These are interesting ideas. It looks like you intend the except clause of the for loop to *only* cover the iter() and next() calls that are implicit in the for loop. You're right that it's awkward to catch exceptions there. However, I worry that when people see this syntax, they will think that the except clause is for handling exceptions in the loop body. (That's certainly what I assumed when I read just your subject line. :-)
On Fri, Jul 26, 2019 at 11:26 AM Serhiy Storchaka <storch...@gmail.com> wrote: > Python allows you to write code in tight and readable form. Consider the > following example. > > with connect() as stream: # connect() or __enter__() can fail. > for data in stream: # __next__() can fail > write(data) # write() can fail > > The problem is that different lines can raise an exception of the same > type (for example OSError). We want to catch and handle exceptions > raised when open a connection, when read a data and when write a data in > different ways. Currently you need to expand so convenient Python > statements "with" and "for" (see PEP 343) > > _mgr = connect() > _enter = type(_mgr).__enter__ > _exit = type(_mgr).__exit__ > _value = _enter(_mgr) > _exc = True > try: > stream = _value > _it = iter(stream) > while True: > try: > data = next(_it) > except StopIteration: > break > write(data) > except: > _exc = False > if not _exit(_mgr, *sys.exc_info()): > raise > finally: > if _exc: > _exit(_mgr, None, None, None) > > and then add "try ... except" around corresponding explicit calls of > `__enter__()` and `next()`. > > try: > _mgr = connect() > _enter = type(_mgr).__enter__ > _exit = type(_mgr).__exit__ > _value = _enter(_mgr) > _exc = True > except OSError: > handle_connection_error() > else: > try: > stream = _value > try: > _it = iter(stream) > except OSError: > handle_read_error() > else: > while True: > try: > data = next(_it) > except StopIteration: > break > except OSError: > handle_read_error() > break > try: > write(data) > except OSError: > handle_write_error() > except: > _exc = False > if not _exit(_mgr, *sys.exc_info()): > raise > finally: > if _exc: > _exit(_mgr, None, None, None) > > Does not it look ugly? > > I propose to add "except" clause to "for" and "with" statement to catch > exceptions in the code that can't be wrapped with "try ... except". > > for VAR in EXPR: > BLOCK > except EXC: > HANDLER > > should be equivalent to > > try: > _it = iter(EXPR) > except EXC: > HANDLER > else: > while True: > try: > VAR = next(_it) > except StopIteration: > break > except EXC: > HANDLER > break > BLOCK > > and > > with EXPR as VAR: > BLOCK > except EXC: > HANDLER > > try: > _mgr = EXPR > _enter = type(_mgr).__enter__ > _exit = type(_mgr).__exit__ > _value = _enter(_mgr) > _exc = True > except EXC: > HANDLER > else: > try: > VAR = _value > BLOCK > except: > _exc = False > if not _exit(_mgr, *sys.exc_info()): > raise > finally: > if _exc: > _exit(_mgr, None, None, None) > > And correspondingly for asynchronous versions "async for" and "async with". > > So you will be able to add errors handling like in: > > with connect() as stream: > for data in stream: > try: > write(data) > except OSError: > handle_write_error() > except OSError: > handle_read_error() > except OSError: > handle_connection_error() > _______________________________________________ > Python-ideas mailing list -- python-ideas@python.org > To unsubscribe send an email to python-ideas-le...@python.org > https://mail.python.org/mailman3/lists/python-ideas.python.org/ > Message archived at > https://mail.python.org/archives/list/python-ideas@python.org/message/M76F3434TXNO2EUMZZ647EABTCNXYGXA/ > Code of Conduct: http://python.org/psf/codeofconduct/ > -- --Guido van Rossum (python.org/~guido) *Pronouns: he/him/his **(why is my pronoun here?)* <http://feministing.com/2015/02/03/how-using-they-as-a-singular-pronoun-can-change-the-world/>
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