all Python 3Exceptionsconstruct

raise

raising

Raises an exception by hand. raise ... from chains exceptions together. A bare raise re-raises the current one.

Syntax

raise ValueError('msg')
raise RuntimeError('msg') from original

A bare raise is legal only inside an exception handler (otherwise RuntimeError: No active exception to re-raise) and it preserves the original traceback — writing raise e instead just adds an extra stack frame. Inside except, Python already chains exceptions on its own ("During handling of the above exception, another exception occurred"); from states the cause explicitly, and from None hides the earlier exception from the report. You may raise either a class or an instance: raise ValueError and raise ValueError() are equivalent, the class gets instantiated for you.

Examples

def check_age(age):
    if age < 0:
        raise ValueError(f'Age cannot be negative: {age}')
    return age
try:
    check_age(-1)
except ValueError as e:
    print(e)  # → Age cannot be negative: -1

# raise from
try:
    try:
        int('abc')
    except ValueError as e:
        raise RuntimeError('conversion failed') from e
except RuntimeError as err:
    print(type(err.__cause__).__name__)  # → ValueError — исходная причина сохранена

# Re-raise
def safe():
    try:
        return 1/0
    except ZeroDivisionError:
        print('caught, re-raising')
        raise  # передаёт исходное исключение
try:
    safe()
except ZeroDivisionError:
    print('handled')  # → caught / handled

# raise с кастомным классом
class ValidationError(ValueError):
    pass
def validate(x):
    if x < 0:
        raise ValidationError(f'Invalid value: {x}')
try:
    validate(-5)
except ValidationError as e:
    print(e)  # → Invalid value: -5

# raise без аргументов в except
try:
    try:
        1/0
    except ZeroDivisionError:
        raise  # пробрасываем
except ZeroDivisionError:
    print('outer caught')  # → outer caught

# Цепочка __cause__ и __context__
try:
    raise ValueError('first') from TypeError('cause')
except ValueError as e:
    print(type(e.__cause__).__name__)  # → TypeError

raise

Syntax

Examples

See also