Exception Handling in Python

🟡 Intermediate

📖 Definition

Exception Handling is a mechanism that allows Python programs to intercept, process, and recover from runtime errors (like division by zero, invalid input parsing, or missing files) gracefully without crashing application execution.

🇮🇳 Hindi

Program crash hone se bachane ke liye Exception Handling ka use hota hai. Risky code ko try block mein rakha jata hai, error aane par except block chalta hai, else block tabhi chalta hai jab koi error na ho, aur finally block hamesha execute hota hai (cleanup ke liye).

🚩 Marathi

Application crash hone pasun vachvanyasathi try...except block vaparatat. Cleanup sathi finally block hamesha run hoto.

📝 1. Exception Handling Architecture

  • try: Encloses code that might raise an exception during execution.
  • except ExceptionType as err: Catches and handles specific exception types.
  • else: Executes ONLY if no exception occurred in the try block.
  • finally: ALWAYS executes (whether an exception occurred or not) for resource cleanup.
  • raise ExceptionType("message"): Manually triggers/raises an exception.
       +------------------------------------+

       | try: Risky Operation Execution      |
       +-----------------+------------------+

                         |
           Has Exception?|
          +--------------+--------------+

          |                             |
          ▼ YES                         ▼ NO
+-------------------------+   +-------------------------+

| except ExceptionType:   |   | else:                   |
| Handles Error           |   | Executes if NO exception|
+------------+------------+   +------------+------------+

             |                             |
             +--------------+--------------+

                            |
                            ▼
               +--------------------------+

               | finally: ALWAYS Runs!    |
               +--------------------------+

📝 2. Common Built-In Python Exceptions

  • ValueError: Invalid value passed to function (e.g. int("abc")).
  • TypeError: Invalid operation on incompatible types (e.g. "2" + 2).
  • ZeroDivisionError: Division by zero (10 / 0).
  • KeyError: Non-existent dictionary key accessed.
  • IndexError: Sequence index out of bounds.
  • FileNotFoundError: Trying to open a missing file.

💡 Complete Example: Robust Order Processing System

# Custom Exception Definition
class InsufficientBalanceError(Exception):
    """Raised when account balance is less than transaction amount."""
    pass

def process_withdrawal(account_balance, withdrawal_amount_str):
    """Processes withdrawal with comprehensive error handling & validation."""
    try:
        # 1. Parsing numeric input (Can raise ValueError)
        amount = float(withdrawal_amount_str)
        
        # 2. Input Validation
        if amount <= 0:
            raise ValueError("Withdrawal amount must be greater than zero.")
            
        if amount > account_balance:
            raise InsufficientBalanceError(
                f"Requested ₹{amount:.2f} exceeds available balance ₹{account_balance:.2f}."
            )
            
        account_balance -= amount
        
    except ValueError as err:
        print(f"[VAL_ERROR] Invalid Input : {err}")
        return account_balance, False
        
    except InsufficientBalanceError as err:
        print(f"[FUND_ERROR] Balance Issue: {err}")
        return account_balance, False
        
    except Exception as err:
        print(f"[UNEXPECTED_ERROR] Unexpected: {err}")
        return account_balance, False
        
    else:
        # Runs ONLY if withdrawal succeeded without errors!
        print(f"[SUCCESS] Withdrew ₹{amount:.2f}. Remaining: ₹{account_balance:.2f}")
        return account_balance, True
        
    finally:
        # Always runs for audit trail logging!
        print("[AUDIT LOG] Withdrawal processing attempt complete.")

# Testing Test Cases
print("=== TESTING EXCEPTION HANDLING SYSTEM ===")

current_balance = 5000.00

print("\n--- Test Case 1: Non-numeric String Input ---")
current_balance, _ = process_withdrawal(current_balance, "abc")

print("\n--- Test Case 2: Insufficient Funds ---")
current_balance, _ = process_withdrawal(current_balance, "7500.00")

print("\n--- Test Case 3: Valid Transaction ---")
current_balance, _ = process_withdrawal(current_balance, "1200.00")

👀 Output

=== TESTING EXCEPTION HANDLING SYSTEM ===

--- Test Case 1: Non-numeric String Input ---
[VAL_ERROR] Invalid Input : could not convert string to float: 'abc'
[AUDIT LOG] Withdrawal processing attempt complete.

--- Test Case 2: Insufficient Funds ---
[FUND_ERROR] Balance Issue: Requested ₹7500.00 exceeds available balance ₹5000.00.
[AUDIT LOG] Withdrawal processing attempt complete.

--- Test Case 3: Valid Transaction ---
[SUCCESS] Withdrew ₹1200.00. Remaining: ₹3800.00
[AUDIT LOG] Withdrawal processing attempt complete.

⚠️ Common Mistakes

  • Using bare except: without specifying an exception class (catches ALL exceptions including KeyboardInterrupt and SystemExit, making terminal cancellation impossible!).
  • Swallowing exceptions silently with except Exception: pass without logging error context, making debugging impossible!

🛡️ Safety / Important Notes

Always catch specific exception types (ValueError, FileNotFoundError) before catching broader parent exceptions (Exception).

🌍 Real-World Usage

Handling API network connection timeouts, catching invalid JSON payload parsing, handling database connection retries, and validating user input forms.

🧪 Try It Yourself

  1. Write a program that prompts the user for two numbers and divides them inside a try...except block, catching ZeroDivisionError and ValueError.
  2. Add a finally block printing "Calculation attempted.".

🎯 Mini Challenge

Create a custom exception InvalidEmailError and write a function validate_email(email) that raises it if the email string does not contain @ or ..

🔗 Related Topics

🧭 Navigation

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