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

[Talin]

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