On Thu, Sep 30, 2021 at 11:03 AM Soni L. <fakedme+py@gmail.com> wrote:
So uh, this is a hardly at all fleshed out idea, but one thing we really don't like about python is having to do stuff like this so as to not swallow exceptions:
def a_potentially_recursive_function(some, args): """ Does stuff and things. Raises ExceptionWeCareAbout under so and so conditions. """ try: some.user_code() except ExceptionWeCareAbout as exc: raise RuntimeError from exc code_we_assume_is_safe() if args.something and some_condition: raise ExceptionWeCareAbout
It'd be nice if there was a way to... make this easier to deal with. Perhaps something where, something like this:
def a_potentially_recursive_function(some, args) with ExceptionWeCareAbout: """ Does stuff and things. Raises ExceptionWeCareAbout under so and so conditions. """ some.user_code() code_we_assume_is_safe() if args.something and some_condition: raise ExceptionWeCareAbout
becomes:
def a_potentially_recursive_function(some, args): """ Does stuff and things. Raises ExceptionWeCareAbout under so and so conditions. """ try: some.user_code() code_we_assume_is_safe() if args.something and some_condition: let_exception_through = True raise ExceptionWeCareAbout except ExceptionWeCareAbout as exc: if let_exception_through: raise else: raise RuntimeError from exc
(or equivalent)
and something like:
def foo() with Bar: try: baz() except Bar: raise
becomes:
def foo(): try: try: baz() except Bar: allow_exception = True raise except Bar as exc: if allow_exception: raise else: raise RuntimeError from exc
(thus allowing explicit exception propagation)
Additionally, note that it would only apply to the raise itself - something like `raise may_raise_the_same_error()` would desugar as if:
exception = may_raise_the_same_error() allow_exception = True raise exception
Obviously this doesn't solve exception handling, doesn't require the caller to catch the exceptions, etc etc. It does, however, encourage better exception hygiene. Chances are something like this would significantly reduce the amount of swallowed exceptions, if it gets widely adopted. We know something like this would've saved us a lot of trouble, so we're sharing the idea in the hopes it can, in the future, help others.
Can you give a realistic example of how this works, and how you could accidentally leak the exact same exception that you'd be intentionally raising? It looks to me like you possibly should be splitting the function into the recursive part and the setup part, where only the setup is capable of raising that exception. I've seen a LOT of bad Python code caused by an assumption that "recursion" must always mean "calling the external API". A much better model, in a lot of cases, is: def _somefunc_recursive(x, y, z): ... _somefunc_recursive(x, y + 1, z - 1) def somefunc(x, y, z=4): ... _somefunc_recursive(x, y, z) return someresult ChrisA