Introduction to Python If ... Else
Python If ... Else
The if-else
statement allows you to conditionally execute a block of code based on a given condition.
The if
block is executed if the condition is true, and the else
block is executed if the condition is false.
Here's the basic syntax of the if-else
statement:
if condition:
# code block to be executed if the condition is true
else:
# code block to be executed if the condition is false
Here's an example that demonstrates the if-else
statement:
age = 20
if age >= 18:
print("You are an adult.")
else:
print("You are a minor.")
In this example:
- If the
age
is greater than or equal to 18, the code inside theif
block is executed, and it prints "You are an adult." If theage
is less than 18, the code inside theelse
block is executed, and it prints "You are a minor."
You can also include multiple conditions using the elif
statement, which stands for "else if".
The elif
statement allows you to check for additional conditions after the initial if
statement.
As an example:
score = 75
if score >= 90:
print("You got an A.")
elif score >= 80:
print("You got a B.")
elif score >= 70:
print("You got a C.")
elif score >= 60:
print("You got a D.")
else:
print("You failed.")
In this example:
- The code checks the value of the
score
variable and prints the corresponding grade based on the score. - The conditions are checked in order, and the first condition that is true is executed.
- If none of the conditions are true, the code inside the else block is executed.
Nested if-else
Statements
You can nest if-else
statements within each other to handle more complex conditions. This allows you to check for multiple conditions and execute different code blocks accordingly.
As an example:
x = 10
y = 5
if x > y:
print("x is greater than y.")
else:
if x < y:
print("x is less than y.")
else:
print("x is equal to y.")
Ternary Operator
Python supports a shorthand syntax called the ternary operator, which allows you to write a simple if-else
statement in a single line.
It has the following syntax:
value_if_true if condition else value_if_false
As an example:
x = 10
y = 5
result = "x is greater than y." if x > y else "x is less than or equal to y."
print(result)
Chained Comparison Operators
You can use chained comparison operators to simplify complex conditions. Instead of using multiple if
statements, you can chain the comparison operators together.
As an example:
x = 10
if 0 < x < 100:
print("x is between 0 and 100.")
Truthy and Falsy Values
In Python, certain values are considered "truthy" or "falsy" when used in conditions.
Truthy values evaluate to True
, while falsy values evaluate to False
.
It can be useful in if-else
statements. Some examples of falsy values include False
, None
, 0
, ""
(empty string), and empty data structures such as empty lists, sets, dictionaries, and tuples.
value = 0
if value:
print("Value is truthy.")
else:
print("Value is falsy.")
Short-Circuit Evaluation
Python uses short-circuit evaluation for logical operators (and
and or
).
If the result of the expression can be determined by evaluating only the first operand, the second operand is not evaluated.
This can be useful in conditions that involve costly operations or function calls.
x = 5
y = 10
if x > 0 and y / x > 2:
print("Condition is true.")
In this example:
- The
y / x
division is not evaluated ifx
is less than or equal to 0, preventing a potential division by zero error.