Python  – Learn from Basic to Advanced

Python: Variables

Variable:

  • Variable is an identifier whose value can change. For example, variable age can have a different value for a different person.

  • Variable name should be unique in a program.

  • Variable declaration is implicit in Python, means variables are automatically declared and defined when they are assigned a value the first time.

  • Variables must always be assigned values before they are used in the program, otherwise, it will lead to an error.

  • Wherever a variable name occurs in the program, the interpreter replaces it with the value of that particular variable.

Value of a variable can be a string (for example, ‘A’, ‘Excellent’), number (for example 98.5) or any combination of alphanumeric (alphabets and numbers for example ‘B10’) characters.

In Python, we can use an assignment statement to create new variables and assign specific values to them.

The basic assignment statement has this form:

<variable> = <expr>

Here, <variable> is an identifier and <expr> is an expression.

For Example:

Class = 12

Remark = "Keep it up!"

Percentage = 98.5

# Variable created of numeric (integer) type

# Variable created of string type

# Variable created of numeric (floating point) type

There are some rules you have to follow for naming variables:

  • A variable starts with a letter A to Z or a to z or an underscore (_) followed by zero or more letters, underscores and digits (0 to 9).

  • Python does not allow special characters

  • Variable must not be a keyword of Python.

  • No blank spaces are allowed between the variable name.

  • Python is a case sensitive programming language.

Thus, Sname and sname are two different variables in Python.

The following variable names are valid:
roll_no

salary

sep98_score

index_age

percentage
amount

dob

GrossPay

Cust_no

D_o_j

__my_name

name_23

a1

b2_c3

Some invalid variable names which produce syntax errors are as follows:​​

>>> 98Sep_sco

>>> Your Age

>>> while

>>> myname@

SyntaxError: invalid syntax because variable start with a digit

SyntaxError: invalid syntax because variable contains space

SyntaxError: invalid syntax because variable is a reserve word

SyntaxError: invalid syntax because variable contains a special  character

Variables can also be assigned values in different forms. These are

 

1. Multiple Assignments:

  • Assigning same value to multiple variables:

          x = y = z = 100

  • It will assign value 100 to all three variables x, y and z.

  • Assigning multiple value to multiple variable in single line

         p, q, r = 10, 20, 30

  • It will assign the value order wise that is value 10 assign to variable p, value 20 assign to variable q and value 30 assign to variable r.

2. Assigning an expression.

An expression can be assigned to a variable. For example,

x = 10                 # x is an L-value
x = 3.9 * x * (1 - x)  # (3.9 * x * (1 – x)) is an R-value

print(x)
Celsius = 40
Fahrenheit = 9.0 / 5.0 * Celsius + 32

print(Celsius, Fahrenheit)

OUTPUT:

-351.0
40 104.0

3. A variable can be assigned many times.

It always retains the value of the most recent assignment. For example,

>>> Var1 = 7 # Var1 is an L-value

>>> Var1

    7        # OUTPUT

>>> Var1 = 0 # L-value can be modified

>>> Var1

    0        # OUTPUT

>>> Var1 = Var1 + 10 # (Var1 + 10) is an R-value

>>> Var1

    10       # OUTPUT

Notice that the above lines assigned many values to the same variable and produces different results. But the last assigned value is the correct value for Var1. The last assignment shows how the current value of a variable can be used to update its value.

4. Using sequences to assign multiple values at once.

Using the assignment operator, we can assign sequences like tuple and list values into multiple values. For example,

>>> days = (31,28,31,30,31,30,31,31,30,31,30,31)
>>> Jan,Feb,Mar,Apr,May,Jun,Jul,Aug,Sep,Oct,Nov,Dec) = days
>>> Jan
31   
# OUTPUT
>>> Feb
28   
# OUTPUT

Concepts of L-Value and R-Value

  • Lvalue refers to object to which you can assign value. It refers to memory location. It can appear LHS or RHS of assignment

  • Rvalue refers to the value we assign to any variable. It can appear on RHS of assignment

For Example:

# Num1 is an L-value

num1 = 100

# Num1 is an L-value

num2 = 200

#(Num1 +Num2) is an R-value

result = num1 + num2

print(result)

Example: Write a Python program to find the area of a triangle given that its base is 20 units and height is 40 units.

base = 20

height = 40

area = 0.5 * base * height

print("The Area is: ", area)

OUTPUT:

The Area is: 400

 
 
 

Python Basic Tutorial

 
 
 

From the above declarations, days is a tuple of twelve elements, and (Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec) is a tuple of three variables. Assigning one to the other assigns each of the values of days to each of the variables (Jan, Feb, ….) in order.

OUTPUT:

300

 

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