MacPython/FourCharacterCode

By convention, Apple uses four character codes in place of integer enumerations in almost all of its software, protocols, and file formats.

Notes:

Four character codes are literally four ../MacRoman characters (including \x00, but usually four 7-bit ASCII safe characters).
When "packed" as an integer, they are almost always stored in big endian format as a 32bit signed integer (e.g. strnAsInteger = struct.unpack(">i", 'strn')).
You may see crazy stuff like © or in four character codes (or maybe just garbage, depending on the text editor). The text encoding used is always MacRoman. Fortunately, the general case is the 7-bit subset of ../MacRoman that overlaps with ASCII.
Four character codes are supposed to be globally unique in context, but not in general.
Apple maintains a registry of application signatures via Developer Technical Support. This list is not publically available.
Apple maintains exclusive "ownership" of all lowercase four character codes for their software and APIs.

Examples:

'hook' - the application signature for iTunes (notice that it is an Apple product and is therefore lowercase)
'8BIM' - the application signature for Adobe Photoshop.
'shor' - the ../AEDesc type code for a 16bit signed integer, also known as typeSInt16, typeSMInt, or typeShortInteger

In Python, four character codes are passed around as four character str (primarily for introspection purposes). Some Python code will automatically interchange four character codes with str, unicode, int, or long but that will not be the general case until Python 2.4 at the earliest. ../bgen may output constants that look like FOUR_CHAR_CODE('shor'), however FOUR_CHAR_CODE is (currently) a no-op and just returns the input string unchanged.

In C, four character codes are interchangable with 32 bit integer types and will usually be defined in an enumerator:

/*  
    This is how you will see them in Universal Interfaces format 
*/
enum {
    MyConstant = FOUR_CHAR_CODE('MooV'),
    MyOtherConstant = FOUR_CHAR_CODE('©xxx')
}

/*  
    This is the "new way" that you will see in OS X frameworks
*/
enum {
    MyConstant = 'MooV',
    MyOtherConstant = (long)0xA9787878 /* so that the source isn't tainted with high ascii */
}