Python Try...Except

Python Exception Handling: try/except/else/finally

Exceptions represent runtime errors that disrupt program execution. Python handles errors gracefully using try, except, else, and finally blocks, along with custom exception hierarchies.

Exception Execution Lifecycle

flowchart TD
    A["Enter try Block"] --> B{"Exception Raised?"}
    B -- "Yes" --> C{"Matches except Clause?"}
    C -- "Yes" --> D["Execute except Block"]
    C -- "No" --> E["Propagate Upward"]
    B -- "No" --> F["Execute else Block"]
    D --> G["Execute finally Block"]
    F --> G
    G --> H["Continue Execution"]

Block Responsibilities

• try: Contains code that might trigger an exception.
• except ExceptionType as err: Catches and handles specific exception instances.
• else: Executes ONLY if NO exception was raised in the try block.
• finally: Executes ALWAYS (regardless of whether an exception occurred or was handled).

Practical Code Example

class InsufficientFundsError(Exception):
    """Custom domain exception for account withdrawals."""
    def __init__(self, balance: float, amount: float) -> None:
        super().__init__(f"Cannot withdraw ${amount:.2f}; available balance is ${balance:.2f}.")
        self.balance = balance
        self.amount = amount

def process_withdrawal(balance: float, amount: float) -> float:
    if amount > balance:
        raise InsufficientFundsError(balance, amount)
    return balance - amount

def safe_execution_wrapper() -> None:
    current_balance = 100.0
    
    try:
        new_balance = process_withdrawal(current_balance, 150.0)
    except InsufficientFundsError as err:
        print(f"Caught Custom Exception: {err}")
    except Exception as general_err:
        print(f"Unexpected System Error: {general_err}")
    else:
        print(f"Withdrawal Successful! Remaining Balance: ${new_balance}")
    finally:
        print("Audit Log: Transaction attempt closed.")

if __name__ == "__main__":
    safe_execution_wrapper()

Best Practices & Gotchas

• Do Not Catch Bare except:: Catching bare except: traps system signals like KeyboardInterrupt and SystemExit, making scripts impossible to stop cleanly with Ctrl+C. Catch except Exception: instead.
• Keep try Blocks Narrow: Wrap only the specific lines of code that can raise the anticipated exception inside the try block.
• Use Exception Chaining: Use raise NewException(...) from original_error to preserve original tracebacks.

Self-Check Challenge

Write a function safe_divide(a: float, b: float) -> float | None that catches ZeroDivisionError and prints a warning instead of crashing.