Good Code Series (i): Lazyobject

site-packages/django/utils/functional.py

1 defNew_method_proxy (func):2     defInner (self, *args):3         ifself_wrapped isEmpty:4 Self._setup ()5         returnFunc (self._wrapped, *args)6     returnInner7 8 classLazyobject (object):9     """Ten A Wrapper for another class The can is used to delay instantiation of the wrapped class. One  A by subclassing, you had the opportunity to intercept and alter the - instantiation. If you don ' t need to do, use Simplelazyobject. -     """ the  -     #Avoid Infinite recursion when tracing __init__ (#19456). -_wrapped =None -  +     def __init__(self): -self._wrapped =Empty +  A     __getattr__=new_method_proxy (getattr) at  -     def __setattr__(self, Name, value): -         ifName = ="_wrapped": -             #Assign to __dict__ to avoid infinite __setattr__ loops. -Self.__dict__["_wrapped"] =value -         Else: in             ifself._wrapped isEmpty: - Self._setup () to setattr (self._wrapped, name, value) +  -     def __delattr__(self, name): the         ifName = ="_wrapped": *             RaiseTypeError ("can ' t delete _wrapped.") $         ifself._wrapped isEmpty:Panax Notoginseng Self._setup () - delattr (self._wrapped, name) the  +     def_setup (self): A         """ the must is implemented by subclasses to initialize the wrapped object. +         """ -         RaiseNotimplementederror ('subclasses of Lazyobject must provide a _setup () method') $  $     #Because We have messed with __class__ below, we confuse pickle as to what -     #class we are pickling. We ' re going to initialize the wrapped -     #object to successfully pickle it, so we might as well just pickle the the     #wrapped object since they ' re supposed to act the same. -     #Wuyi     #Unfortunately, if we try to simply act like the wrapped object, the ruse the     #When would break , Pickle gets our ID (). Thus we end up with pickle -     #thinking, in effect, that we is a distinct object from the wrapped Wu     #object, but with the same __dict__. This can cause problems (see #25389). -     # About     #So instead, we define our own __reduce__ method and custom Unpickler. We $     #Pickle The wrapped object as the Unpickler ' s argument, so that pickle -     #Would pickle it normally, and then the Unpickler simply returns its -     #argument. -     def __reduce__(self): A         ifself._wrapped isEmpty: + Self._setup () the         return(Unpickle_lazyobject, (self._wrapped,)) -  $     #We have the explicitly override __getstate__ so, older versions of the     #pickle don ' t try to pickle the __dict__ (which in the case of a the     #Simplelazyobject may contain a lambda). The value would end up being the     #ignored by our __reduce__ and custom Unpickler. the     def __getstate__(self): -         return {} in  the     def __deepcopy__(Self, Memo): the         ifself._wrapped isEmpty: About             #We have a to use type (self), not self.__class__, because the the             #latter is proxied. theresult =type (self) () theMemo[id (self)] =result +             returnresult -         returnCopy.deepcopy (self._wrapped, Memo)

Good Code Series (i): Lazyobject