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For 2023 - 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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))
Python dictionaries - mutable and immutable keys and values

Lists, Tuples, and dictionaries are the conventional collection variables in Python - but when you stop to consider it, objects and strings are collections too. All of these structures bundle together other elements (members) in various ways.

In first dictionary demonstrations, we usually use strings as both the key and the value; strings can be quickly set up so the demos are short, and strings are far and away the most common keys anyway. There's such a first example from our training notes [here], and an example that goes a lot further in accessing / sorting all the elements (you can't sort a dictionary, but you can sort the keys!) [here].

During yesterday's Python course in Oxford, I set (and later answered) an exercise that created a dictionary of objects, keyed on a string - the code is [here]. That's a very common use indeed of a dictionary, where a whole series of things (objects) is reference by some sort of unique string - a "primary key" in database terms.

A further example, in which the keys are other objects, may be found [here], with the class that's used as keys defined in a separate file [here]. You'll note that we've added two objects set up with identical parameters as keys, and they have formed separate members of the dictionary. That's because objects of our type coin are "muttable" - i.e. changeable, as opposed to strings which are not changeable in situ - "immutable"; if you use the same immutable key twice, you'll be referring to one and the same object and the second one added will overwrite the first.

More about mutable and immutable on my next Python courses - you may have missed Oxford this week, and the course in Frankfurt in 2 weeks time is private (single company) one. But we do have a public course coming up in about a month. See Learning to Program in Python and Python Programming.
(written 2011-01-29, updated 2011-02-10)

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)
  [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)

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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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