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

[Drew Warwick]

The struct unpack API is inconvenient to use with files. I must do:

struct.unpack(fmt, file.read(struct.calcsize(fmt))

every time I want to read a struct from the file. I ended up having to
create a utility function for this due to how frequently I was using
struct.unpack with files:

def unpackStruct(fmt, frm):
    if isinstance(frm, io.IOBase):
        return struct.unpack(fmt, frm.read(struct.calcsize(fmt)))
    else:
        return struct.unpack(fmt, frm)

This seems like something that should be built into the default
implementation -- struct.unpack already has all the information it needs
with just the struct format and open binary file. Current behavior is an
error since struct.unpack only supports bytes-like objects, so this should
be backwards compatible except in the case where a developer is relying on
that to error in a try block instead of verifying the buffer type
beforehand.

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