[Edu-sig] a question on java attributes vs. python

[Leif Johnson]

On Tue, 29 Apr 2003, ali wrote:

> is there a python counterpart for the declaration of class attributes in
> java like static, private, protected???

AFAIK there's no protected attribute. however, here's a class definition
with a static and a private variable :

class MyObject:
    ## this is shared among all class instances (static). it's accessed
    ## using self.instances

    instances = 0

    def __init__(self, attr):
        self.instances += 1

        ## this is a private variable because it starts with '__'. within
        ## this class instance it's accessed using self.__myvar, but the
        ## interpreter does some name mangling so external objects can't
        ## have direct access, for example :
        ##
        ## >>> obj = MyObject('a')
        ## >>> print obj.__myvar # (error)

        self.__myvar = attr

    def get_myvar(self): return self.__myvar
    def set_myvar(self, val): self.__myvar = val

    def instance_info(self):
        print self.instances, 'instances of MyObject have been created'

As I learn more Python I'm increasingly pleased with it. In this case, you
can think of all function definitions in a class as being static, just like
the static 'instances' variable above ; the function and data symbols are
simply shared among MyObject instances (although this is a bit more complex
with inheritance involved ; that's why class functions require the 'self'
parameter first, from what I understand it's how the static function code
is tied to a real class instance when called).

I think this is another good example of how Python and Scheme are really
not so different---functions are data are functions.

leif

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Leif Morgan Johnson . http://ambient.2y.net/leif/
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