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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
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
  [296] Using a Python dictionary as a holder of object attributes - (2005-04-30)
  [383] Overloading of operators on standard objects in Python - (2005-07-19)
  [477] Class, static and unbound variables - (2005-10-25)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [903] Pieces of Python - (2006-10-23)
  [964] Practical polymorphism in action - (2006-12-04)
  [1146] __new__ v __init__ - python constructor alternatives? - (2007-04-14)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1517] Python - formatting objects - (2008-01-24)
  [1644] Using a utility method to construct objects of different types - Python - (2008-05-17)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [1819] Calling base class constructors - (2008-10-03)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2409] TypeError: super() argument 1 must be type, not classobj (Python) - (2009-09-18)
  [2485] How do I set up a constant in Python? - (2009-10-31)
  [2693] Methods that run on classes (static methods) in Python - (2010-03-25)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2720] Multiple inheritance in Python - complete working example - (2010-04-14)
  [2722] Mixins example in Python - (2010-04-14)
  [2764] Python decorators - your own, staticmethod and classmethod - (2010-05-14)
  [2785] The Light bulb moment when people see how Object Orientation works in real use - (2010-05-28)
  [2889] Should Python classes each be in their own file? - (2010-07-27)
  [2905] Defining static methods in Python - (2010-08-05)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [3442] A demonstration of how many Python facilities work together - (2011-09-16)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3524] Metaclasses (Python) and Metatables (Lua) - (2011-11-17)
  [3796] Backquote, backtic, str and repr in Python - conversion object to string - (2012-07-05)
  [3887] Inheritance, Composition and Associated objects - when to use which - Python example - (2012-10-10)
  [4028] Really Simple Class and Inheritance example in Python - (2013-03-04)
  [4094] Python Properties - how and why - (2013-05-18)
  [4344] Python base and inherited classes, test harness and unit testing - new examples - (2014-12-07)
  [4356] Object factories in C++, Python, PHP and Perl - (2014-12-19)
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)
  [4410] A good example of recursion - a real use in Python - (2015-02-01)
  [4449] Spike solution, refactoring into encapsulated object methods - good design practise - (2015-03-05)
  [4450] Deciding whether to use parameters, conditional statements or subclasses - (2015-03-05)
  [4541] Setting up and tearing down with the Python with keyword - (2015-10-16)
  [4649] Object and Static methods - what is the difference; example in Python 3 - (2016-02-17)
  [4717] with in Python - examples of use, and of defining your own context - (2016-11-02)
  [4718] Defining an object that is a modified standard type in Python - (2016-11-02)
  [4719] Nesting decorators - (2016-11-02)


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Some other Articles
Santa announcement, 5th December 2010, Melksham
Lots of ways of doing it in Perl - printing out answers
Increment operators for counting - Perl, PHP, C and others
What will we be teaching in six years?
A list of special method and attribute names in Python
How will we present courses over the coming years?
Looking forward - the next 3000
2999 - looking back
Using an exception to initialise a static variable in a Python function / method
3D graphics - web site usage - simple matplotlib and python example
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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