Python If Statements
The Python if statement is a statement which is used to test
specified condition. We can use if statement to perform conditional operations
in our Python application.
The if statement executes only when specified condition is true. We can pass any valid expression into the if parentheses.
There are various types of if statements in Python.
- if statement
- if-else statement
- nested if statement
Python if Statement Syntax
if test expression:
statement(s)
Here, the program evaluates the
test
expression and will execute statement(s) only if the text expression
is True.
If the text expression is
False, the statement(s) is
not executed.
In Python, the body of the
if statement is
indicated by the indentation. 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.
Example: Python if Statement
# 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.")
In the above
example, num
> 0 is the test
expression.
The body of if is executed only if this evaluates to True. When variable num is equal to 3,
test expression is true and body inside body of if is executed.If variable num is equal to -1, test expression is false and body inside
body of if is skipped. The print() statement falls outside of the if block (unindented). Hence, it is executed regardless of
the test expression.
Python if...else Statement
Syntax of if...else
if test expression:
Body of if
else:
Body of else
The
if..else statement
evaluates test expression and will execute
body of if only when test condition is True.
If the condition is
False, body of else is executed.
Indentation is used to separate the blocks.
Example of if...else
# 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")
In the above example, when num is equal to 3, the
test expression is true and body of
ifis executed and body of else is
skipped.
If num is equal to -5,
the test expression is false and body of
else is executed and
body of if is skipped.
If num is equal to 0, the
test expression is true and body of
if is executed
and body of else is skipped.
Python
if...elif...else Statement / Nested If
Else Statement
In python, we can use
nested If Else to check multiple conditions. Python provides elif keyword
to make nested If statement.
This
statement is like executing a if statement inside a else statement.
Syntax of if...elif...else
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, 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.
Example of if...elif...else
# 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
Python Nested if statements
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. This can get
confusing, so must be avoided if we can.
Python Nested if Example
# 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
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
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