raise — Intermediate Examples

Throws an exception manually

EasyIntermediateAdvancedExpert

Re-raising and chaining exceptions

Using raise in except blocks.

python
# Re-raise the current exception
def process(data):
    try:
        return int(data)
    except ValueError:
        print(f"Warning: could not process {data!r}")
        raise  # re-raise the same exception

try:
    process("abc")
except ValueError as e:
    print(f"Caught re-raised: {e}")

# Exception chaining with 'from'
class AppError(Exception):
    pass

def load_config(path):
    try:
        raise FileNotFoundError(f"No such file: {path}")
    except FileNotFoundError as e:
        raise AppError("Configuration failed") from e

try:
    load_config("config.yaml")
except AppError as e:
    print(f"Error: {e}")
    print(f"Caused by: {e.__cause__}")

Bare 'raise' re-throws the current exception. 'raise X from Y' chains exceptions, setting __cause__ for traceback clarity.

Custom exception hierarchies

Defining your own exception classes.

python
class APIError(Exception):
    def __init__(self, message, status_code=500):
        super().__init__(message)
        self.status_code = status_code

class NotFoundError(APIError):
    def __init__(self, resource):
        super().__init__(f"{resource} not found", 404)

class AuthError(APIError):
    def __init__(self):
        super().__init__("Authentication required", 401)

errors = [NotFoundError("User"), AuthError(), APIError("Server crash")]
for err in errors:
    print(f"[{err.status_code}] {err}")
Expected Output
[404] User not found
[401] Authentication required
[500] Server crash

Custom exception hierarchies let you catch broad categories (APIError) or specific ones (NotFoundError). Store extra data as attributes.

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