Exception Handling

Introduction

Use try / except / else / finally. Catch specific exceptions, not bare except:. Define custom exception classes for your domain.

Understanding the topic

Core concepts to understand:

• try / except / else / finally
• Catch specific exceptions: except ValueError:
• raise ... from e chains exceptions.
• Custom exceptions inherit from Exception.

Syntax reference

Visual flow / code:

class PaymentError(Exception):
    """Domain-specific exception."""

def charge(amount: float) -> None:
    try:
        if amount <= 0:
            raise ValueError("amount must be positive")
        gateway_call(amount)
    except ValueError as e:
        raise PaymentError("invalid amount") from e
    except ConnectionError:
        logger.exception("Gateway down")
        raise
    else:
        logger.info("OK")
    finally:
        cleanup()

Execution workflow

Real-world use

Custom exception types let upstream code distinguish 'card declined' from 'gateway down' without parsing strings — essential in microservices.

Best practices

• Catch specific exceptions only.
• Use raise ... from to preserve cause.
• Log before re-raise.

Common mistakes

• except: swallows KeyboardInterrupt, SystemExit — never use bare except.

Hands-on exercise

Interview preparation — practice these questions:

• try/except/else/finally — when does else run?
• Why chain exceptions?
• EAFP vs LBYL?

Summary

In summary: Be specific; chain causes. EAFP is Pythonic.