with using "mydie.value = ..." vs. "mydie = ...", the latter of which
will never work the way you want. with the addition of the above
code, you can leave __init__() and roll() alone as the self.value =
... assignment will still call your property methods to do the
assigning. (also note that roll() does not have to return self.value
unless that is desired.)
now, even with properties, the bad news is that someone can be sneaky
and do something like "mydie.set_value(200)" to try and get around
doing "mydie.value = 200". in other words, you cannot restrict access
to the property methods.
the good news is that there is a workaround to this. i have an
example in one of the newly-written sections in (the 2nd ed of) my
book that was inspired by the following cookbook recipe:
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/205183
the crux of this recipe is that it is very much like using a closure
to get your cake and eat it too. you stick your getter (and perhaps
setter) into another scope which is then rendered inaccessible to the
instance.
hope this helps!
-- wesley
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
"Core Python Programming", Prentice Hall, (c)2007,2001
http://corepython.com
wesley.j.chun :: wescpy-at-gmail.com
python training and technical consulting
cyberweb.consulting : silicon valley, ca
http://cyberwebconsulting.com
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