Mutable v Immuatble objects in Python, and the implication

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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))

In python, "everything is an object" - well, at least everything you name, and lots of standard system things too, are!

Some object types are immutable. That means tha once they have been created, they cannot be modified and any code that looks like it modifies them in reallity produces a new object that's an amdened copy with the new data.

Other object types are mutable. And in that case, they can be amended in situ once they have been created.

One of the effects of "mutable v immutable" is that is you pass an immutable object into a function or method and (attempt to) change it there, you will NOT change it in the main code. But if you pass a mutable object into a function or method, you WILL change it in the main code. See [example source].

  def one (first):

    first += 5

    return first

  f = 27

  x = one(f)

  print f

result - the ORIGINAL 27

  def two (second):

    second += [5]

    return second

  s = [27]

  y = two(s)

  print s

result - a MODIFIED list [27,5]

• Immutable obects provided by Python ... int, float, str, tuple
• Muttable objects ... most others!
(written 2015-02-24)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles

Y111 - Python - More on Collections and Sequences
  [61] Python is a fabulous language - (2004-09-24)
  [386] What is a callback? - (2005-07-22)
  [633] Copying a reference, or cloning - (2006-03-05)
  [899] Python - extend v append on a list - (2006-10-20)
  [1304] Last elements in a Perl or Python list - (2007-08-16)
  [1310] Callbacks - a more complex code sandwich - (2007-08-19)
  [1869] Anonymous functions (lambdas) and map in Python - (2008-11-04)
  [1873] List Comprehensions in Python - (2008-11-06)
  [2718] Python - access to variables in the outer scope - (2010-04-12)
  [2894] Sorting people by their names - (2010-07-29)
  [2920] Sorting - naturally, or into a different order - (2010-08-14)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3150] Python dictionaries - mutable and immutable keys and values - (2011-01-29)
  [3348] List slices in Python - 2 and 3 values forms, with an uplifting example - (2011-07-06)
  [3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
  [3797] zip in Python - (2012-07-05)
  [4398] Accessing variables across subroutine boundaries - Perl, Python, Java and Tcl - (2015-01-18)

Y102 - Python - Fundamentals
  [328] Making programs easy for any user to start - (2005-05-29)
  [748] Getting rid of variables after you have finished with them - (2006-06-06)
  [956] Python security - trouble with input - (2006-11-30)
  [1430] Integer v float - Python - (2007-11-12)
  [1448] Question on division (Java) - Also Perl, PHP, Python ... - (2007-11-28)
  [1461] Python - input v raw input - (2007-12-06)
  [1878] Pascals Triangle in Python and Java - (2008-11-10)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2442] Variable storage - Perl, Tcl and Python compared - (2009-10-08)
  [2778] Learning to program in Python 2 ... and / or in Python 3 - (2010-05-24)
  [3083] Python - fresh examples from recent courses - (2010-12-11)
  [3181] Beware - a=a+b and a+=b are different - Python - (2011-02-23)
  [3278] Do I need to initialise variables - programming in C, C++, Perl, PHP, Python, Ruby or Java. - (2011-05-05)
  [3551] Some terms used in programming (Biased towards Python) - (2011-12-12)
  [3886] Formatting output - why we need to, and first Python example - (2012-10-09)
  [3917] BODMAS - the order a computer evaluates arithmetic expressions - (2012-11-09)
  [4324] Learning to program - variables and constants - (2014-11-22)
  [4712] A reminder of the key issues to consider in moving from Python 2 to Python 3 - (2016-10-30)

Back to
Reading command line parameters in Python

Previous and next
or
Horse's mouth home

Forward to
Seventh stay away this year - and it's still only February!

Some other Articles

Combining tests into suites, and suites into bigger suites - Python and unittest
Graphing presentations in Python - huge data, numpy and matplotlib
Elements of an exception in Python - try, except, else, finally
Seventh stay away this year - and it's still only February!
Mutable v Immuatble objects in Python, and the implication
Reading command line parameters in Python
A first graph with Matplotlib in Python
Json is the new marshall, pickle and cPickle / Python
Loving programming in Python - and ready to teach YOU how
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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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