Many developers believe that
__init__ is the first method invoked in the life-cycle of an object. When we create an object or instantiate a class, the first method that gets called
__new__, which actually creates an objects and then passed the arguments to to
__init__, the initializer.
__new__ gets invoked before
The first argument to
__new__ is the class and followed by other arguments to be passed to
__new__ is very rarely called, but is used when sub classing immutable types like tuple. The responsibility of
__new__ is to return the instance of the class. There are cases where instances are created without calling
__init__ for example, loading from pickle where as there is no way to create an instance without calling
__new__. Don't get me wrong, we can skip calling
__new__ of base class in some cases, but that is not in the scope of this article.
Let us say, we want a type to convert meter to inch. In this case, we can subclass float and override the
__new__ method as below.
class meter(float): "Convert from inch to meter" def __new__(cls, arg=0.0): return float.__new__(cls, arg*0.0254)
Though this is not the right method for unit conversion, let us use it.
>>> print inch(12) 0.3048
There are some points regarding
__new__is a static method. When defining it, you don't need to use the phrase "
__new__ = staticmethod(__new__)"
__new__must be a class; the remaining arguments are the arguments as seen by the constructor call.
__new__method that overrides a base class's
__new__method may call that base class's
__new__method. The first argument to the base class's
__new__method call should be the class argument to the overriding
__new__method, not the base class; if you were to pass in the base class, you would get an instance of the base class.
__new__method must call its base class's
__new__method; that's the only way to create an instance of your object. The subclass
__new__can do two things to affect the resulting object: pass different arguments to the base class
__new__, and modify the resulting object after it's been created
__new__must return an object. There's nothing that requires that it return a new object that is an instance of its class argument, although that is the convention. If you return an existing object, the constructor call will still call its
__init__method. If you return an object of a different class, its
__init__method will be called. If you forget to return something, Python will unhelpfully return None, and your caller will probably be very confused.
__new__may return a cached reference to an existing object with the same value; this is what the int, str and tuple types do for small values. This is one of the reasons why their
__init__does nothing: cached objects would be re-initialized over and over. (The other reason is that there's nothing left for
__new__returns a fully initialized object.)
__init__method and leave
__init__to accept the new signature. However, most built-in types ignore the arguments to the method they don't use; in particular, the immutable types (int, long, float, complex, str, unicode, and tuple) have a dummy
__init__, while the mutable types (dict, list, file, and also super, classmethod, staticmethod, and property) have a dummy
__new__. The built-in type 'object' has a dummy
__new__and a dummy
__init__(which the others inherit). The built-in type 'type' is special in many respects; see the section on metaclasses.
__new__, but is handy to know anyway.) If you subclass a built-in type, extra space is automatically added to the instances to accomodate
__dict__is not initialized until you use it though, so you shouldn't worry about the space occupied by an empty dictionary for each instance you create.) If you don't need this extra space, you can add the phrase "
__slots__ = " to your class. (See above for more about
__new__is a static method, not a class method. I initially thought it would have to be a class method, and that's why I added the classmethod primitive. Unfortunately, with class methods, upcalls don't work right in this case, so I had to make it a static method with an explicit class as its first argument. Ironically, there are now no known uses for class methods in the Python distribution (other than in the test suite). I might even get rid of classmethod in a future release if no good use for it can be found!
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