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!

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[Rene Pijlman]

Daniel Ortmann:

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

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

 

[Lawrence Oluyede]

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

Take a look at "Thinking in python"
http://www.mindview.net/Books/TIPython

 

[Yermat]

Daniel Ortmann a écrit :

Hello,

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

Thanks!

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]

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

 

[John Roth]

Daniel Ortmann:

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

 

[Peter Hansen]

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

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]

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 .

 

[Aahz]

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

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.

 

[Jeff Epler]

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

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

class Singleton(object):
"""All methods in a Singleton object are made class methods"""
__metaclass__ = SingletonClassMeta

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

An example:.... x = []
.... def f(self): return self.x
.... def g(self, arg): self.x.append(arg)
....

C.f() []
C.g(3)
C.f()

[3]

 

[Aahz]

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.

That's assuming one needs methods, of course; many times, the singleton
is just used for storing attributes.