class - Understanding Borg Singleton Pattern in Python -

i saw borg singleton pattern code couldn't head around how new members add singleton object gets appended __shared_state = {} dictionary.

here singleton code

class borg(object):     _shared_state = {}      def __new__(cls,*args,**kwargs):         obj = super(borg,cls).__new__(cls,*args,**kwargs)         obj.__dict__ = cls._shared_state         return obj  class child(borg):     pass  if __name__ == '__main__':     borg = borg()     another_borg = borg()      print borg another_borg     child = child()      borg.only_one_var = "i'm 1 var"         print child.only_one_var 

so question when object borg.only_one_var created how appended _shared_state dictionary

by default each instance gets own dictionary , hence assigning attribute 1 instance doesn't affect other instances.

but can make instance's dictionary point new dict , when internally used there on store items.

in case every time instance being created you're assigning dictionary point borg. _shared_state. hence, instances use same dict fetch , set attributes.

it's equivalent to:

shared = {}  class a(object):     def __init__(self):         self.__dict__ = shared 

demo:

>>> ins = [a() _ in range(5)] >>> ins[0].x = 100 >>> in ins: ...     print(i.x) ... 100 100 100 100 100  >>> shared {'x': 100} 

in cpython assignment of new dictionary __dict__ happens inside pyobject_genericsetdict:

int pyobject_genericsetdict(pyobject *obj, pyobject *value, void *context) {     pyobject **dictptr = _pyobject_getdictptr(obj);     ...     if (!pydict_check(value)) {         pyerr_format(pyexc_typeerror,                      "__dict__ must set dictionary, "                      "not '%.200s'", py_type(value)->tp_name);         return -1;     }     py_incref(value);     py_xsetref(*dictptr, value);  # set dict point new dict     return 0; } 

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note since arrival of key-sharing dictionaries in python 3.3+ dictionaries of instances of same class can share internal state save space.