when and how do you use Self?

[Fredrik Lundh]

"Tieche Bruce A MSgt USMTM/AFD" <bruce.tieche at usmtm.sppn.af.mil> wrote:

> Could someone explain (in English) how and when to use self?
>
> I have been reading, and haven't found a good example/explanation

consider a class C:

    >>> class C:
    ...     def method(self):
    ...         print self
    ...
    >>> C
    <class __main__.C at 0x0091D7E0>

you can create unique instances of this class by "calling" the class itself:

    >>> a = C()
    >>> a
    <__main__.C instance at 0x00925FD0>

    >>> b = C()
    >>> b
    <__main__.C instance at 0x00927030>

here, "a" and "b" are two separate objects, that both refer to the same
class object, but are otherwise distinct (the cryptic codes are object
identities).

now, if you call a method on one of those objects, Python will use the
method code from the class, but the method will get a reference to the
instance as its first argument (self).

when you call the method via the "a" object, the method gets a reference
to the "a" object as the first argument:

    >>> a.method()
    <__main__.C instance at 0x00925FD0>

when you call the method via the "b" object, the method gets a reference
to the "b" object as the first argument:

    >>> b.method()
    <__main__.C instance at 0x00927030>

the instance object is usually used to store instance-specific variables
(usually known as "attributes" or "members").  an example:

    >>> class Counter:
    ...     def __init__(self):
    ...         self.value = 0
    ...     def increment(self):
    ...         self.value = self.value + 1
    ...         return self.value
    ...
    >>> a = Counter()
    >>> b = Counter()
    >>> a.increment()
    1
    >>> a.increment()
    2
    >>> a.increment()
    3
    >>> b.increment()
    1

(the __init__ method is automatically called for each new instance)

you can also access the instance attributes "from the outside":

    >>> print a.value
    3
    >>> print b.value
    1

for more on this, see:

http://docs.python.org/tut/node11.html#SECTION0011300000000000000000

</F>