Python 2.7 quick reference

The .__new__() method is called when an instance is created.

Method .__new__() is always a static method (see Section 26.4, “Static methods”), even if you do not specifically make it a static method.

A constructor call for some class C has this general form:

C(*p, **k)

That is, it can have any number of positional arguments and any number of keyword arguments.

The equivalent call to the .__new__() method will look like this:

    def __new__(cls, *p, **k):
        ...              

The first argument cls must be the class being created.

The .__new__() method must call the parent class's .__new__() method to create the instance.

For example, if your class inherits directly from object, you must call:

        object.__new__(cls)

The value returned by that call is the new instance.

In most cases, the .__new__() method will return a new instance of cls, and that class's .__init__() will then be called with that instance as its self argument, and the positional and keyword arguments p and k will be passed to that constructor as well.

>>> class Test(object):
...     def __new__(cls, *p, **k):
...         inst = object.__new__(cls)
...         return inst
...     def __init__(self, *p, **k):
...         print "p={0} k={1}".format(p, k)
... 
>>> t=Test('egg', 'kale', sauce='Bearnaise')
p=('egg', 'kale') k={'sauce': 'Bearnaise'}

However, if the .__new__() method does not return an instance of class cls, the constructor method .__init__() will not be called. This allows the class more control over how new instances are created and initialized. You can return an instance of an entirely different class if you like.