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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.
How do I set up a constant in Python?

You may think of values like pi being a constant, but in Python they're really just another object that's defined in the namespace of the module from which they're loaded!

So ...

>>> import math
>>> math.pi
3.1415926535897931
>>> math.pi *= 4
>>> math.pi
12.566370614359172


You are fully entitled to think that what I have just done as an illustration would be dreadful in a real program - and I would agree with you 110%. I've done it to show you that in Python you do not have constants, you have variables that you are strongly advised not to change. You'll find that exacly the same behaviour happens with variables that look like python constants because they are capitalised:

>>> import re
>>> re.IGNORECASE
2
>>> re.IGNORECASE+=4
>>> re.IGNORECASE
6
>>>


Why does Python behave this way? Because it's a trusting language that's written for programmers who think a bit about what they're doing and don't overstape the mark with 'sillies'. This approach allows for a slimmer, sleaker language that is coded and operationally faster, without the need to have private, public, protected, final, abstract and interface keywords. These are all keywords which are around in languages like Java (mandatory) and recent PHP (optional in use) and when you think about it they're just policemen, there to enforce rules. But don't most of us stick to the rules most of the time anyhow, once we understand that they're good for us! A really clever thing about Python is the way the language was designed to encourage us to write good code without the need for policemen like P.C. John Private, Sergant Tom Protected and Inspector Bill Public; the languages's block inset system though you may hate it at first does lead to code that in naturally inset, and the language's structures of working on lists ensure that they are never going to be sparse (you should use a dictionary).

It's actually a very good idea in any programming language to set up a file of globals or constants for you application. Here's a Python example - let's see how we might use it first:

>>> from constant import *
>>> print "This page " + whc.COPYRIGHT
This page Copyright Well House Consultants Ltd, 2009
>>>


And what might it contain?

>>> dir (whc)
['COMPANY', 'COPYRIGHT', 'FREEPHONE', 'PHONE', 'VAT', '__doc__', '__module__']
>>>


And of course it contains a self-documenting test harness:

Dorothy-2:~ grahamellis$ python constant.py
COMPANY : Well House Consultants Ltd
COPYRIGHT : Copyright Well House Consultants Ltd, 2009
FREEPHONE : 0800 043 8225
PHONE : 01225 708225
VAT : 15
__doc__ : A file of values to be used as globals or constants
throughout our company's Python programs. By using this
file, you are able to change the VAT rate (which is 15%
as I write this in late 2009, but goes up to 17.5% on
the 1st January 2010 in ONE PLACE only
__module__ : __main__


The source code for our constant module may be found here
(written 2009-10-31)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y116 - Python - Applying OO design techniques and best practise
  [340] Code and code maintainance efficiency - (2005-06-08)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [668] Python - block insets help with documentation - (2006-04-04)
  [836] Build on what you already have with OO - (2006-08-17)
  [945] Code quality counts - (2006-11-26)
  [1181] Good Programming practise - where to initialise variables - (2007-05-09)
  [2363] Alpaca Case or Camel Case - (2009-08-16)
  [2407] Testing code in Python - doctest, unittest and others - (2009-09-16)
  [2523] Plan your application before you start - (2009-12-02)
  [2604] Tips for writing a test program (Ruby / Python / Java) - (2010-01-29)
  [3887] Inheritance, Composition and Associated objects - when to use which - Python example - (2012-10-10)
  [4028] Really Simple Class and Inheritance example in Python - (2013-03-04)
  [4118] We not only teach PHP and Python - we teach good PHP and Python Practice! - (2013-06-18)
  [4359] How to avoid too many recalculations within an object - (2014-12-21)
  [4718] Defining an object that is a modified standard type in Python - (2016-11-02)

Y112 - Python - Objects - Intermediate
  [296] Using a Python dictionary as a holder of object attributes - (2005-04-30)
  [383] Overloading of operators on standard objects in Python - (2005-07-19)
  [477] Class, static and unbound variables - (2005-10-25)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [903] Pieces of Python - (2006-10-23)
  [964] Practical polymorphism in action - (2006-12-04)
  [1146] __new__ v __init__ - python constructor alternatives? - (2007-04-14)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1517] Python - formatting objects - (2008-01-24)
  [1644] Using a utility method to construct objects of different types - Python - (2008-05-17)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [1819] Calling base class constructors - (2008-10-03)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2409] TypeError: super() argument 1 must be type, not classobj (Python) - (2009-09-18)
  [2693] Methods that run on classes (static methods) in Python - (2010-03-25)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2720] Multiple inheritance in Python - complete working example - (2010-04-14)
  [2722] Mixins example in Python - (2010-04-14)
  [2764] Python decorators - your own, staticmethod and classmethod - (2010-05-14)
  [2785] The Light bulb moment when people see how Object Orientation works in real use - (2010-05-28)
  [2889] Should Python classes each be in their own file? - (2010-07-27)
  [2905] Defining static methods in Python - (2010-08-05)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [3002] A list of special method and attribute names in Python - (2010-10-17)
  [3442] A demonstration of how many Python facilities work together - (2011-09-16)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3524] Metaclasses (Python) and Metatables (Lua) - (2011-11-17)
  [3796] Backquote, backtic, str and repr in Python - conversion object to string - (2012-07-05)
  [4094] Python Properties - how and why - (2013-05-18)
  [4344] Python base and inherited classes, test harness and unit testing - new examples - (2014-12-07)
  [4356] Object factories in C++, Python, PHP and Perl - (2014-12-19)
  [4366] Changing what operators do on objects - a comparison across different programming languages - (2014-12-26)
  [4410] A good example of recursion - a real use in Python - (2015-02-01)
  [4449] Spike solution, refactoring into encapsulated object methods - good design practise - (2015-03-05)
  [4450] Deciding whether to use parameters, conditional statements or subclasses - (2015-03-05)
  [4541] Setting up and tearing down with the Python with keyword - (2015-10-16)
  [4649] Object and Static methods - what is the difference; example in Python 3 - (2016-02-17)
  [4717] with in Python - examples of use, and of defining your own context - (2016-11-02)
  [4719] Nesting decorators - (2016-11-02)


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Parallel Pinging, using Python Threads or Expect spawn lists
A variable number of arguments in a PHP function
Melksham Forum
Santa Special - Trowbridge and Melksham to Swindon
How do I set up a constant in Python?
Finding text and what surrounds it - contextual grep
Clustering on Tomcat
Load balancing with sticky sessions (httpd / Tomcat)
Sample code with errors in it on our web site
Pantomimes around Melksham - 2009/2010 season
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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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