If…else branching control structure in Python

In this article, we will learn how to use the if…else structure in Python. The if…else statement is used to execute code if a specific condition is met. It allows us to make a decision in the code flow. The logic of the branching control structure if…else in Python is similar to other languages like C++, Java,…

1. If statement in Python

The syntax of the if statement in Python:

if condition:
    #statement(s)

If condition is True, then the statements in the body of the if statement will be executed. After condition is the colon (:) and the body of the if starts with indentation.

Note: Python’s compiler will understand any condition value different from 0 as True. While None or 0 will be False.

Example:

a = 5
if a>1:
    print("a > 1:", True)
a = 5
if a:
    print("a:", True)
a = '/'
if a:
    print("a = '/':", True)
a = 5
# False, not print True
if a>7:
    print("a > 7:", True)
a = 0
# False, not print True
if a:
    print("a:", True)
a = None
# False, not print True
if a:
    print("a:", True)

Result

a > 1: True
a: True
a = '/': True

2. If…else statement in Python

The syntax of the if…else statement in Python:

if condition:
    #Body of if
else:
    #Body of else

If condition is True, then the statements in the body of the if statement will be executed. Otherwise, if condition is False, the statements in the body of the else statement will be executed.

Example:

num = 3
if num >= 0:
    print("num is greater than 0 or equal to 0")
else:
    print("num is smaller than 0")

num = -1
if num >= 0:
    print("num is greater than 0 or equal to 0")
else:
    print("num is smaller than 0")

Result

num is greater than 0 or equal to 0
num is smaller than 0

3. If…elif…else statement in Python

If there are multiple conditions to compare and choose, we can use the if…elif…else statement. The syntax of the if…elif…else statement in Python:

if condition1:
    #Body of if
elif condition2:
    #Body of elif
else:
    #Body of else

In case of condition1 is True, the body of the if will executed. However, if condition1 is False, we move to condition2. If condition2 is True, it will execute elif statement. On the other hand, the else statement will be executed.

Example:

num = 0
if num > 0:
    print("num is greater than 0")
elif num == 0:
    print("num is equal to 0")
else:
    print("num is smaller than 0")

Result

num is equal to 0

In case there are more conditions, we can use more elif. Example:

mark = 7.5
if mark >= 9:
    print("excellent")
elif mark >= 8 and mark < 9:
    print("very good")
elif mark >= 6.5 and mark < 8:
    print("good")
elif mark >= 5 and mark < 6.5:
    print("average")
elif mark >= 3.5 and mark < 5:
    print("weak")
else:
    print("poor")

Result

good

4. Nested if statements in Python

If statements can be in another if statement. This is called a nested if statement. Example:

price = 50
quantity = 5
amount = price*quantity

if amount > 100:
    if amount > 500:
        print("Amount is greater than 500")
    else:
        if amount < 500 and amount > 400:
            print("Amount is")
        elif amount < 500 and amount > 300:
            print("Amount is between 300 and 500")
        else:
            print("Amount is between 200 and 500")
elif amount == 100:
    print("Amount is 100")
else:
    print("Amount is less than 100")

Result

Amount is between 200 and 500

5. Shortened if statement in Python

We can write a shortened if statement in 1 line with the following syntax:

if condition: statement

Example:

i = 15
if i % 3 == 0 : print(i,"is divisible by 3.")

Result

15 is divisible by 3.

We can write a shortened if…else statement in 1 line with the following syntax:

statement_when_True if condition else statement_when_False

Example:

i = 16
print(i,"is divisible by 3.") if  i % 3 == 0 else print(i,"is not divisible by 3.")

Result

16 is not divisible by 3.

Try to master the if…else branching control structure because you will use it a lot in your programs.