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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 Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own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))
Multiple identical keys in a Python dict - yes, you can!

If you have a list, you can only have one element at each number - there's just a single positon [0], a single [1] and so on. That's clear, and natural to understand. With a dict in Python (like a hash in Perl or an associative array in PHP), it's peceived wisdom that you can only have one element with a paricular name.

Let's see an example - setting up a dict:
  control = {"Andrew" : "Cambridge", "Barabara" : "Bloomsbury", "Andrew": "Corsica"}
  print control


Which sets up a dict called "control" ... adds an element called Andrew, another element called Barbara, and then replaces the original Andrew element with a new element of the same name. So that although we've added three elements, we only have two when we print it out:
  {'Barabara': 'Bloomsbury', 'Andrew': 'Corsica'}

But ... what if I wanted to store multiple "Andrew"s? In Python, if I use mutable objects as my keys, then I can create multiple identical objects, and each can be a separate key. Let's use a very simple object - with just a single member that's a string:
  class person(object):
    def __init__(self,name):
      self.name = name


I can then set up my dict as follows:
  alternate = {person("Andrew") : "Cambridge", person("Barabara") : "Bloomsbury", person("Andrew"): "Corsica"}
  print alternate

and when I print out the results, I do get two (identical but different) Andrews:
  {'Barabara': 'Bloomsbury', 'Andrew': 'Cambridge', 'Andrew': 'Corsica'}

Newcomers to this sort of structure worry about how they can access individual elements once the dict has been set up. That's understandable but not really a problem - if two key objects are identical, it doesn't matter which one you refer to. And if they're not identical, then you can differentiate between the keys somehow. And you can loop through all the keys via the keys method:
  for staff in alternate.keys():
    print staff, "lives in", alternate[staff]


If you refer to an element with an existing name, though, you now need to be careful. For example:
  alternate[person("Andrew")] = "Tignabruiach"
Will add yet another "Andrew" to our dict.

There are further examples within the source code from which the above examples are taken [here].




A further approach to the need to multiple identical keys is to store a list at each member of a dict. There's a new example of setting one of these up [here].

In my example, I've provided an add method to store new data, and I would need to use a loop to access members with the same key.

Examples from our new Intermediate Python course which ran for the first time last week.
(written 2012-11-24, updated 2012-11-25)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y107 - Python - Dictionaries
  [103] Can't resist writing about Python - (2004-10-29)
  [955] Python collections - mutable and imutable - (2006-11-29)
  [1144] Python dictionary for quick look ups - (2007-04-12)
  [1145] Using a list of keys and a list of values to make a dictionary in Python - zip - (2007-04-13)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2915] Looking up a value by key - associative arrays / Hashes / Dictionaries - (2010-08-11)
  [2986] Python dictionaries - reaching to new uses - (2010-10-05)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [3464] Passing optional and named parameters to python methods - (2011-10-04)
  [3488] Python sets and frozensets - what are they? - (2011-10-20)
  [3554] Learning more about our web site - and learning how to learn about yours - (2011-12-17)
  [3555] Football league tables - under old and new point system. Python program. - (2011-12-18)
  [3662] Finding all the unique lines in a file, using Python or Perl - (2012-03-20)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)
  [4409] Setting up and using a dict in Python - simple first example - (2015-01-30)
  [4469] Sorting in Python 3 - and how it differs from Python 2 sorting - (2015-04-20)
  [4661] Unique word locator - Python dict example - (2016-03-06)
  [4668] Sorting a dict in Python - (2016-04-01)


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Whether you have programmed before or not, we can teach you Python
Some other Articles
A long overdue meeting - a steeping stone towards coordinate transport user inputs
Agenda for Wiltshire Link Travel groups meeting, Saturday 1st December 2012
Lisa is home today ...
Whether you have programmed before or not, we can teach you Python
Multiple identical keys in a Python dict - yes, you can!
Behind Melksham Spa - Mood Mist over wet fields.
River nearly bursting its banks in Melksham
Optional positional and named parameters in Python
Reporting the full stack trace when you catch a Python exception
Melksham Bus Issues - to be raised at First Bus Customer Panel
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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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