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# Copyright (C) 2006-2011 Dan Pascu. See LICENSE for details.
#
"""Types and metaclasses"""
from __future__ import absolute_import
__all__ = ['Singleton', 'NullType']
from new import instancemethod
from application.python.decorator import preserve_signature
class Singleton(type):
"""Metaclass for making singletons"""
def __init__(cls, name, bases, dic):
from types import FunctionType, UnboundMethodType
if type(cls.__init__) is UnboundMethodType:
initializer = cls.__init__
elif type(cls.__new__) is FunctionType:
initializer = cls.__new__
else:
def initializer(self, *args, **kw): pass
@preserve_signature(initializer)
def instance_creator(cls, *args, **kw):
key = (args, tuple(sorted(kw.iteritems())))
try:
hash(key)
except TypeError:
raise TypeError("cannot have singletons for classes with unhashable arguments")
if key not in cls.__instances__:
cls.__instances__[key] = super(Singleton, cls).__call__(*args, **kw)
return cls.__instances__[key]
super(Singleton, cls).__init__(name, bases, dic)
cls.__instances__ = {}
cls.__instantiate__ = instancemethod(instance_creator, cls, type(cls))
def __call__(cls, *args, **kw):
return cls.__instantiate__(*args, **kw)
class NullTypeMeta(type):
def __init__(cls, name, bases, dic):
super(NullTypeMeta, cls).__init__(name, bases, dic)
cls.__instance__ = super(NullTypeMeta, cls).__call__()
def __call__(cls, *args, **kw):
return cls.__instance__
class NullType(object):
"""Instances of this class always and reliably "do nothing"."""
__metaclass__ = NullTypeMeta
__name__ = 'Null'
def __init__(self, *args, **kw): pass
def __call__(self, *args, **kw): return self
def __reduce__(self): return (self.__class__, (), None)
def __repr__(self): return self.__name__
def __str__(self): return self.__name__
def __len__(self): return 0
def __nonzero__(self): return False
def __eq__(self, other): return self is other
def __ne__(self, other): return self is not other
def __contains__(self, item): return False
def __getattr__(self, name): return self
def __setattr__(self, name, value): pass
def __delattr__(self, name): pass
def __getitem__(self, key): return self
def __setitem__(self, key, value): pass
def __delitem__(self, key): pass
def __get__(self, obj, type): return self
def __set__(self, obj, value): pass
def __delete__(self, obj): pass
def __enter__(self): return self
def __exit__(self, exc_type, exc_value, traceback): pass
def __iter__(self): return self
def next(self): raise StopIteration
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