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# (c) 2024 E. Castedo Ellerman <castedo@castedo.com>
# Released under the MIT License (https://spdx.org/licenses/MIT)
"""Helper functions for returning and raising unexceptional exceptions.
Unexceptional exceptions are Exception objects that are returned as "normal" error
objects, with return type hints, for "normal" non-try-except execution flow in some,
but not necessarily all, layers of a Python application.
With return type hints, type checkers, such as Mypy, can identify when unexceptional
exceptions are not being handled properly.
See the README.md in https://gitlab.com/castedo/unexceptional for more information.
"""
from __future__ import annotations
from types import TracebackType
from typing import TYPE_CHECKING, TypeVar, cast
if TYPE_CHECKING:
ExceptionT = TypeVar('ExceptionT', bound='Exception')
NonExceptionT = TypeVar('NonExceptionT')
def cast_or_raise(ret: NonExceptionT | Exception) -> NonExceptionT:
"""Cast a value to a non-Exception type or raise an Exception.
Example:
def do_fancy_math(x: float) -> float | ValueError:
...
try:
...
y = cast_or_raise(do_fancy_math(x))
# mypy knows y is a float here
...
except ValueEror:
...
"""
if isinstance(ret, Exception):
raise ret
return ret
def unexceptional(ex: ExceptionT, cause: Exception | None = None) -> ExceptionT:
"""Return an Exception object with a stack trace and optional cause.
Example:
if bad_case:
return unexceptional(ValueError("Bad case"))
"""
try:
raise ex
except Exception as ret:
ret.__cause__ = cause
if not ret.__traceback__:
return cast('ExceptionT', ret)
frame = ret.__traceback__.tb_frame
frame = frame.f_back or frame
tb = TracebackType(None, frame, frame.f_lasti, frame.f_lineno)
return cast('ExceptionT', ret.with_traceback(tb))
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