Python If...Else

Python Conditionals: Control Flow & Pattern Matching

Conditional statements control program execution paths based on boolean evaluations (if, elif, else). Python 3.10+ also introduces structural pattern matching (match/case) for complex data destructuring.

Decision Flow Diagram

flowchart TD
    A["Start Evaluation"] --> B{"Condition 1 True?"}
    B -- "Yes" --> C["Execute If Block"]
    B -- "No" --> D{"Condition 2 True?"}
    D -- "Yes" --> E["Execute Elif Block"]
    D -- "No" --> F["Execute Else Block"]
    C --> G["Continue Program Execution"]
    E --> G
    F --> G

Syntax Overview

1. Standard Conditional: if condition: ... elif condition: ... else: ...
2. Ternary Operator: value = true_val if condition else false_val
3. Pattern Matching (Python 3.10+): match subject: case pattern: ...

Practical Code Example

from typing import Any, Dict

def handle_http_status(status_code: int) -> str:
    """Demonstrate Python 3.10+ Structural Pattern Matching."""
    match status_code:
        case 200 | 201:
            return "Success: Resource processed."
        case 400:
            return "Client Error: Bad Request."
        case 401 | 403:
            return "Security Alert: Unauthorized or Forbidden."
        case 404:
            return "Client Error: Resource Not Found."
        case 500 | 502 | 503:
            return "Server Error: Internal system failure."
        case _:
            return f"Unhandled Status Code: {status_code}"

def evaluate_discount(user_score: int, is_vip: bool) -> float:
    # Ternary Conditional Operator
    discount_rate = 0.20 if is_vip or user_score >= 100 else 0.05
    return discount_rate

if __name__ == "__main__":
    print(handle_http_status(200))
    print(handle_http_status(404))
    print(handle_http_status(999))
    
    rate = evaluate_discount(120, False)
    print(f"Applicable Discount Rate: {rate * 100}%")

Best Practices & Gotchas

• Keep Guard Clauses Flat: Avoid deep nested if statements. Return early (guard clause) to keep code readable.
• Match/Case vs If/Elif: Use match/case when matching data structures, types, or enumerations; use if/elif for range checks (x > 50).
• Careful with Assignment inside Conditions: Python does not allow assignment inside standard if x = 5: (SyntaxError). Use the walrus operator if (x := 5) > 2: if intentional.

Self-Check Challenge

Write a function categorize_age(age: int) -> str using if/elif/else that returns "Minor" (age < 18), "Adult" (18 <= age < 65), or "Senior" (age >= 65).