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Mutable v Immuatble objects in Python, and the implication

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
Y102 - Python - Fundamentals
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  [3917] BODMAS - the order a computer evaluates arithmetic expressions - (2012-11-09)
  [3886] Formatting output - why we need to, and first Python example - (2012-10-09)
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  [2442] Variable storage - Perl, Tcl and Python compared - (2009-10-08)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [1878] Pascals Triangle in Python and Java - (2008-11-10)
  [1461] Python - input v raw input - (2007-12-06)
  [1448] Question on division (Java) - Also Perl, PHP, Python ... - (2007-11-28)
  [1430] Integer v float - Python - (2007-11-12)
  [956] Python security - trouble with input - (2006-11-30)
  [748] Getting rid of variables after you have finished with them - (2006-06-06)
  [633] Copying a reference, or cloning - (2006-03-05)
  [328] Making programs easy for any user to start - (2005-05-29)

Y111 - Python - More on Collections and Sequences
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  [3797] zip in Python - (2012-07-05)
  [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)
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  [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)
  [386] What is a callback? - (2005-07-22)
  [61] Python is a fabulous language - (2004-09-24)


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