[Python-3000] struni and the Apple four-character-codes

[Guido van Rossum]

Make them bytes literals (some code already does this), and convert
them to integers when they're needed to be used as hash keys. (Does
this happen a lot? I haven't seen it yet.)

I would endorse an API to create an int from a bytes array (or
arbitrary length) and vice versa -- that would be a useful way to
marshal long integers, too. There's probably already a C API to do
something like that.

--Guido

On 7/24/07, Talin <talin at acm.org> wrote:
> Jeffrey Yasskin wrote:
> > I'm looking through a couple of the OS X tests and have run into the
> > question of what to do with four-character codes. (For those of you
> > who are unfamiliar with these, Apple, around the dawn of time, decided
> > that C constants like 'TEXT' (yes, those are single quotes) would
> > compile to the uint32_t 0x54455854 (or maybe the other-endian version
> > of that) so they could use these as cheap-but-readable type
> > identifiers.) In Python 2, these are represented as 'str' instances,
> > which PyMac_GetOSType() in Python/mactoolboxglue.c converts to the
> > native int format. For Python 3, right now they're str8's, but str8 is
> > theoretically supposed to go away. Because they're binary constants
> > displayed as ASCII, not unicode text, I initially thought that 'bytes'
> > was the appropriate type. Unfortunately, bytes is mutable, and I think
> > it makes sense to hash these constants (and some code in aepack.py
> > does).
> >
> > So, I'm stuck and wanted to ask the list for input. I see 5 options:
> >  1) Make these str instances so they're immutable and just rely on
> > convention and runtime errors to keep them in ascii.
> >  2) Make them bytes, and cast them to something else when you want to
> > make them keys in a dict.
> >  3) Keep them str8 and give up on getting rid of it.
> >  4) Make bytes immutable, add a 'buffer' type which acts like the
> > current bytes type, and make these codes instances of bytes. [probably
> > impossible this late in the game]
> >  5) Make a new hashable class for these codes which converts them to
> > and from ints and bytes and becomes the general argument type for the
> > apple platform interface. [Cleanest, but lots of work that I'm not
> > volunteering to do]
> >
> > Thoughts?
> > Jeffrey
>
> Yeah. I like the idea of converting them to integers, but I don't think
> you need a special hash table class for that. Instead, create a wrapper
> class for the four character codes:
>
>     TextId = FourCharId("TEXT")
>     i = int(TextId) # Integer value
>     s = str(TextId) # String representation
>     some_map[TextId] = "Some Text" # Can use as dict key
>
> The wrapper class is an immutable class that handles conversion to
> integer form in the constructor, hashing, and has a __str__ and __repr__
> method that produces the original input string. Then you can use that as
> a key to a regular dict.
>
> -- Talin
>
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-- 
--Guido van Rossum (home page: http://www.python.org/~guido/)