问题
I'm trying to implement class inherited singleton as described here (Method 2). Going over the question and the extensive chosen answer I tried to implement the following:
class Singleton(object):
_instance = None
def __new__(cls, *args, **kwargs):
if not isinstance(cls._instance, cls):
cls._instance = object.__new__(cls, *args, **kwargs)
cls._instance._initialized = False
return cls._instance
class A(Singleton):
def __init__(self):
print "Init is called"
class B(Singleton):
def __init__(self):
print "Init is called"
As you may guess, whenever I create Class A
I get the same object, but __init__
is called. This is problematic as the Class A
can have all it's members changed due to this.
Doing:
a1 = A()
a2 = A()
print a1 == a2
Will result in:
>> Init is called
>> Init is called
>> True
This question poses a similar issue but I would prefer not to use the solution there as it doesn't include the inheritance and I have at least 2 classes that needs the Singleton
inheritance. I tried to implement the solution here but it didn't work. The solution here works but it involves changing Class A
and Class B
which I would prefer not to.
Is there a way to change the Singleton
implementation so that __init__
won't be called on every creation? (I can't use metaclasses as both A
and B
inherit other classes e.g. abstract classes with their own metaclasses).
Thank you
回答1:
In modern Python this could be done by writting a __init_subclass__
method that could decorate __init__
to make it check cls._instance
before running.
As it is, (i.e. you need it for Python 2 as well) - I think the simpler thing is for __new__
to patch __init__
with a NOP method if the instance already exists:
_nop_init = lambda self, *args, **kw: None
class Singleton(object):
_instance = None
def __new__(cls, *args, **kwargs):
if not isinstance(cls._instance, cls):
cls._instance = object.__new__(cls, *args, **kwargs)
cls._instance._initialized = False
# Python 2 have to check in the cls.__dict__ - Py3 could check the attribute directly:
elif cls.__dict__.get("__init__", None) is not _nop_init:
cls.__init__ = _nop_init
return cls._instance
class A(Singleton):
def __init__(self):
print "Init is called"
class B(Singleton):
def __init__(self):
print "Init is called"
extra info The language mechanism to call __init__
is built-in the __call__
method of type
- the metaclass for all classes in Python. It will call the __new__
and __init__
method of a target class when instantiating it, and thus, with a metaclass, it is easy to control these calls from a custom metaclass. It is also interesting to note that when __new__
does not return an instance of the target class, __init__
is not called. (In this singleton case, the singleton is an instance of te class, and thus it is called).
real world example: last time I coded a singleton, I opted for avoiding re-run __init__
in its own code - sine it is the only singleton in the project, tere was no need for generic code for that in a __new__
:
https://github.com/jsbueno/pythonchain/blob/1f9208dc8bd2741a574adc1bf745d218e4314e4a/pythonchain/block.py#L276
来源:https://stackoverflow.com/questions/59316642/python-class-inherited-singleton-inits-instance-on-every-call