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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)

 
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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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