Creating and iterating through Python lists

Archive - Originally posted on "The Horse's Mouth" - 2009-07-12 07:07:09 - Graham Ellis
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

places.append("Hastings")

places.append("Dungeness")

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."

Main Content

Creating and iterating through Python lists