[Tutor] Deleting singleton objects

[dman]

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On Thu, May 23, 2002 at 01:15:33PM +0930, BELSEY, Dylan wrote:
=20
| 	BTW is there a more efficient or standard way of implementing a
| singleton in Python, that is a lot "cleaner".

Yeah, normally singletons aren't destroyed at all during the lifetime
of a program.  (see the GoF book)

Here's how it is typically done in Python :


class _single_class :
    # whatever the implementation is
    pass

# make the one and only instance
_single =3D _single_class()

# we don't need the class hanging around to tempt people to misbehave
del _single_class

# a function to hide the fact that the class is a singleton
def single() : return _single


This does 3 things :
    1)  uses underscores to indicate, via convention, private names

    2)  creates the instance at the module level (since modules exist
        in python, though C++ and Java lack them)

    3)  destroys the (easy to get to) handle to the class so that it
        can't be constructed any more

    4)  Uses a function to hide the fact that the class is a
        singleton.  This is another area where Python shines -- in
        C++/Java, instantiating a class does not look like a function
        call, so this sort of thing can't be done transparently.

HTH,
-D

--=20

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