>> That's because of the _PyIO_ConvertSsize_t converter, which silently
>> converts None to -1.
>> There's probably a good reason for doing this in the _io module
>
> I'm not sure about the original reason, but I find None as the default "omitted" value prettier than -1 myself, so I think it's a good thing :)
>

If being pretty is the only reason for this choice, then I think that
documenting the method as

method:: read([n])

is simpler and cleaner .

But you've got much more experience than me, so I won't argue any further :-)

> Can I use _PyIO_ConvertSsize_t? Or should I duplicate its
> functionality in mmapmodule.c?

I let Antoine answer that.