Working with variables in Python

A variable in Python represents named location that refers to a value and whose values can be used and processed during program run.

Example :

Name=”Lovejot”
Percentage=91

There are two variables Name and Percentage with values “Lovejot” and 91 respectively.

Creating a Variable

In Python , we can simply write a statement containing a valid identifier assigned with a valid value. Python automatically creates the variable of the same data type as the data type of value assigned. Example 1:

marks=90

In above statement, variable marks is an integer variable containing value 90.

Example 2:

Sname=”Lovejot”

In above statement, variable Sname is a string variable containing value “Lovejot”.

Example 3:

Balance=1000.50

In above statement, variable Balance is a float variable containing value 1000.50.

Lvalues and Rvalues

lvalue : it is the variable or expressions that can come on the left hand side of an assignment statement.
rvalue : it is the expression that can come on the right hand side of an assignment statement.
Example:

Name=”Lovejot”

Here Name is lvalue and “Lovejot” is rvalue.
“Lovejot”=Name is wrong as rvalue “Lovejot” can’t come at left hand side of assignment statement.
** Lvalues can come on left hand side as well as right hand side of an assignment statement.
**Rvalues can only appear at right hand side of assignment statement.

Multiple Assignments

1. Assigning same value to multiple variables
We can assign same value to multiple variables in a single statement
Example:

a=b=c=20

It will assign value 20 to variables a, b, c i.e. all three variables a,b and c will refer to same location with value 20.

2. Assigning multiple values to multiple variables
We can even assign multiple values to multiple variables in single statement.
Example:

x,y,z = 10, 20, 30

First variable is x is given first value i.e. 10, second variable y gets second value i.e. 20 and third variable z gets value 30.
Because x is having value 25 and y is having 50. Now, if you want to swap values of x and y, you just need to write :

3. Interchanging values of two variables.
We can also interchange values of two variables in single statement.
Example:

x,y=10,20 #x gets 10 and y gets 20
x, y = y , x #x gets values of y i.e. 20 and y gets values of x i.e. 10
print (x,y) will give output as interchanged values i.e. 20 10

Variable Definition

A variable in Python is not created until some value is assigned to it.
Example:

print (x) #Line 1
x = 20
print (x)

When we run the above code, it will produce an error for the first statement (Line 1) i.e. name ‘x’ not defined.
This is because x has not been assigned any value but it is printed in line 1.
So, to correct the above code, we need to assign some value to x before using it in a statement as

x = 0 #variable x created now
print (x)
x = 20
print (x)

Now the above code will run without any error.

Dynamic Typing

Dynamic typing means that data type of a variable can change during program execution.
Example:

x = 10 #Statement 1
x = “Hello” #Statement2
print(x)

In Statement1 variable x is referring to an integer value 10.
In Statement2 variable x has been assigned a string value ”Hello”.
The output of above code will be
Hello
Because value of variable x has been modified by value ‘Hello’

Initially variable x is first referring to an integer value 10 and then to a string value “Hello”.
This is not an error as Python will accept it . This is known as Dynamic Typing.
Caution with Dynamic Typing
There is no problem in Python to change data type of a variable but programmer needs to be careful while assigning values as wrong assignment may generate errors .
Example

X = 10
Y = X/2
X = ‘Hello’
Y = X/2 #this lines will give error as string value can’t be divided .