On 21 April 2015 at 16:09, Chris Angelico <ros...@gmail.com> wrote: > Pretty accurate, yeah. Here's how I see it: > > def incremental_parser(input: FileLike) -> List[Token]: > tokens = [] > data = "" > while True: > if not data: > data = input.read(64) > token = Token(data[0]); data = data[1:] > while token.wants_more(): > token.give_more(data[0]); data = data[1:] > tokens.append(token) > if token.is_end_of_stream(): break > input.seek(-len(data), 1) > return tokens > > If you were to exhaustively stipulate the requirements on the > file-like object, you'd have to say: > > * Provides a read() method which takes an integer and returns up to > that many bytes > * Provides a seek() method which takes two integers > * Is capable of relative seeking by at least 63 bytes backward > * Is open for reading > * Etcetera > > That's not the type system's job. Not in Python. Maybe in Haskell, but > not in Python. So how much _should_ go into the type hint? I'm happy > with "FileLike" or however it's to be spelled; maybe separate readable > files from writable ones, as those are two fairly clear variants, but > that's about all you really need. If you provide incremental_parser() > with an input file that's non-seekable, it's going to have problems - > and your editor may or may not even be able to detect that (some files > are seekable but only in the forward direction, but they'll have the > exact same seek() method).
Ok, that makes sense to me. =) Looking at mypy's source code, I see shutil.copyfileobj has the following signature: def copyfileobj(fsrc: IO[AnyStr], fdst: IO[AnyStr], length: int = 16*1024) -> None: so it seems that mypy defines a general IO datatype here. I continue to be worried about the lack of a *general* solution to this problem, but I'm glad that some specific solutions exist. I still think library authors will not do much to take up this proposal. =) _______________________________________________ Python-Dev mailing list Python-Dev@python.org https://mail.python.org/mailman/listinfo/python-dev Unsubscribe: https://mail.python.org/mailman/options/python-dev/archive%40mail-archive.com
