Using just literal constants can soon become boring since your program will give the same results everytime. We need some way of storing information and manipulating them so that we can do interesting things. We can achieve this using variables. Variables are exactly what they mean, their value can vary - you can store any value using a variable. Variables are just parts of your computer's memory which you are using to store some information. Unlike literal constants, we need some way to use these variables and hence we give names for variables.
Variables are examples of identifiers. Identifiers are names given to identify something. We can choose any name for identifiers as long as they satisfy these simple rules:
The name can consist of alphabets (both lower and uppercase), underscore ('_') and digits (0-9).
Only the first character of the name cannot be a digit.
Names are case-sensitive. For example,
myname
and myName
are
not the same names, they are
two different variables. Note the lowercase
n
in the former and the uppercase
N
in the latter.
Examples of valid identifier names
are i
, __my_name
,
name_23
and
a1b2_c3
.
Examples of invalid identifier
names are 2things
, this is spaced
out
, and my-name
.
Variables can hold values of different types, which are called as data types.. The basic types are numbers and strings which we have already discussed.
In later chapters, we will learn how to create our own types using classes.
Python refers to anything used in a program as an object. What it really means is that Python is strongly object-oriented in the sense that everything in Python is an object including numbers, strings and even functions.
We have not yet explored what an object really means, we will explore it in later chapters. However, it is very important to keep the idea of an object in mind. You can picture an object like a bag where you can put in stuff or take out stuff.
First of all, notice that we have a set of steps in the program and each step is run one after the other in the order that we have listed the steps. Also, notice that each step is on its own line. We can consider each step to be a statement since we are basically stating to Python about our intentions in this program.
Let us now consider the statement i= 5
.
Remember, that we had previously learnt that
we should consider objects
as bags. In this case, consider our program as a bag. We
are putting the name i
into this bag. More
appropriately, we are putting the name i
into
the space of our program, this is why our program is referred to
as having a namespace. This is
one of the most important concepts that you
will learn about Python - when you create a new name, you are
entering it into the namespace. That name can hold a value of any
type. In this case, it happens to be the name
i
which is holding the value
5
.
We use the print
statement to display
the value of i
. In the statement i =
i + 1
, the portion to the right side of the equal sign
is first calculated. Here, we just add the value of the literal
constant 1 to the value of the variable i
(which happens to be 5). The sum is 6
. Now,
let us observe what is to the left side of the equal sign - we
have i
again - so we give the value
6
to i
, this is called assigning the
value.
Similarly, we assign a triple-quoted string to the name
s
and then display its value using the
print
statement.
Also notice the use of comments to explain parts of the program.
Variable names are defined when first used. No declaration or data type definition is required.