The constructor you'll normally override (and call when you create an object) in Python is called __init__, but there is also a method called __new__ available to the class author.

• If __new__ is defined on a class, it is called in preference to __init__
  
• With __new__ you return the object you have constructed, whereas with __init__ there is an implicit return of the object in the current class.

Here's a code sample showing three classes with a mixture of different constructors.

class A(object):
  def __new__(cls, *args, **kwds):
    print "one"
    print "A.__new__", args, kwds
    return object.__new__(B, *args, **kwds)
  def __init__(cls, *args, **kwds):
    print "two"
    print "A.__init__", args, kwds
class B(object):
  def __new__(cls, *args, **kwds):
    print "three"
    print cls
    print B
    print "B.__new__", args, kwds
    return object.__new__(cls, *args, **kwds)
  def __init__(cls, *args, **kwds):
    print "four"
    print "B.__init__", args, kwds
class C(object):
  def __init__(cls, *args, **kwds):
    print "five"
    print "C.__init__", args, kwds
print C()
print "====================="
print A()
print "====================="
print B()

And here's the result of running that code:

40:~/ndfi grahamellis$ python newvinit
five
C.__init__ () {}
<__main__.C object at 0x6fc10>
=====================
one
A.__new__ () {}
<__main__.B object at 0x6fc10>
=====================
three
<class '__main__.B'>
<class '__main__.B'>
B.__new__ () {}
four
B.__init__ () {}
<__main__.B object at 0x6fc10>
40:~/ndfi grahamellis$

In class C, there's only an __init__ so that's run. Class A has both, and so the __new__ take precedence. In class B, the __new__ takes precendence but it makes an apparent recursive call that's been diverted to __init__ as something of a safeguard - for illustrative purposes, this one!

When would you use __new__?

1. When you want you constructor to return an object of a different type to the class in which it is defined. For example, you have a class "animal" with subclasses "farmanimal" and "pet" and you want the animal cosntructor to be able to examine the data passed in to it and return an animal ... OR a farmanimal OR a pet depending on that data.

2. It has been suggested that experienced base class programmers override __new__ to discourage programmers who subclass them from overwriting their constructors. Personally, I would suggest that a good training course is better than this form of security through obscurity.
(written 2007-04-14 07:20:47)

 Y112 - Python - Objects - Intermediate
  [2485] How do I set up a constant in Python? - (2009-10-31)
  [2409] TypeError: super() argument 1 must be type, not classobj (Python) - (2009-09-18)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [1819] Calling base class constructors - (2008-10-03)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [1644] Using a utility method to construct objects of different types - Python - (2008-05-17)
  [1517] Python - formatting objects - (2008-01-24)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [964] Practical polymorphism in action - (2006-12-04)
  [903] Pieces of Python - (2006-10-23)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [477] Class, static and unbound variables - (2005-10-25)
  [383] Overloading of operators on standard objects in Python - (2005-07-19)
  [296] Using a Python dictionary as a holder of object attributes - (2005-04-30)

Y212 - Further advanced features of Python
  [2616] Defining a static method - Java, Python and Ruby - (2010-02-01)
  [2123] Using Python with OpenOffice - (2009-04-09)
  [1555] Advanced Python, Perl, PHP and Tcl training courses / classes - (2008-02-25)
  [1148] Python decorators - wrapping a method call in extra code - (2007-04-15)
  [1140] Python GTK - Widget, Packing, Event and Feedback example - (2007-04-09)
  [235] Preparation for a day's work - (2005-03-04)

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