an example of a singleton design pattern in python?

[Daniel Ortmann]

Hello,

Does anyone have an example of a singleton design pattern in python?

Thanks!

--
Daniel Ortmann, LSI Logic, 3425 40th Av NW, Suite 200, Rochester MN 55901
work: Daniel.Ortmann@ lsil.com / 507.535.3861 / 63861 int / 8012.3861 gdds
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[Rene Pijlman]

Re: an example of a singleton design pattern in python?

Daniel Ortmann:[color=blue]
>Does anyone have an example of a singleton design pattern in python?[/color]

http://aspn.activestate.com/ASPN/search?query=singleton&section=PYTHONCKBK&type=Subsection

--
René Pijlman

[Lawrence Oluyede]

Re: an example of a singleton design pattern in python?

Daniel Ortmann <dortmann@lsil. com> writes:
[color=blue]
> Does anyone have an example of a singleton design pattern in python?[/color]

Take a look at "Thinking in python"

http://www.mindview.net/Books/TIPython

--
Lawrence "Rhymes" Oluyede

http://loluyede.blogspot.com

[Yermat]

Re: an example of a singleton design pattern in python?

Daniel Ortmann a écrit :
[color=blue]
> Hello,
>
> Does anyone have an example of a singleton design pattern in python?
>
> Thanks!
>[/color]

http://www.python.org/cgi-bin/moinmoin/SingletonProgramming

Look for links too on page :

http://www.python.org/cgi-bin/moinmoin/PatternProgramming

Hope it will help !
Yermat

[Ville Vainio]

Re: an example of a singleton design pattern in python?

>>>>> "Daniel" == Daniel Ortmann <dortmann@lsil. com> writes:

Daniel> Hello, Does anyone have an example of a singleton design
Daniel> pattern in python?


This trivial snippet might be enough for your needs:

class Foo():
pass

_inst = None

def fooinstance():
if not _inst:
_inst = Foo()
return _inst

--
Ville Vainio http://tinyurl.com/2prnb

[John Roth]

Re: an example of a singleton design pattern in python?


"Rene Pijlman" <reply.in.the.n ewsgroup@my.add ress.is.invalid > wrote in
message news:645r00hics lg2qn55sncentdk 8alnbevab@4ax.c om...[color=blue]
> Daniel Ortmann:[color=green]
> >Does anyone have an example of a singleton design pattern in python?[/color]
>
>[/color]

http://aspn.activestate.com/ASPN/search?query=singleton&section=PYTHONCKBK&type=Subsection

The "cannonical " singleton for new style classes is in:

http://www.python.org/2.2.3/descrintro.html

See the section on overriding the __new__() method.

John Roth[color=blue]
>
> --
> René Pijlman[/color]

[Peter Hansen]

Re: an example of a singleton design pattern in python?

Ville Vainio wrote:[color=blue]
>[color=green][color=darkred]
> >>>>> "Daniel" == Daniel Ortmann <dortmann@lsil. com> writes:[/color][/color]
>
> Daniel> Hello, Does anyone have an example of a singleton design
> Daniel> pattern in python?
>
> This trivial snippet might be enough for your needs:
>
> class Foo():
> pass
>
> _inst = None
>
> def fooinstance():
> if not _inst:
> _inst = Foo()
> return _inst[/color]

Looks like at least two errors, one being the parentheses on the
class name, and the other being a failure to flag _inst as being
global...

-Peter

[Ville Vainio]

Re: an example of a singleton design pattern in python?

[color=blue][color=green]
>> This trivial snippet might be enough for your needs:
>>
>> class Foo(): pass
>>
>> _inst = None
>>
>> def fooinstance(): if not _inst: _inst = Foo() return _inst[/color][/color]

peter> Looks like at least two errors, one being the parentheses
peter> on the class name, and the other being a failure to flag
peter> _inst as being global...

True. I guess the idea got through anyway. It was late :-P.

--
Ville Vainio http://tinyurl.com/2prnb

[Aahz]

Re: an example of a singleton design pattern in python?

In article <hc63caapdui.fs f@praxis.lsil.c om>,
Daniel Ortmann <dortmann@lsil. com> wrote:[color=blue]
>
>Does anyone have an example of a singleton design pattern in python?[/color]

It appears that none of the other responses mentioned the two simplest
techniques:

* Within a single module, use a global class as your singleton object.

* Across modules, use a module as your singleton object.
--
Aahz (aahz@pythoncra ft.com) <*> http://www.pythoncraft.com/

A: No.
Q: Is top-posting okay?

[Jeff Epler]

Re: an example of a singleton design pattern in python?

On Wed, Jan 21, 2004 at 07:38:26PM -0500, Aahz wrote:[color=blue]
> * Within a single module, use a global class as your singleton object.[/color]

This is a little bit inconvenient, because you must either mark all
methods as staticmethod (and refer to the class by name), or you must
mark all methods as classmethod.

This metaclass can help. It makes all functions into staticmethods at
the time the class is created:

from types import FunctionType

class SingletonClassM eta(type):
def __new__(cls, name, bases, dict):
for k, v in dict.iteritems( ):
if isinstance(v, FunctionType):
dict[k] = classmethod(v)
return (super(Singleto nClassMeta, cls).
__new__(cls, name, bases, dict))

class Singleton(objec t):
"""All methods in a Singleton object are made class methods"""
__metaclass__ = SingletonClassM eta

Probably __init__ should be made to raise an exception (RuntimeError,
"Singletons cannot be instantiated")) ...

An example:[color=blue][color=green][color=darkred]
>>> class C(Singleton):[/color][/color][/color]
.... x = []
.... def f(self): return self.x
.... def g(self, arg): self.x.append(a rg)
....[color=blue][color=green][color=darkred]
>>> C.f()[/color][/color][/color]
[][color=blue][color=green][color=darkred]
>>> C.g(3)
>>> C.f()[/color][/color][/color]
[3]

[Aahz]

Re: an example of a singleton design pattern in python?

In article <mailman.625.10 74734700.12720. python-list@python.org >,
Jeff Epler <jepler@unpytho nic.net> wrote:[color=blue]
>On Wed, Jan 21, 2004 at 07:38:26PM -0500, Aahz wrote:[color=green]
>>
>> * Within a single module, use a global class as your singleton object.[/color]
>
>This is a little bit inconvenient, because you must either mark all
>methods as staticmethod (and refer to the class by name), or you must
>mark all methods as classmethod.[/color]

That's assuming one needs methods, of course; many times, the singleton
is just used for storing attributes.
--
Aahz (aahz@pythoncra ft.com) <*> http://www.pythoncraft.com/

A: No.
Q: Is top-posting okay?