Training, Open Source Programming Languages

This is page ... lists.html

Our email: • Phone: 01225 708225

Retiring, March 2020 - sorry, you have missed our final public course.
The Coronavirus situation has lead us to suspend public training - which was on the cards anyway, with no plans to resume

Please ask about private 'maintenance' training for Python, Tcl, Perl, PHP, Lua, etc
Happily continuing private consultancy / programming work
Creating and iterating through Python lists

In Python, you create lists by putting a series of values in square brackets - and the program creates a list with that number of elements. You can change an element by referring to the element by number in square brackets, remembering that Python starts counting at 0. Unlike Perl, you can't extend a list simply by referring to a number above the current range (very prone to programming errors), but you can use the append or extend methods to add extra elements on the end (flexible, whilst encouraging good coding practise).

places = ["London","Melksham","Farnborough"]
places[1] = "A Beautiful old town in Wiltshire"
# Use the append method to extend a list

When you want to iterate through all the elements of a list, there are a number of ways of doing so and the most straightforward is for loop as follows ... although in this case you don't even get the index number, so if the position of each value is important you'll want to use one of the following alternatives.

# Quick and easy - but unable to get at posn no
for place in places:
   print place

You can list individual places and loop through positions numbers - but that isn't extensible. Better to use a range function to get a list of position numbers of (in the case of a long list) xrange which is an iterator - returning one value at a time and not stuffing memory with a large intermediate list.

# Not extensible
for posn in [0,1,2]:
   print posn,places[posn]
# Extensible BUT intermediate list may get long
for posn in range(len(places)):
   print posn,places[posn]
# Extensible and uses an iterator
for posn in xrange(len(places)):
   print posn,places[posn]

Python 3 (actually 3.1) has now been released, and in Python 3 there are some slight syntax changes. print is now a function so you would add brackets around the parameters, and you would always use range rather than xrange as it will automatically convert to an iterator for you.

Why am I still using a Python 2 example? Because the recommended upgrade route is to keep maintaining code in Python 2 until all the platforms that you run on have been converted, using the automated converter while you are supporting both. And because most of the delegates on our Leaning to program in Python and Python Programming courses are following that approach. We have both versions available during the course, and we ensure that delegates leave with a knowledge of the conversion process without switching back and forth all through the course and leading to confusion.

The example above is taken from the Python course I am running this weekend - more normally, the course runs during the week, with the next available dates in mid August. As one of the delegates who is booked on that next course wrote about our update strategy "Aha, perfect ... I was told that I should concentrate on python 2.x as python 3.x has an awful lot of changes and all our existing scripts would have to be updated."
(written 2009-07-12)

Associated topics are indexed as below, or enter for individual articles
Y104 - Python - Lists and Tuples
  [4722] Embedding more complex code into a named block - (2016-11-04)
  [4368] Shuffling a list - Ruby and Python - (2014-12-28)
  [4027] Collections in Python - list tuple dict and string. - (2013-03-04)
  [3763] Spike solutions and refactoring - a Python example - (2012-06-13)
  [3669] Stepping through a list (or an array) in reverse order - (2012-03-23)
  [3348] List slices in Python - 2 and 3 values forms, with an uplifting example - (2011-07-06)
  [3257] All possible combinations from a list (Python) or array (Ruby) - (2011-04-23)
  [3181] Beware - a=a+b and a+=b are different - Python - (2011-02-23)
  [3118] Arrays of arrays - or 2D arrays. How to program tables. - (2011-01-02)
  [2996] Copying - duplicating data, or just adding a name? Perl and Python compared - (2010-10-12)
  [2719] Traffic lights in Python - (2010-04-13)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2284] Strings as collections in Python - (2009-07-12)
  [1789] Looking for a value in a list - Python - (2008-09-08)
  [1641] Tektronix 4010 series / Python Tuples - (2008-05-13)
  [1220] for loop - how it works (Perl, PHP, Java, C, etc) - (2007-06-06)
  [955] Python collections - mutable and imutable - (2006-11-29)
  [899] Python - extend v append on a list - (2006-10-20)
  [657] The ternary operator in Python - (2006-03-25)
  [383] Overloading of operators on standard objects in Python - (2005-07-19)

Back to
Understanding the new local government structure in Wiltshire
Previous and next
Horse's mouth home
Forward to
Python - using exceptions to set a fallback
Some other Articles
Everyone is in the customer relations business
Checking robots.txt from Python
Python - using exceptions to set a fallback
Creating and iterating through Python lists
Understanding the new local government structure in Wiltshire
First courses for 2010
Python classes / courses - what version do we train on?
Who is Marc Schneider of Multilingual Search Engine Optimization Inc
Debugging multipage (session based) PHP applications
4759 posts, page by page
Link to page ... 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96 at 50 posts per page

This is a page archived from The Horse's Mouth at - 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.

Link to Ezine home page (for reading).
Link to Blogging home page (to add comments).

© WELL HOUSE CONSULTANTS LTD., 2020: 48 Spa Road • Melksham, Wiltshire • United Kingdom • SN12 7NY
PH: 01225 708225 • EMAIL: • WEB: • SKYPE: wellho

PAGE: ... lists.html • PAGE BUILT: Sat May 27 16:49:10 2017 • BUILD SYSTEM: WomanWithCat