Beware - a=a+b and a+=b are different

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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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

Beware - a=a+b and a+=b are different - Python

It's commonly stated the the += operator is simply a more efficient and shorter to code alternative to using a + operator and saving back to the same variable ... in other words that
  original += extra
and
  original = original + extra
do the same thing.

But - in Python at least - that's not quite the case. If we had given the original variable another name prior to the operation above:
  aka = original
and we modify the alternative name after the operation:
  aka[1] = 5
we will get different results when we print out original again.

Using "+" original is unchanged by the reassignment on an element:
  [2, 4, 6, 8, 10, 12, 14, 16]
But using +=, aka remains an alias to the variable, so it is changed:
  [2, 5, 6, 8, 10, 12, 14, 16]

There are complete samples using += [here] and using reassignment [here] in our source library.

Note - if you are defining your own class, you can set up the + and += operator to perform completely different actions if you like, by defining the __add__ and __iadd__ methods with different code.
(written 2011-02-23, updated 2011-02-24)

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

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)
  [899] Python - extend v append on a list - (2006-10-20)
  [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)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [3257] All possible combinations from a list (Python) or array (Ruby) - (2011-04-23)
  [3348] List slices in Python - 2 and 3 values forms, with an uplifting example - (2011-07-06)
  [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)

Y102 - Python - Fundamentals
  [328] Making programs easy for any user to start - (2005-05-29)
  [633] Copying a reference, or cloning - (2006-03-05)
  [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)
  [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)
  [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)
  [4442] Mutable v Immuatble objects in Python, and the implication - (2015-02-24)
  [4712] A reminder of the key issues to consider in moving from Python 2 to Python 3 - (2016-10-30)

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