[Python-ideas] struct.unpack should support open files

[James Edwards]

Here's a snippet of semi-production code we use:

    def read_and_unpack(handle, fmt):
        size = struct.calcsize(fmt)
        data = handle.read(size)
        if len(data) < size: return None
        return struct.unpack(fmt, data)

which was originally something like:

    def read_and_unpack(handle, fmt, offset=None):
        if offset is not None:
            handle.seek(*offset)
        size = struct.calcsize(fmt)
        data = handle.read(size)
        if len(data) < size: return None
        return struct.unpack(fmt, data)

until we pulled file seeking up out of the function.

Having struct.unpack and struct.unpack_from support files would seem
straightforward and be a nice quality of life change, imo.

On Mon, Dec 24, 2018 at 9:36 AM Dan Sommers <
2QdxY4RzWzUUiLuE at potatochowder.com> wrote:

> On 12/24/18 7:33 AM, Steven D'Aprano wrote:
>  > On Mon, Dec 24, 2018 at 03:01:07PM +0200, Andrew Svetlov wrote:
>
>  >> Handling files overcomplicates both implementation and mental space
>  >> for API saving.
>
>  > I haven't thought about this very deeply, but at first glance, I like
>  > Drew's idea of being able to just pass an open file to unpack and have
>  > it read from the file.
>
> The json module has load for files, and loads for bytes and strings,
> That said, JSON is usually read and decoded all at once, but I can see
> lots of use cases for ingesting "unpackable" data in little chunks.
>
> Similarly (but not really), print takes an optional destination that
> overrides the default destination of stdout.
>
> Ironically, StringIO adapts strings so that they can be used in places
> that expect open files.
>
> What about something like gzip.GzipFile (call it struct.StructFile?),
> which is basically a specialized file-like class that packs data on
> writes and unpacks data on reads?
>
> Dan
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