What you will learn in this section

You’ll learn about how to store data in your program. One of the keys to becoming a better Python programmer is to have a sound knowledge of Python's built-in data structures. Since this is just a beginner course, we won't cover in-depth of data structures but readers are requested to learn data structures in depth if you want to become good python developer.

Introduction

Data strucutre is an structures which can hold some data together. In other words they are used to store a collection of related data. There are four built-in data structures in Python:

• list
• tuple
• dictionary
• set

We will see how to use each of them and how they help us store data properly.

List

In 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 probably 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.

List can have any number of items and they may be of different types (integer, float, string etc.).

Nested list

A list can even have another list as an item. That list are called nested list.

Let's do some practice to understand list in much easier way. In your python shell, define a variable called "django" and store some elements value into it.

# empty list
django = []

# list of integers
django = [1, 2, 3]

# list with mixed datatypes
django = [1, "Hello", 3.4]

# nested list
django = ["mouse", [8, 4, 6]]

Accessing Values in Lists

To access values in lists, use the square brackets for slicing along with the index or indices to obtain value available at that index. For example:

>>> django = [1, 2, 3]
>>> print django[0]
1
>>> django = ["mouse", [8, 4, 6]]
>>> print django[0]
mouse
>>>

Updating Lists

You can update single or multiple elements of lists by giving the slice on the left-hand side of the assignment operator, and you can add to elements in a list with the append() method. Let's open a terminal and update a list.

>>> nescode = ["hello django", "apps", "ERP"]
>>> print nescode[2]
ERP
>>> nescode[2] = "apps"
>>> print nescode[2]
apps
>>>

In above example, you have seen how to update list in python. Similarly, you can delete and index. There are various built-in list function and method in python, some of the examples are:

Tuple

Tuples are sequences, just like lists. The differences between tuples and lists are, the tuples cannot be changed unlike lists and tuples use parentheses, whereas lists use square brackets. Creating a tuple is as simple as putting different comma-separated values. Let's define a tuple:

>>> best_os = ("ubuntu", "macos", "solaris" "windows")

Accessing tuple is same as list. Since, we defined a tuple, let's access the best operating system in same python shell.

>>> print best_os[0]
ubuntu
>>>

Dictionary

Dictionary is another data type built into python. Unlike sequences, which are indexed by a range of numbers, dictionaries are indexed by keys, which can be any immutable type; strings and numbers can always be keys.

Tuples can be used as keys if they contain only strings, numbers, or tuples; if a tuple contains any mutable object either directly or indirectly, it cannot be used as a key. You can’t use lists as keys, since lists can be modified in place using index assignments, slice assignments, or methods like append() and extend().

set

In simplest terms set can be defined as - set is an unordered collections of unique elements. Or Sets are lists with no duplicate entries. Let's say you want to collect a list of words used in a paragraph:

>>> print set("Hello Django is an open initiatives for python classes.".split())
set(['initiatives', 'for', 'python', 'is', 'an', 'classes.', 'Django', 'open', 'Hello'])
>>>

Sets are a powerful tool in Python since they have the ability to calculate differences and intersections between other sets. For example, say you have a list of participants in events A and B:

>>> a = set(["Sunil", "Sunita", "Arundhati", "Munender"])
>>> b = set(["Sunil", "Amit"])

Now, Let's find out which members attended both events, you may use the intersection method here:

>>> a.intersection(b)
set(['Sunil'])
>>> b.intersection(a)
set(['Sunil'])
>>>

Practice drills and recommendation

Do this experiment before moving to next chapter.

• Read modules chapter from official python documentation https://docs.python.org/2.7/tutorial/datastructures.html
• Make mistake knowingly while practicing above program and analyse the errors.
• Do Googling with a keywords like - Python data structures, Data structure
• Write comment for every single line of code.
• As I said in beginning of this chapter, Data structure knowledge is very very important to be a good developer. Buy and read books on Data structure.