Conditional Statements

Conditional statements allow your program to make decisions based on conditions_a. Python uses if, elif (else if), and else statements to control the flow of execution.

if Statement

The if statement evaluates a condition and executes a block of code if the condition is True:

age = 18

if age >= 18: # Check if age is greater than or equal to 18, if it is, it will run the block of code below (the indented region)
    print("You are an adult")

if-else Statement

When writing an if statement_b, you may want to provide an alternative block of code to execute when the condition is False. This is done using the else statement. Imagine the else statement as a backup plan, which will run if the condition is not met.

age = 16

if age >= 18: # Checking if the age is greater than or equal to 18 - (This is False because the age is 16)
    print("You are an adult")
else: # because the condition following the if statement is False, it will run the block of code below (the indented region)
    print("You are a minor")

if-elif-else Statement

The elif statement allows you to check multiple conditions in a single if statement. If the first condition is False, it checks the next condition, and so on. If none of the conditions are True, it executes the else block. You can also not include the else statement; however, it is not recommended as it can lead to some issues.

score = 85 

if score >= 90:   # This condition is skipped because it is False (the score is 85)
    grade = "A"
elif score >= 80: # This condition is True (the score is 85)
    grade = "B"   #this will be the block of code that is run
elif score >= 70: # Because the previous condition is True, this condition is skipped
    grade = "C"
elif score >= 60: # Because the previous condition is True, this condition is skipped
    grade = "D"
else:             # Because the previous condition(s) are True, the else block is not ran.
    grade = "F"

print(f"Your grade is {grade}")

Nested if Statements

You can nest conditional statements inside other conditional statements:

age = 25
income = 50000

if age >= 18: # Check if age is greater than or equal to 18, if it is, it will run the block of code below (the indented region)
    # Nested if statement
    if income >= 40000: 
        print("You are eligible for a premium credit card") # This condition is True (the income is 50000)
    else:
        print("You are eligible for a standard credit card")
else:
    print("You are not eligible for a credit card")

Note: Nesting too deeply can make your code hard to read. Try to keep your code as flat (less indentions) as possible and use functions to break down complex logic.

Conditional Expressions (Ternary Operator)

Python also provides a conditional expression (also known as a ternary operator) that allows you to write a simple if-else statement in a single line. This is useful for assigning values based on conditions without using multiple lines of code.

age = 20
status = "adult" if age >= 18 else "minor" # The condition is evaluated as True (the age is 20), so the value of status will be "adult"
print(status)  # "adult"

This is equivalent to:

# Equivalent to the if-else statement above
age = 20
if age >= 18:
    status = "adult"
else:
    status = "minor"
print(status)  # "adult"

Here is another sample:

# Another example
x = 15
result = "Even" if x % 2 == 0 else "Odd" # The condition is evaluated as False (x is 15), so the value of result will be "Odd"
print(f"{x} is {result}")

# Can be used in assignment, return values, etc.
temperature = 28
message = "It's hot" if temperature > 25 else "It's cool" # "It's hot"
print(message)

Truthy and Falsy Values

In Python, certain values are considered "truthy" or "falsy". This means that they evaluate to True or False when used in a conditional statement, even if they are not explicitly boolean values (True or False).

Falsy Values

Falsy values are values that evaluate to False in a boolean context. They include:

False
None
Zero (0, 0.0, 0j)
Empty sequences ('', [], (), {}, set())

Truthy Values

Truthy values are values that evaluate to True in a boolean context. They include:

True
Non-zero numbers (1, -1, 3.14)
Non-empty sequences ('hello', [1], (1,), {1: 'a'}, set([1]))
Any object that is not None or False

# Truthy and falsy values
print("Falsy values:")
print(f"bool(False): {bool(False)}") 
print(f"bool(None): {bool(None)}")
print(f"bool(0): {bool(0)}")
print(f"bool(''): {bool('')}")
print(f"bool([]): {bool([])}")
print(f"bool({{}}): {bool({})}")

print("\nTruthy values:")
print(f"bool(True): {bool(True)}")
print(f"bool(1): {bool(1)}")
print(f"bool(-1): {bool(-1)}")
print(f"bool('hello'): {bool('hello')}")
print(f"bool([0]): {bool([0])}") # List with one element (0), which is a non-empty list, you will learn about lists in the next unit
print(f"bool({{'key': 'value'}}): {bool({'key': 'value'})}") # Dictionary, which is a data structure that you will learn about in the next unit

This also means that you can use these values directly in conditional statements without explicitly comparing them to True or False (WE WILL USE THIS LATER):

# Using truthy and falsy values in conditionals
x = 0

if x:  # This condition is False because x is 0 (falsy value)
    print("x is truthy")
else:
    print("x is falsy")  # This will be printed

Logical Operators in Conditionals

You can also combine multiple conditions using logical operators: and, or, and not. These operators allow you to create more complex conditions.

Operator	Description	Example
`and`	True if both conditions are true (a AND b)	`if a and b:`
`or`	True if at least one condition is true (a OR b)	`if a or b:`
`not`	Inverts the boolean value (`True` -> `False`; `False` -> `True`)	`if not a:`

age = 25
has_license = True

# Using 'and' operator - both conditions must be True
if age >= 18 and has_license: #  This condition is True (age is 25 and has_license is True)
    print("You can drive")
else:
    print("You cannot drive")

# Using 'or' operator - at least one condition must be True
has_credit_card = True
has_cash = False 

if has_credit_card or has_cash: # this will run because ATLEAST one is true
    print("You can make a purchase")
else:
    print("You cannot make a purchase")

# Using 'not' operator - inverts the boolean value
is_weekend = False # This value will be flipped to True
if not is_weekend: # If is_weekend is False, this condition is True.
    print("Time to work")
else:
    print("Time to relax")

Membership and Identity Tests

Membership and identity tests are used to check if a value is part of a collection or if two variables point to the same object in memory.

Operator	Description	Example
`in`	True if the value is found in the collection (list, tuple, string, etc.)	`if value in collection:`
`is`	True if both variables point to the same object in memory	`if a is b:`

# Membership tests with 'in' and 'not in'
fruits = ["apple", "banana", "cherry"] 
# imagine a list as a collection of items in a box
# We are checking if this box contains a specific item (fruit) or not 
# We will cover this in more detail in the next unit; however, it is a important concept within control flows.
if "banana" in fruits:
    print("Yes, banana is in the list")

if "orange" not in fruits: # you can also chain other operators together, in this case, not in is used to check if the value is not in the list
    print("No, orange is not in the list")

# Identity tests with 'is' and 'is not'
a = [1, 2, 3]
b = [1, 2, 3]
c = a
# a and b are two different objects with the same content, while c is a reference to the same object as a

print(f"a == b: {a == b}")  # True - same values
print(f"a is b: {a is b}")  # False - different objects in memory
print(f"a is c: {a is c}")  # True - same object in memory

Best Practices

Be explicit: Make your conditions clear and easy to understand
Use parentheses when combining multiple conditions to make the logic clearer
Simplify nested conditions when possible
Avoid comparing directly to True/False/None:
- Use if x: instead of if x == True:
- Use if not x: instead of if x == False:
- Use if x is None: instead of if x == None:

# Best practices
x = 5
items = [1, 2, 3]

# Bad:
if x == True:
    print("x is True")

if items != []:
    print("Items is not empty")

# Good: (True is implied here)
if x:
    print("x is truthy")
if items:
    print("Items is not empty")

# Using parentheses for complex conditions and precedence
age = 25
income = 50000
has_debt = False

if (age >= 18 and income >= 40000) or (age >= 25 and not has_debt):
    print("Loan application approved")
else:
    print("Loan application denied")

Conditional statements are fundamental to creating dynamic and responsive programs. In the next lesson, we'll explore loops, which allow you to repeat code execution.

Footnotes

a - A condition is an expression that evaluates to either True or False.
b - A statement is a line of code that performs an action.

if Statement​

if-else Statement​

if-elif-else Statement​

Nested if Statements​

Conditional Expressions (Ternary Operator)​

Truthy and Falsy Values​

Falsy Values​

Truthy Values​

Logical Operators in Conditionals​

Membership and Identity Tests​

Best Practices​

Footnotes​