Using if-else Conditions in Python

Table of Contents

Introduction to Conditional Statements {#introduction}

Conditional statements in Python allow programs to make decisions based on different conditions. They control the flow of execution by evaluating expressions that return Boolean values (True or False). The primary conditional statements in Python are if, elif, and else.

Key Characteristics

| Feature | Description | |---------|-------------| | Syntax | Uses keywords if, elif, else followed by colons | | Indentation | Code blocks are defined by consistent indentation | | Boolean Context | Conditions are evaluated in Boolean context | | Multiple Conditions | Support for complex logical expressions | | Nested Structure | Conditions can be nested within other conditions |

Truth Values in Python

Python evaluates various data types in Boolean context:

| Data Type | True Values | False Values | |-----------|-------------|--------------| | Numbers | Non-zero numbers | 0, 0.0, 0j | | Strings | Non-empty strings | Empty string "" | | Collections | Non-empty lists, tuples, sets, dicts | Empty collections | | None | Never | None | | Boolean | True | False |

Basic if Statement {#basic-if}

The if statement is the simplest form of conditional execution. It executes a block of code only when a specified condition is True.

Syntax Structure

`python if condition: # Code block to execute when condition is True statement1 statement2 # More statements... `

Basic Examples

`python

Simple numeric condition

String condition

Boolean variable

List condition (checking if not empty)

Notes on Basic if Statements

- The condition after if must evaluate to a Boolean value - Indentation is crucial in Python - typically 4 spaces - Multiple statements can be executed within the if block - If the condition is False, the entire block is skipped

if-else Statement {#if-else}

The if-else statement provides an alternative path of execution when the condition is False.

Syntax Structure

`python if condition: # Code executed when condition is True true_statements else: # Code executed when condition is False false_statements `

Comprehensive Examples

`python

Age verification system

Temperature control system

File processing example

Practical Applications

| Use Case | Example Scenario | |----------|------------------| | User Authentication | Check credentials and grant/deny access | | Input Validation | Verify user input meets requirements | | Error Handling | Handle success/failure scenarios | | Configuration | Set different parameters based on conditions |

if-elif-else Statement {#if-elif-else}

The elif (else if) statement allows checking multiple conditions in sequence. It's more efficient than multiple separate if statements.

Syntax Structure

`python if condition1: # Code for condition1 statements1 elif condition2: # Code for condition2 statements2 elif condition3: # Code for condition3 statements3 else: # Default code when no conditions are met default_statements `

Detailed Examples

`python

Grade calculation system

Traffic light system

Season determination

Nested Conditions {#nested-conditions}

Nested conditions involve placing if-else statements within other if-else statements, allowing for more complex decision-making logic.

Structure and Examples

`python

Complex user access control system

Banking transaction validation

Comparison Operators {#comparison-operators}

Comparison operators are used to compare values and return Boolean results.

Operator Reference Table

| Operator | Description | Example | Result | |----------|-------------|---------|---------| | == | Equal to | 5 == 5 | True | | != | Not equal to | 5 != 3 | True | | < | Less than | 3 < 5 | True | | <= | Less than or equal | 5 <= 5 | True | | > | Greater than | 7 > 3 | True | | >= | Greater than or equal | 5 >= 3 | True |

Practical Examples

`python

Numeric comparisons

String comparisons

Date and time comparisons

def check_event_eligibility(birth_date, event_date, min_age=18): age = (event_date - birth_date).days // 365 if age >= min_age: if event_date > date.today(): return { "eligible": True, "message": f"Eligible for event (Age: {age})", "days_until_event": (event_date - date.today()).days } elif event_date == date.today(): return { "eligible": True, "message": f"Event is today! (Age: {age})", "days_until_event": 0 } else: return { "eligible": False, "message": "Event has already passed", "days_until_event": -1 } else: return { "eligible": False, "message": f"Too young (Age: {age}, Required: {min_age})", "days_until_event": None } `

Logical Operators {#logical-operators}

Logical operators combine multiple conditions and return Boolean values.

Logical Operators Table

| Operator | Description | Truth Table | Example | |----------|-------------|-------------|---------| | and | Returns True if both conditions are True | True and True = True
True and False = False
False and True = False
False and False = False | age >= 18 and has_license | | or | Returns True if at least one condition is True | True or True = True
True or False = True
False or True = True
False or False = False | is_weekend or is_holiday | | not | Returns the opposite Boolean value | not True = False
not False = True | not is_banned |

Complex Examples

`python

Advanced user authentication system

Weather-based activity recommendation

Membership and Identity Operators {#membership-identity}

These operators check for membership in sequences and object identity.

Operators Table

| Operator | Description | Example | Use Case | |----------|-------------|---------|----------| | in | Returns True if value is found in sequence | 'a' in 'apple' | Check if item exists | | not in | Returns True if value is not found in sequence | 'z' not in 'apple' | Verify absence | | is | Returns True if variables point to same object | x is None | Identity comparison | | is not | Returns True if variables point to different objects | x is not None | Identity comparison |

Comprehensive Examples

`python

Content filtering system

System configuration validator

Conditional Expressions (Ternary Operator) {#ternary}

Python's conditional expression provides a concise way to write simple if-else statements in a single line.

Syntax

`python value_if_true if condition else value_if_false `

Examples and Use Cases

`python

Basic ternary operations

Mathematical operations with ternary

String formatting with ternary

List comprehension with ternary

Best Practices {#best-practices}

Code Organization and Readability

| Practice | Good Example | Poor Example | |----------|--------------|--------------| | Avoid Deep Nesting | Use early returns | Multiple nested if statements | | Use Descriptive Conditions | if user_is_authenticated: | if x == True: | | Consistent Indentation | 4 spaces throughout | Mixed tabs and spaces | | Logical Grouping | Related conditions together | Scattered condition checks |

Improved Code Examples

`python

Good practice: Early returns to reduce nesting

Good practice: Extract complex conditions into functions

def is_premium_customer(customer): return customer.get('subscription_type') == 'premium' and customer.get('account_status') == 'active'

def calculate_support_priority(ticket, customer, current_time): if ticket['severity'] == 'critical': return 'immediate' if is_premium_customer(customer): if is_business_hours(current_time): return 'high' else: return 'medium' if ticket['severity'] == 'high' and is_business_hours(current_time): return 'medium' return 'low'

Good practice: Use constants for magic numbers

def calculate_order_total(items, customer_type="standard"): subtotal = sum(item['price'] * item['quantity'] for item in items) # Apply bulk discount total_quantity = sum(item['quantity'] for item in items) if total_quantity >= OrderConstants.BULK_ORDER_QUANTITY: subtotal *= (1 - OrderConstants.BULK_DISCOUNT_RATE) # Calculate shipping shipping = 0 if subtotal >= OrderConstants.FREE_SHIPPING_THRESHOLD else OrderConstants.STANDARD_SHIPPING_COST # Calculate tax tax = subtotal * OrderConstants.TAX_RATE return { 'subtotal': round(subtotal, 2), 'shipping': shipping, 'tax': round(tax, 2), 'total': round(subtotal + shipping + tax, 2) } `

Common Patterns and Examples {#patterns}

Pattern Matching Alternatives

`python

Dictionary-based dispatch pattern

State machine pattern

Validation chain pattern

def validate_username(username): if len(username) < 3: return {"valid": False, "message": "Username must be at least 3 characters"} elif len(username) > 20: return {"valid": False, "message": "Username must be less than 20 characters"} elif not username.isalnum(): return {"valid": False, "message": "Username must contain only letters and numbers"} else: return {"valid": True, "message": "Username is valid"}

def validate_email(email): if "@" not in email: return {"valid": False, "message": "Email must contain @ symbol"} elif "." not in email.split("@")[1]: return {"valid": False, "message": "Email domain must contain a dot"} else: return {"valid": True, "message": "Email format is valid"} `

Error Handling with Conditions {#error-handling}

Defensive Programming Patterns

`python

Comprehensive error handling with conditions

File processing with comprehensive error handling

This comprehensive guide covers all aspects of using if-else conditions in Python, from basic syntax to advanced patterns and best practices. The examples demonstrate real-world applications and provide a solid foundation for understanding conditional logic in Python programming.