exception_handling

Exception Handling

Structured error handling with try/except/else/finally, custom exceptions, and exception chaining.

Files

File	Description
example.py	Custom exceptions, chaining, else/finally

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Descriptive Example

Scenario

Parse user age from input — convert errors into a domain-specific ValidationError with clear context.

class ValidationError(Exception):
    def __init__(self, field, message):
        self.field = field
        super().__init__(f"{field}: {message}")

def parse_age(value):
    try:
        age = int(value)
    except ValueError as e:
        raise ValidationError("age", f"must be integer, got {value!r}") from e

    if age < 0 or age > 150:
        raise ValidationError("age", "must be between 0 and 150")
    return age

parse_age("twenty")   # ValidationError: age: must be integer, got 'twenty'
                      # __cause__ links to original ValueError

try/except/else/finally flow

try:
    result = risky_operation()
except SpecificError as e:
    handle(e)
else:
    on_success(result)      # runs only if NO exception
finally:
    cleanup()               # ALWAYS runs

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Interview Q&A

Q1: Difference between except Exception and bare except:?
A: Bare except: catches everything including KeyboardInterrupt and SystemExit. Never use it. Catch specific exceptions or Exception when re-raising.

Q2: When is the else clause useful?
A: Code that should run only on success — kept out of try so its own exceptions aren't caught by the handler above.

Q3: What does raise NewError(...) from original do?
A: Sets __cause__ on the new exception, preserving the original traceback chain for debugging.

Q4: What is the exception hierarchy?
A: BaseException → Exception → specific errors (ValueError, TypeError, etc.). Catch subclasses, not BaseException.

Q5: When should you create custom exception classes?
A: For domain-specific errors (validation, auth, payment) that callers can catch specifically. Add attributes like field or code.

Q6: finally vs context manager?
A: Both guarantee cleanup. Context managers (with) are preferred — cleaner syntax and composable via ExitStack.

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Run

python3 example.py