Re: [Pyobjc-dev] NSString & mutability

[<bb...@ma...>]

On Tuesday, Feb 4, 2003, at 18:28 US/Eastern, Just van Rossum wrote:
> Let's focus on making NSString instances work as much as Python strings
> as possible.
>
> For fun, I removed the conversion, and (quite obviously) as it stands
> stuff breaks horribly. The first error thrown by starting a random
> PythonCocoa app is in NibClassBuilder, where an NSString is fed to an
> os.path function. I _think_ for many cases it's enough if we implement
> all string methods. There aren't even that many:
>
>     'capitalize', 'center', 'count', 'encode', 'endswith', 
> 'expandtabs',
>     'find', 'index', 'isalnum', 'isalpha', 'isdecimal', 'isdigit',
>     'islower', 'isnumeric', 'isspace', 'istitle', 'isupper', 'join',
>     'ljust', 'lower', 'lstrip', 'replace', 'rfind', 'rindex', 'rjust',
>     'rstrip', 'split', 'splitlines', 'startswith', 'strip', 'swapcase',
>     'title', 'translate', 'upper', 'zfill'

In objc_support.m, I changed the thing that converts from NSString to 
PythonStrings to this:

         else if ([obj isKindOfClass:[NSString class]])
         {
	    retobject = buffer_from_nsstring(obj);
         }

It is really just a convenience hook via which I can try different 
recipes for converting between.   I current implement 
buffer_from_nsstring() as:

static PyObject* buffer_from_nsstring(NSString *aString)
{
     PyObject* result;
     NSData* data = [aString dataUsingEncoding:NSASCIIStringEncoding];
     if (data == NULL) {
	const char* utf8 = [aString UTF8String];
	result = PyUnicode_DecodeUTF8(utf8, strlen(utf8), "strict");
     } else {
	result = PyString_FromStringAndSize([data bytes], [data length]);
     }
     return result;
}

This works-- it is basically *exactly* like the implementation in CVS.  
  I tried an implementation that returns a character buffer, but a unit 
test that does...

self.assertEquals("foo", NSString.stringWithString_("foo"))

... fails because "foo" cannot be compared to an instance of <character 
buffer>.

Nuts.

What we really need is a way to attach the NSString instance to the 
PythonString instance such that any crossing of the bridge can directly 
grab the NSString on entry into ObjC.

I have realized that I'm wrong in regards to avoiding conversion -- 
conversion isn't the problem.  The problem is the lack of preserving 
the identity as a string crosses from Python to ObjC and back again.   
That the developer will always have to access the NSString methods via 
unbound methods is annoying, but not catastrophically so.

Example:  By 'preserving the identity', I mean that the assertion error 
in the following should not happen [should behave like the Python 
equivalent]:

 >>> from Foundation import *
 >>> a = NSMutableArray.array()
 >>> s = "foo bar baz"
 >>> a.addObject_(s)
 >>> id(s)
1669344
 >>> id(a[0])
6531696
 >>> assert(id(s) == id(a[0]))
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
AssertionError
 >>> k = [s]
 >>> assert(id(s) == id(k[0]))
 >>>

Mutable strings raise other issues -- mainly that we have to deal with 
them at all. :-)

b.bum