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Metaclasses (Python) and Metatables (Lua)

"A Metaclass (Python) or Metatable (Lua) adds extra code / data / behaviour to an object, table, or class."

What does that mean? It means that I can define a behavior that I want to be shared, that I want to define at run-time, that I want to amend ... in a separate module / zone / table from my main class. It's flexible, powerful. In Lua, it's a vital feature if you're looking to write code using an Object Oriented paradigm, as your metatable provides you with your methods which are able to be shared between objects. In Python, it's rather esoteric - yes, useful but you'll tend to write code which all but the most hardened of programmers will end up scratching their heads over.

Anyway - new Python examples [here]. Lua example [here], and a further description with examples [here]. That latter example makes good reading on the principles for Python users as well as Lua users.

Our Lua Courses all cover Metatables. Our Python Courses do not at present cover metaclasses; your instructor will be happy to answer questions on them, demonstrate them if you ask, cover them on extra days or after class. He will raise them if they look to be useful based on your description of your applications .. but for the most part they're something that few Python application programmers will be using, however internally important they are - and it's part of a trainer's job to pick out the important things for YOU!
(written 2011-11-17, updated 2011-12-03)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
U107 - Object Orientation - the Lua way
  [4753] Lua, Tcl, Python, C and C++ courses - at our Melksham HQ or on your site - forward from July 2017 - (2017-07-02)
  [4573] Classic style OO code - in Lua - (2015-11-05)
  [4572] Tables with values and code in Lua - looks like an object? - (2015-11-05)
  [4273] Dot or Colon separator between table name and member in Lua - what is the difference? - (2014-05-06)
  [4248] Metatables, Metamethods, classes and objects in Lua - (2014-03-18)
  [4117] Is Lua an Object Oriented language? - (2013-06-15)
  [3730] What is a metatable? How do I set one up? How do I use them? Lua - (2012-05-12)
  [3727] Using Lua tables as objects - (2012-05-11)
  [3694] Special __ methods you can use in Lua metatables - (2012-04-12)
  [3683] Weak references in Lua - what are they, and why use them? - (2012-04-04)
  [3396] Tables as Objects in Lua - a gentle introduction to data driven programming - (2011-08-17)
  [3142] Private and Public - and things between - (2011-01-22)
  [2710] __index and __newindex in Lua - metatable methods - (2010-04-05)
  [2703] Lua Metatables - (2010-04-02)
  [2701] Is Lua an Object Oriented language? - (2010-04-01)
  [2455] Lua examples - coroutines, error handling, objects, etc - (2009-10-15)
  [2359] A fresh example - objects the Lua way - (2009-08-13)
  [2318] For Lua Programmers AND for Town Planners - (2009-08-02)
  [1819] Calling base class constructors - (2008-10-03)
  [1743] First class functions in Lua lead to powerful OO facilities - (2008-08-07)
  [1699] If you are learning Lua, here are some more examples - (2008-07-06)
  [1692] Towards Object Oriented Programming in Lua - (2008-06-30)

Y112 - Python - Objects - Intermediate
  [4719] Nesting decorators - (2016-11-02)
  [4718] Defining an object that is a modified standard type in Python - (2016-11-02)
  [4717] with in Python - examples of use, and of defining your own context - (2016-11-02)
  [4649] Object and Static methods - what is the difference; example in Python 3 - (2016-02-17)
  [4541] Setting up and tearing down with the Python with keyword - (2015-10-16)
  [4450] Deciding whether to use parameters, conditional statements or subclasses - (2015-03-05)
  [4449] Spike solution, refactoring into encapsulated object methods - good design practise - (2015-03-05)
  [4410] A good example of recursion - a real use in Python - (2015-02-01)
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)
  [4356] Object factories in C++, Python, PHP and Perl - (2014-12-19)
  [4344] Python base and inherited classes, test harness and unit testing - new examples - (2014-12-07)
  [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)
  [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)
  [3002] A list of special method and attribute names in Python - (2010-10-17)
  [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)
  [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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Some other Articles
December courses - PHP, Python, Perl - and a weekend Lua course
Melksham - changing times, an opportunity for the future
Melksham has a Heart - Montage picture for sale
Melksham has a heart - come and see the pictures. Sunday, 11 to 3, Assembly Hall
Metaclasses (Python) and Metatables (Lua)
When competition is not a good idea - Melksham Bus Absurdity
Networking - North and West Wilts FSB Style
Emergency First Aid at Work (HSE) training in Melksham - places available for 19.1.2012
Quintessentially English - scenes from our Wiltshire town
Python - current versions and implementations (CPython, Jython, IronPython etc)
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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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