Changing the exception type in a context manager

Question

I currently have code that looks like this scattered throughout my codebase:

try: 
    something()
except Exception as exc:
    raise SomethingError from exc

I would like to write a context manager that would remove some of this boiler-plate:

with ExceptionWrapper(SomethingError):
    something()

It looks like it is possible to suppress exceptions inside a context manager - see: contextlib.suprress. It doesn't look like it is possible to change what exception is being raised.

However, I haven't been able to find clear documentation on what the return value of the __exit__ function of a context manager is.

Answer 1

This sort of simple context manager is easiest to implement using contextlib.contextmanager, which creates a context manager out of a generator. Simply wrap the first yield statement in the generator with a try block and raise the desired exception in the except block:

import contextlib
from typing import Iterator


class SomethingError(Exception):
    pass


@contextlib.contextmanager
def exception_wrapper(message: str) -> Iterator[None]:
    """Wrap Exceptions with SomethingError."""
    try: 
        yield
    except Exception as exc:
        raise SomethingError(message) from exc

This can be used like so:

>>> with exception_wrapper("and eggs!"):
...     raise Exception("spam")

Traceback (most recent call last):
  File "/path/to/script.py", line 11, in exception_wrapper
    yield
  File "/path/to/script.py", line 17, in <module>
    raise Exception("spam")
Exception: spam

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/path/to/script.py", line 16, in <module>
    with exception_wrapper("and eggs!"):
  File "/usr/lib/python3.10/contextlib.py", line 153, in __exit__
    self.gen.throw(typ, value, traceback)
  File "/path/to/script.py", line 13, in exception_wrapper
    raise SomethingError(message) from exc
__main__.SomethingError: and eggs!