[Python-Dev] Type hints -- a mediocre programmer's reaction

[Cory Benfield]

On 21 April 2015 at 16:09, Chris Angelico <rosuav at 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. =)