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For 2021 - online Python 3 training - see ((here)).

Our plans were to retire in summer 2020 and see the world, but Coronavirus has lead us into a lot of lockdown programming in Python 3 and PHP 7.
We can now offer tailored online training - small groups, real tutors - works really well for groups of 4 to 14 delegates. Anywhere in the world; course language English.

Please ask about private 'maintenance' training for Python 2, Tcl, Perl, PHP, Lua, etc.
Copying - duplicating data, or just adding a name? Perl and Python compared

When you copy a list in Perl, you're duplicating the data and you end up with two distinct copies ... but when you copy a list in Python, you're copying the reference so that you end up with two names for the same variable - almost like an alias.

So in Perl - with two different copies - you end up with two sets of data that can diverge as you modify them, but in Python, if you modify the variable under one name, you're also modifying it under the other name.

If you want the Perl behaviour in Python, you can use a list slice to do the copy - replacing
  tea = lunch
with
  chucked = lunch[:]
but even that does only a "shallow" copy - it duplicates the references to objects within the list, so that if those objects are mutable and you change their contents, you're still chaning both.

So in my example above:
  lunch[0] = "Mints"
also changes tea, but does not alter chucked whereas
  lunch[2][1] = "Beans"
also changes tea and chucked

There are source code examples here - [source in Perl] and [source in Python]

Original Data:
["Sausages","Mash",["Sweetcorn","Peas"],"Brocolli"]

In @tea, in Perl ... unchanged:
["Sausages","Mash",["Sweetcorn","Peas"],"Brocolli"]

In tea, in Python ... Mints and Beans have replaced Sausages and Peas:
['Mints', 'Mash', ['Sweetcorn', 'Beans'], 'Brocolli']

and in chucked, in Python ... Beans have replaced peas:
['Sausages', 'Mash', ['Sweetcorn', 'Beans'], 'Brocolli']
(written 2010-10-12, updated 2010-10-30)

 
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)
  [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)
  [4442] Mutable v Immuatble objects in Python, and the implication - (2015-02-24)

Y104 - Python - Lists and Tuples
  [383] Overloading of operators on standard objects in Python - (2005-07-19)
  [657] The ternary operator in Python - (2006-03-25)
  [955] Python collections - mutable and imutable - (2006-11-29)
  [1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
  [1641] Tektronix 4010 series / Python Tuples - (2008-05-13)
  [1789] Looking for a value in a list - Python - (2008-09-08)
  [2280] Creating and iterating through Python lists - (2009-07-12)
  [2284] Strings as collections in Python - (2009-07-12)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2719] Traffic lights in Python - (2010-04-13)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [3181] Beware - a=a+b and a+=b are different - Python - (2011-02-23)
  [3257] All possible combinations from a list (Python) or array (Ruby) - (2011-04-23)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3763] Spike solutions and refactoring - a Python example - (2012-06-13)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [4368] Shuffling a list - Ruby and Python - (2014-12-28)
  [4722] Embedding more complex code into a named block - (2016-11-04)

P217 - Perl - More than Simple Lists and Hashes!
  [43] Hash of lists in Perl - (2004-09-09)
  [293] Course follow-ups - (2005-04-27)
  [1514] Autovivification - the magic appearance of variables in Perl - (2008-01-21)
  [2241] Perl references - $$var and \$var notations - (2009-06-15)
  [2840] Just pass a pointer - do not duplicate the data - (2010-06-30)
  [2877] Further more advanced Perl examples - (2010-07-19)
  [3007] Setting up a matrix of data (2D array) for processing in your program - (2010-10-21)
  [3072] Finding elements common to many lists / arrays - (2010-11-26)
  [3105] Adventure with references to lists and lists of references - (2010-12-26)
  [3399] From fish, loaves and apples to money, plastic cards and BACS (Perl references explained) - (2011-08-20)
  [3406] Not multidimentional arrays - but lists of lists. Much more flexible. Perl! - (2011-08-26)
  [3444] Take the dog on a lead - do not carry her. Perl references. - (2011-09-17)
  [3577] How to do multidimensional arrays (or rather lists and hashes) in Perl - (2012-01-14)
  [3906] Taking the lead, not the dog, for a walk. - (2012-10-28)

P208 - Perl - Lists
  [28] Perl for breakfast - (2004-08-25)
  [140] Comparison Chart for Perl programmers - list functions - (2004-12-04)
  [230] Course sizes - beware of marketing statistics - (2005-02-27)
  [240] Conventional restraints removed - (2005-03-09)
  [355] Context in Perl - (2005-06-22)
  [463] Splitting the difference - (2005-10-13)
  [560] The fencepost problem - (2006-01-10)
  [622] Queues and barrel rolls in Perl - (2006-02-24)
  [762] Huge data files - what happened earlier? - (2006-06-15)
  [773] Breaking bread - (2006-06-22)
  [928] C++ and Perl - why did they do it THAT way? - (2006-11-16)
  [968] Perl - a list or a hash? - (2006-12-06)
  [1316] Filtering and altering Perl lists with grep and map - (2007-08-23)
  [1703] Perl ... adding to a list - end, middle, start - (2008-07-09)
  [1828] Perl - map to process every member of a list (array) - (2008-10-09)
  [1917] Out of memory during array extend - Perl - (2008-12-02)
  [1918] Perl Socket Programming Examples - (2008-12-02)
  [2067] Perl - lists do so much more than arrays - (2009-03-05)
  [2226] Revision / Summary of lists - Perl - (2009-06-10)
  [2295] The dog is not in trouble - (2009-07-17)
  [2484] Finding text and what surrounds it - contextual grep - (2009-10-30)
  [2813] Iterating over a Perl list and changing all items - (2010-06-15)
  [2833] Fresh Perl Teaching Examples - part 2 of 3 - (2010-06-27)
  [3400] $ is atomic and % and @ are molecular - Perl - (2011-08-20)
  [3548] Dark mornings, dog update, and Python and Lua courses before Christmas - (2011-12-10)
  [3870] Writing more maintainable Perl - naming fields from your data records - (2012-09-25)
  [3939] Lots of ways of doing the same thing in Perl - list iteration - (2012-12-03)
  [4609] Mapping an array / list without a loop - how to do it in Perl 6 - (2016-01-03)


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A river in Melksham is not just for boaters.
Previous and next
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Horse's mouth home
Forward to
3D graphics - web site usage - simple matplotlib and python example
Some other Articles
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
Copying - duplicating data, or just adding a name? Perl and Python compared
A river in Melksham is not just for boaters.
Python - some common questions answered in code examples
Arrays v Lists - what is the difference, why use one or the other
Matplotlib - graphing in Python - teaching examples
Loading and saving data - Python / numpy
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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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