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A list of special method and attribute names in Python

If I write
  a = b + c
in Python, I'm really writing
  a = b.__add__(c)
(see source code example [here]

In other words, every variable is an object and every operator is a method. It's just the icing on the cake that makes the language as powerful as it is - with the clever engine usually hidden just below the covers.

However - if you're defining your own objects, you need to define your own operators too, and I have often been asked if I have a complete list of all the special method names and what they do. Well - here's an overview of what they do in Python 2.7 (a few extras get added from time to time); you can find an authoritative description of each in the Python Language Reference Manual - data model chapter

Update - Feb 2011 - I've long been looking for a good description page for all of these magic methods - just come across one on Rafe Kettler's site.

Special Method Names

__new__ True constructor - usually wraps __init__
__init__ Object constructor
__call__
__getattr__ Handling object attributes [example] and [calling example]
__setattr__
__delattr__
__getattribute__
__getitem__ subscripting with [..] [example]
__setitem__
__delitem__
__del__ Destructor / wrapper around del
__repr__ Convert to Python source
__str__ Convert to printable string
__cmp__ Compare two objects
__lt__ Less Than
__gt__ Greater Than
__eq__ Equal to
__ne__ Not Equal to
__le__ Less Than or Equal
__ge__ Greater Than or Equal
__hash__ Calculate an (integer) hash value
__nonzero__ Is it nonzero
__unicode__ Convert to Unicode String
__get__
__set__
__delete__
__instancecheck__ isinstance builtin function
__subclasscheck__
__getslice__ Working with slices .. [..:..][example]
__setslice__
__delslice__
__len__ len building function
__add__ + [example]
__mul__ *
__contains__
__coerce__
__iter__
__reversed__
__sub__ -
__div__ /
__floordiv__ //
__mod__ %
__divmod__
__pow__ ^
__lshift__ <<
__rshift__ >>
__and__ &
__xor__ ~
__or__ |
__truediv__ __future__ > /
__radd__ "r" methods operate on object to right
__rmul__
__rsub__
__rdiv__
__rtruediv__
__rfloordiv__
__rmod__
__rdivmod__
__rpow__
__rlshift__
__rrshift__
__rand__
__rxor__
__ror__
__iadd__ +=
__imul__ *=
__isub__ -=
__idiv__ /=
__itruediv__ __future__ > /=
__ifloordiv__ //=
__imod__ %=
__ipow__ ^=
__ilshift__ <<=
__irshift__ >>=
__iand__ &=
__ixor__
__ior__ |=
__neg__ monadic -
__pos__ monadic +
__abs__ abs built in function
__invert__ monadic ~
__complex__ complex built in function
__int__ int built in function
__long__ long built in function
__float__ float built in function
__oct__ oct built in function
__hex__ hex built in function
__index__
__enter__
__exit__

Special Attribute Names

These are not callable methods - rather they are built in variables. Some are readonly and other may be modified.

__doc__ or func_doc Documentation String
__name__ or func_name Name of method
__module__ Name of module
__self__
__func__
__dict__
__file__
__bases__
__slots__
__weakref__
__metaclass__
func_defaults Defaulted parameters
func_code
func_globals Globals used
func_dict Funtion namespace
func_closure

That's quite an extensive list - and it makes you realise just how much the authors of language wrappers such as numpy have had to do, and why there's so a time lag between a new version of Python being released and upgrades to numpy

We cover the use of a number of these on our Python Programming Course - although not all of them; some are niche and easily understood from the manual ... and once you have worked out __add__ you can probably work out __sub__, __mul__ and __div__ for yourself ;-)
(written 2010-10-17, updated 2011-02-13)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y112 - Python - Objects - Intermediate
  [4094] Python Properties - how and why - (2013-05-18)
  [4028] Really Simple Class and Inheritance example in Python - (2013-03-04)
  [3887] Inheritance, Composition and Associated objects - when to use which - Python example - (2012-10-10)
  [3796] Backquote, backtic, str and repr in Python - conversion object to string - (2012-07-05)
  [3524] Metaclasses (Python) and Metatables (Lua) - (2011-11-17)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3442] A demonstration of how many Python facilities work together - (2011-09-16)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [2905] Defining static methods in Python - (2010-08-05)
  [2889] Should Python classes each be in their own file? - (2010-07-27)
  [2785] The Light bulb moment when people see how Object Orientation works in real use - (2010-05-28)
  [2764] Python decorators - your own, staticmethod and classmethod - (2010-05-14)
  [2722] Mixins example in Python - (2010-04-14)
  [2720] Multiple inheritance in Python - complete working example - (2010-04-14)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2693] Methods that run on classes (static methods) in Python - (2010-03-25)
  [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)
  [1146] __new__ v __init__ - python constructor alternatives? - (2007-04-14)
  [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)


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