I call my father "Dad" but everyone else calls him "Norman". Same person, different name.

If Dad puts on the hat that he's got from the Christmas cracker before he eats his Christmas pudding, then everyone else sees that Norman has put on his hat before eating his Christmas pudding. Two names for one and the same person.


The same principle applies to the copying of objects in Python (and Java and PHP 5 and other object oriented languages); an assignment statement is just the addition of an extra name to the same object.

Let's see an example - in Python:


When I run that:


By changing "pudding" I've also changed "pie".


In contrast, if I copy a list in Perl (and in PHP 4, Tcl and other languages which are not 100% object oriented), I duplicate the data. Example in Perl:


Which runs:



See also Python programming course

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Python - More on Collections and Sequences
  [3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
  [3348] List slices in Python - 2 and 3 values forms, with an uplifting example - (2011-07-06)
  [3150] Python dictionaries - mutable and immutable keys and values - (2011-01-29)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [2920] Sorting - naturally, or into a different order - (2010-08-14)
  [2894] Sorting people by their names - (2010-07-29)
  [2718] Python - access to variables in the outer scope - (2010-04-12)
  [1873] List Comprehensions in Python - (2008-11-06)
  [1869] Anonymous functions (lambdas) and map in Python - (2008-11-04)
  [1310] Callbacks - a more complex code sandwich - (2007-08-19)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [899] Python - extend v append on a list - (2006-10-20)
  [633] Copying a reference, or cloning - (2006-03-05)
  [386] What is a callback? - (2005-07-22)
  [61] Python is a fabulous language - (2004-09-24)

Object Orientation and General technical topics - Programming Principles
  [3551] Some terms used in programming (Biased towards Python) - (2011-12-12)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3542] What order are operations performed in, in a Perl expression? - (2011-12-07)
  [3456] Stepping stones - early coding, and writing re-usable code quickly - (2011-09-24)
  [3026] Coding efficiency - do not repeat yourself! - (2010-11-02)
  [2964] An introduction to file handling in programs - buffering, standard in and out, and file handles - (2010-09-21)
  [2915] Looking up a value by key - associative arrays / Hashes / Dictionaries - (2010-08-11)
  [2878] Program for reliability and efficiency - do not duplicate, but rather share and re-use - (2010-07-19)
  [2769] Easy - but for whom? - (2010-05-18)
  [2737] Improving your function calls (APIs) - General and PHP - (2010-04-24)
  [2586] And and Or illustrated by locks - (2010-01-17)
  [2550] Do not copy and paste code - there are much better ways - (2009-12-26)
  [2510] The music of the stock market - (2009-11-22)
  [2415] Variable names like i and j - why? - (2009-09-22)
  [2327] Planning! - (2009-08-08)
  [2310] Learning to write high quality code in Lua - (2009-07-30)
  [2228] Where do I start when writing a program? - (2009-06-11)
  [2022] Pre and post increment - the ++ operator - (2009-02-03)
  [2001] I have not programmed before, and need to learn - (2009-01-19)

Object Oriented Python
  [3436] Moving from scripting to Object Orientation in Python - (2011-09-13)
  [3399] From fish, loaves and apples to money, plastic cards and BACS (Perl references explained) - (2011-08-20)
  [3085] Object Oriented Programming for Structured Programmers - conversion training - (2010-12-14)
  [2604] Tips for writing a test program (Ruby / Python / Java) - (2010-01-29)
  [2169] When should I use OO techniques? - (2009-05-11)
  [2017] Python - a truly dynamic language - (2009-01-30)
  [1925] Introduction to Object Oriented Programming - (2008-12-06)
  [1348] Screw it or Glue it? Access to Object variables - a warning - (2007-09-12)
  [1306] Python class rattling around - (2007-08-16)
  [900] Python - function v method - (2006-10-20)
  [834] Python makes University Challenge - (2006-08-15)
  [477] Class, static and unbound variables - (2005-10-25)

resource index - Python
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At Well House Consultants, we provide training courses on subjects such as Ruby, Lua, Perl, Python, Linux, C, C++, Tcl/Tk, Tomcat, PHP and MySQL. We're asked (and answer) many questions, and answers to those which are of general interest are published in this area of our site.