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)
Some other Articles
Parallel Pinging, using Python Threads or Expect spawn listsA variable number of arguments in a PHP functionMelksham ForumSanta Special - Trowbridge and Melksham to SwindonHow do I set up a constant in Python?Finding text and what surrounds it - contextual grepClustering on TomcatLoad balancing with sticky sessions (httpd / Tomcat)Sample code with errors in it on our web sitePantomimes around Melksham - 2009/2010 season