A Byte of Python


A list is a data structure that holds an ordered collection of items i.e. you can store a sequence of items in a list. This is easy to imagine if you can think of a shopping list where you have a list of items to buy, except that you probbly have each item on a separate line in your shopping list whereas in Python you put commas in between them.

The list of items should be enclosed in square brackets so that Python understands that you are specifying a list. Once you have created a list, you can add, remove or search for items in the list. Since, we can add and remove items, we say that a list is a mutable data type i.e. this type can be altered.

Quick introduction to Objects and Classes

Although, I've been generally delaying the discussion of objects and classes till now, a little explanation is needed right now so that you can understand lists better. We will still explore this topic in detail in its own chapter.

A list is an example of usage of objects and classes. When you use a variable i and assign a value to it, say integer 5 to it, you can think of it as creating an object (instance) i of class (type) int. In fact, you can see help(int) to understand this better.

A class can also have methods i.e. functions defined for use with respect to that class only. You can use these pieces of functionality only when you have an object of that class. For example, Python provides an append method for the list class which allows you to add an item to the end of the list. For example, mylist.append('an item') will add that string to the list mylist. Note the use of dotted notation for accessing methods of the objects.

A class can also have fields which are nothing but variables defined for use with respect to that class only. You can use these variables/names only when you have an object of that class. Fields are also accessed by the dotted notation, for example, mylist.field .

Using Lists

Example 9.1. Using lists

# Filename: using_list.py

# This is my shopping list
shoplist = ['apple', 'mango', 'carrot', 'banana']

print 'I have', len(shoplist), 'items to purchase.'

print 'These items are:', # Notice the comma at end of the line
for item in shoplist:
	print item,

print '\nI also have to buy rice.'
print 'My shopping list is now', shoplist

print 'I will sort my list now'
print 'Sorted shopping list is', shoplist

print 'The first item I will buy is', shoplist[0]
olditem = shoplist[0]
del shoplist[0]
print 'I bought the', olditem
print 'My shopping list is now', shoplist


$ python using_list.py
I have 4 items to purchase.
These items are: apple mango carrot banana
I also have to buy rice.
My shopping list is now ['apple', 'mango', 'carrot', 'banana', 'rice']
I will sort my list now
Sorted shopping list is ['apple', 'banana', 'carrot', 'mango', 'rice']
The first item I will buy is apple
I bought the apple
My shopping list is now ['banana', 'carrot', 'mango', 'rice']

How It Works

The variable shoplist is a shopping list for someone who is going to the market. In shoplist, we only store strings of the names of the items to buy but remember you can add any kind of object to a list including numbers and even other lists.

We have also used the for..in loop to iterate through the items of the list. By now, you must have realised that a list is also a sequence. The speciality of sequences will be discussed in a later section

Notice that we use a comma at the end of the print statement to suppress the automatic printing of a line break after every print statement. This is a bit of an ugly way of doing it, but it is simple and gets the job done.

Next, we add an item to the list using the append method of the list object, as already discussed before. Then, we check that the item has been indeed added to the list by printing the contents of the list by simply passing the list to the print statement which prints it in a neat manner for us.

Then, we sort the list by using the sort method of the list. Understand that this method affects the list itself and does not return a modified list - this is different from the way strings work. This is what we mean by saying that lists are mutable and that strings are immutable.

Next, when we finish buying an item in the market, we want to remove it from the list. We achieve this by using the del statement. Here, we mention which item of the list we want to remove and the del statement removes it fromt he list for us. We specify that we want to remove the first item from the list and hence we use del shoplist[0] (remember that Python starts counting from 0).

If you want to know all the methods defined by the list object, see help(list) for complete details.