Python Conditional Statements

Note: This post is based on my old programming study notes when I taught myself.

Basic Syntax

if condition_1:
    Statement_1
    
elif condition_2:
    Statement_2
    
else:
    Statement_3

If condition_1 is True, then execute Statement_1. If both condition_1 and condition_2 are False, then execute Statement_3. elif can be used multiple times if required.

Important Notes

• Always be careful with indentation - Python uses indentation to define code blocks
• Colon at the end of condition - Each condition line must end with :

# Example
age = 18

if age >= 18:
    print("You are an adult")
elif age >= 13:
    print("You are a teenager")
else:
    print("You are a child")

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Comparison Operators

Operator	Description
x < y	Less than
x > y	Greater than
x == y	Equal to
x != y	Not equal to
x >= y	Greater than or equal to
x <= y	Less than or equal to

# Examples
score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

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Logical Operators

and, or, not

Operator	Description
x and y	Both x and y are True, then True
x or y	Either x or y is True, then True
not x	If x is False, then True

# Examples
temperature = 75
sunny = True

if temperature > 70 and sunny:
    print("Perfect weather for a picnic!")
    
if temperature < 32 or temperature > 100:
    print("Extreme weather conditions")
    
if not sunny:
    print("Don't forget your umbrella")

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Membership Operators

in, not in

• x in [List/Tuple/String]
• x not in [List/Tuple/String]

# Examples
fruits = ['apple', 'banana', 'orange']
favorite = 'apple'

if favorite in fruits:
    print("Your favorite fruit is available")
    
if 'grape' not in fruits:
    print("We don't have grapes")

# String membership
text = "Hello World"
if 'World' in text:
    print("Found 'World' in the text")

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pass Statement

When we want “Nothing to happen for a certain condition”, use pass.

names = ['Bruce', 'James', 'Bobby', 'Jessica']

if 'Jessica' in names:
    pass  # Do nothing
else:
    print('There is a stranger in our crew.')

Explanation: If Jessica is in the names list, then nothing happens. If the name is not in the list, execute the else part.

# Another example - placeholder for future code
def process_data(data):
    if len(data) > 100:
        pass  # Placeholder for large-dataset handling
    else:
        print("Processing small dataset")

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One-line if Statement

If the statement after condition is just one line, we can write the if condition and statement on one line.

pocket = ['iPhone', 'money', 'apple']

if 'money' in pocket: pass
else: print("Show me your money")

# More practical examples
age = 20
print("Adult") if age >= 18 else print("Minor")

# Or with variables
status = "eligible" if age >= 18 else "not eligible"

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Conditional Expression (Ternary Operator)

Traditional way:

if score >= 90:
    message = "Pass"
else:
    message = "Fail"

Using conditional expression:

message = "Pass" if score >= 90 else "Fail"

More examples:

# Determine absolute value
number = -5
absolute = number if number >= 0 else -number

# Set default value
username = input_name if input_name else "Anonymous"

# Choose maximum
maximum = a if a > b else b

# Nested conditional expressions
grade = "A" if score >= 90 else "B" if score >= 80 else "C" if score >= 70 else "F"

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Common Patterns

Multiple Conditions

# AND conditions
if age >= 18 and has_license and has_insurance:
    print("You can drive")

# OR conditions  
if payment_method == "cash" or payment_method == "card" or payment_method == "digital":
    print("Payment accepted")

# Better way for OR with multiple values
if payment_method in ["cash", "card", "digital"]:
    print("Payment accepted")

Range Checking

# Check if number is in range
number = 75
if 0 <= number <= 100:
    print("Number is within valid range")

# Grade boundaries
if 90 <= score <= 100:
    grade = "A"
elif 80 <= score < 90:
    grade = "B"
elif 70 <= score < 80:
    grade = "C"

Truthy and Falsy Values

# These evaluate to False
empty_list = []
empty_string = ""
zero = 0
none_value = None

if not empty_list:
    print("List is empty")

# These evaluate to True
non_empty_list = [1, 2, 3]
non_empty_string = "hello"
non_zero = 42

if non_empty_list:
    print("List has items")

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