What You Need to Know:

• Command-line Basics
• Basic Python syntax and datatypes

What You Will Learn:

• How to create a list in Python syntax

Why This is Important:

In this lesson you will learn how to create a Python list.

Lists

A list is the most basic Python data structure. It is a list of objects or values. The syntax for a list is a set of objects enclosed in brackets. To create an empty list, set a variable equal to empty brackets:

>>> empty_list = []

To create a list with some objects in it, just add the elements separated by commas:

>>> groceries = ['Eggs', 'Milk','Butter']

Accessing items in a list

List items have an index and are accessed by calling their index number. Numbering in the list starts at 0.

>>> groceries[0]
'Eggs'
>>> groceries[1]
'Milk'
>>> groceries[0:2]
['Eggs', 'Milk']
>>> groceries[1:]
['Milk', 'Butter']

Modifying a List

The easiest way to modify a list’s content is to just access the list object by its index (numerical place in the list) and use the assignment operator.

>>> groceries
['Eggs', 'Milk','Butter']
>>> groceries[0] = 'Bread'
>>> groceries
['Bread', 'Milk', 'Butter']

Another convenient way to modify a list is the append() method. The append method allows you to add an element at the end of a list.

>>> groceries.append('Asparagus')
>>> groceries
['Bread', 'Milk', 'Butter', 'Asparagus']

List Methods

Using the len function, you can return the number of items in a list:

>>> print len(groceries)
3

Here are some other list methods that will come in handy:

groceries.extend(['Rutabaga', 'Ice Cream'])
['Bread', 'Milk', 'Butter', 'Asparagus','Rutabaga', 'Ice Cream']

del groceries[3] # removes 'Asparagus' from the list
['Bread', 'Milk', 'Butter','Rutabaga', 'Ice Cream']

groceries.remove('Rutabaga')
['Bread','Milk','Butter','Ice Cream']

groceries.sort()
['Bread', 'Butter', 'Ice Cream', 'Milk']

‘In’ and ‘Not’ Operator

What if you have a list of groceries and you want to check if ‘apples’ is in that list? Or a list of names and you want to check to see if someone is present? Use the ‘in’ operator.

if 'albert' in students:
  print 'good, albert is here'
else:
  print 'gosh, where is albert?!'

To check if an element is absent from a list, use the ‘not’ in operator:

if 'albert' not in students:
  print 'where is albert?!'

range(): building lists of numbers easily

It is frequently useful to be able to generate a list of numbers. Rather than have you type out all the numbers you want, Python makes this easy:

>>> print range(0,10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> print range(0,100,10)
[0, 10, 20, 30, 40, 50, 60, 70, 80, 90]

Generally, ranges have the form: range(<start_int>, <end_int>, <interval>)

List Countries Exercise

• Create a new Python file called countries.py. Create a list called four_letter_countries with the following countries (in this order):
  • Chad
  • Cuba
  • Greenland
  • Iraq
  • Mali
  • Oman
  • India

• Add Fiji to the end of the list

• Delete India

• Reverse the order of the list

• Replace Greenland with Togo

• Add Laos to the end of the list

• Reverse the order of the list again

• Add Peru to the start of the list

What does your final list look like?

Conclusion:

Creating, modifying and accessing lists are important for every programmer, as is being able to use for loops and while loops. Practicing these small examples are a great way to build your foundation as a strong developer.