[Types-sig] File Protocol Proposal

[Tim Hochberg]

From: "Paul Prescod" <paulp@ActiveState.com>

> Writefile objects will be required only to have a .write() method.
>
> Readfile objects will be required only to have a .read([size]) method.

This sounds good. For reasons that I'll describe below though it might be
better for them to have different names. perhaps BasicWritefile and
BasicReadfile.

> Users will be strongly encouraged to inherit from base classes that
> define other methods (e.g. readlines) in terms of the required method.
> e.g.
>
> class UserReadFile:
    [A class that implements all file read methods in terms of just read]

This seems like an excellent idea! I'll see if I can come up with an
equivalent for readable and writable sequences.

Here's a couple of obeservations in no particular order:

Again I don't like the name UserReadFile. I think it suggests a false
parallel with UserList and UserDict. Those exist in order to subclass the
builtin list and dictionary classes, the above class has an entirely
different meaning.

If we are going to have a baseclass that users will be encouraged to inherit
from when defining there own file-like object, it should be possible to
require it in a type declararion. I'd suggest we use ReadFile (ReadableFile,
FileReader?) for the wide interface and BasicReadFile for the narrow
interface.

It's tempting to overload the above class to allow it act as a wrapper for a
basic readfile. So that I can use UserReadFile(aBasicReadFile) to get a full
featured ReadFile. I think that would just be a matter of adding an __init__
method. I'm not positive this would be a good idea, but I believe that this
is what would need to be added:

   def __init__(self, wrapped=None):
      if wrapped is not None:
         self.read = wrapped.read

One an unrelated note, I didn't like the idea of using & and | when someone
else mentioned, since it seemed to be leading down the road to static typing
(a road to nowhere IOW). However, if typecheck objects inherited from a
common base class it would be pretty easy to implement in terms of magic
methods. This would mean that "TypeA | TypeB | TypeC" would mean then same
as what I originally called CompositeOf(TypeA, TypeB, TypeC).

-tim