if...else Statement
Decision making is required when we want to execute a code only if a certain condition is satisfied.
The
if…elif…else
statement is used in Python for decision making.if test expression:
statement(s)
Here, the program evaluates the
test expression
and will execute statement(s) only if the test expression is True
.If the test expression is
False
, the statement(s) is not executed.In Python, the body of the
if
statement is indicated by the indentation. The body starts with an indentation and the first unindented line marks the end.Python interprets non-zero values as
True
. None
and 0
are interpreted as False
.Flowchart of if statement in Python programming

Flowchart of if statement in Python programming
# If the number is positive, we print an appropriate message
num = 3
if num > 0:
print(num, "is a positive number.")
print("This is always printed.")
num = -1
if num > 0:
print(num, "is a positive number.")
print("This is also always printed.")
When you run the program, the output will be:
3 is a positive number
This is always printed
This is also always printed.
In the above example,
num > 0
is the test expression.The body of
if
is executed only if this evaluates to True
.When the variable num is equal to 3, test expression is true and statements inside the body of
if
are executed.If the variable num is equal to -1, test expression is false and statements inside the body of
if
are skipped.The
print()
statement falls outside of the if
block (unindented). Hence, it is executed regardless of the test expression.if test expression:
Body of if
else:
Body of else
The
if..else
statement evaluates test expression
and will execute the body of if
only when the test condition is True
.If the condition is
False
, the body of else
is executed. Indentation is used to separate the blocks.Flowchart of if...else statement in Python

Flowchart of if...else statement in Python Programming
# Program checks if the number is positive or negative
# And displays an appropriate message
num = 3
# Try these two variations as well.
# num = -5
# num = 0
if num >= 0:
print("Positive or Zero")
else:
print("Negative number")
Output
Positive or Zero
In the above example, when num is equal to 3, the test expression is true and, the body of
if
is executed and the body
of else is skipped.If num is equal to -5, the test expression is false, and the body of
else
is executed and the body of if
is skipped.If num is equal to 0, the test expression is true and, the body of
if
is executed and body
of else is skipped.if test expression:
Body of if
elif test expression:
Body of elif
else:
Body of else
The
elif
is short for else if. It allows us to check for multiple expressions.If the condition for
if
is False
, it checks the condition of the next elif
block and so on.If all the conditions are
False
, the body of else is executed.Only one block among the several
if...elif...else
blocks is executed according to the condition.The
if
block can have only one else
block. But it can have multiple elif
blocks.Flowchart of if...elif....else statement in Python

Flowchart of if...elif....else in Python programming
'''In this program,
we check if the number is positive or
negative or zero and
display an appropriate message'''
num = 3.4
# Try these two variations as well:
# num = 0
# num = -4.5
if num > 0:
print("Positive number")
elif num == 0:
print("Zero")
else:
print("Negative number")
When variable num is positive, Positive number is printed.
If num is equal to 0, Zero is printed.
If num is negative, Negative number is printed.
We can have a
if...elif...else
statement inside another if...elif...else
statement. This is called nesting in computer programming.Any number of these statements can be nested inside one another. Indentation is the only way to figure out the level of nesting. They can get confusing, so they must be avoided unless necessary.
'''In this program, we input a number
check if the number is positive or
negative or zero and display
an appropriate message
This time we use nested if statement'''
num = float(input("Enter a number: "))
if num >= 0:
if num == 0:
print("Zero")
else:
print("Positive number")
else:
print("Negative number")
Output 1
Enter a number: 5
Positive number
Output 2
Enter a number: -1
Negative number
Output 3
Enter a number: 0
Zero
Last modified 2yr ago