CODEWITHSUNDEEP
pythoninheritancenew-operatornew-style-class
ychaouche
ychaouche

Reputation: 5092

What happens when you call object.__new__?

  1. Old-style class

    class Person():
    _names_cache = {}
    def __init__(self,name):
        self.name = name
    def __new__(cls,name):
        return cls._names_cache.setdefault(name,object.__new__(cls,name))

ahmed1 = Person("Ahmed")
ahmed2 = Person("Ahmed")
print ahmed1 is ahmed2
print ahmed1
print ahmed2


>>> False
<__main__.Person instance at 0xb74acf8c>
<__main__.Person instance at 0xb74ac6cc>
>>>

  2. New-style class

    class Person(object):
    _names_cache = {}
    def __init__(self,name):
        self.name = name
    def __new__(cls,name):
        return cls._names_cache.setdefault(name,object.__new__(cls,name))

ahmed1 = Person("Ahmed")
ahmed2 = Person("Ahmed")
print ahmed2 is ahmed1
print ahmed1
print ahmed2

>>> True
<__main__.Person object at 0xb74ac66c>
<__main__.Person object at 0xb74ac66c>
>>>

I want to understand what happens in the two versions of the code when I call object.__new__?

Upvotes: 2

Views: 115

Answers (1)

Martijn Pieters
Martijn Pieters

Reputation: 1122512

Old-style classes do not support a __new__ method:

>>> class Foo:
...     def __new__(self, *args, **kw):
...         print 'Nope'
... 
>>> Foo()
<__main__.Foo instance at 0x101f5c908>

__new__ is a new-style class feature only.

Upvotes: 3

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