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)

 Y116 - Python - Applying OO design techniques and best practise
  [2604] Tips for writing a test program (Ruby / Python / Java) - (2010-01-29)
  [2523] Plan your application before you start - (2009-12-02)
  [2407] Testing code in Python - doctest, unittest and others - (2009-09-16)
  [2363] Alpaca Case or Camel Case - (2009-08-16)
  [1181] Good Programming practise - where to initialise variables - (2007-05-09)
  [945] Code quality counts - (2006-11-26)
  [836] Build on what you already have with OO - (2006-08-17)
  [668] Python - block insets help with documentation - (2006-04-04)
  [656] Think about your design even if you don't use full UML - (2006-03-24)
  [340] Code and code maintainance efficiency - (2005-06-08)

Y112 - Python - Objects - Intermediate
  [3524] Metaclasses (Python) and Metatables (Lua) - (2011-11-17)
  [3472] Static variables in functions - and better ways using objects - (2011-10-10)
  [3442] A demonstration of how many Python facilities work together - (2011-09-16)
  [3002] A list of special method and attribute names in Python - (2010-10-17)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [2905] Defining static methods in Python - (2010-08-05)
  [2889] Should Python classes each be in their own file? - (2010-07-27)
  [2785] The Light bulb moment when people see how Object Orientation works in real use - (2010-05-28)
  [2764] Python decorators - your own, staticmethod and classmethod - (2010-05-14)
  [2722] Mixins example in Python - (2010-04-14)
  [2720] Multiple inheritance in Python - complete working example - (2010-04-14)
  [2717] The Multiple Inheritance Conundrum, interfaces and mixins - (2010-04-11)
  [2693] Methods that run on classes (static methods) in Python - (2010-03-25)
  [2409] TypeError: super() argument 1 must be type, not classobj (Python) - (2009-09-18)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [1819] Calling base class constructors - (2008-10-03)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [1644] Using a utility method to construct objects of different types - Python - (2008-05-17)
  [1517] Python - formatting objects - (2008-01-24)
  [1217] What are factory and singleton classes? - (2007-06-04)
  [1146] __new__ v __init__ - python constructor alternatives? - (2007-04-14)
  [964] Practical polymorphism in action - (2006-12-04)
  [903] Pieces of Python - (2006-10-23)
  [831] Comparison of Object Oriented Philosophy - Python, Java, C++, Perl - (2006-08-13)
  [477] Class, static and unbound variables - (2005-10-25)
  [383] Overloading of operators on standard objects in Python - (2005-07-19)
  [296] Using a Python dictionary as a holder of object attributes - (2005-04-30)

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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