On 6 Feb 2003, Thomas Heller wrote:
> Guido van Rossum <guido@python.org> writes:
> > Can you suggest a concrete syntax to do this? Maybe setting __slots__
> > to a dictionary mapping names to type specifiers would do it -- and it
> > would even be backwards compatible: currently, if __slots__ is a dict,
> > its keys are used as slot names and its values are ignored, by virtue
> > of the way the type constructor iterates over __slots__.
>
> May I suggest to use an ordered sequence (of pairs) instead of a dict
> for __slots__. Then you also can completely control the layout that
> C code sees and construct C compatible structures or other data types
> from pure Python code.
I'm happy with this approach, so long as the tuples can also be keys in a
dictionary. ;)
Here is an example of what my metaclass does:
> <plug>
> Here's how ctypes does it: It's named _fields_ instead of __slots__,
> and typically you write something like this
>
> class Point(Structure):
> _fields_ = [("x", "i"),
> ("y", "i")]
>
> which defines a Python objects having x and y instance vars, which
> must be integers.
My metaclass does something similar, though it does not affect the storage,
only the validity constraints:
def enforce_int(x):
if not isinstance(x, (int,long)):
raise TypeError
def enforce_str(x):
if not isinstance(x, basestring):
raise TypeError
class Foo(object):
__metaclass__ = ConstraintObject
__slots__ = { 'a' : enforce_int,
'b' : enforce_str,
'c' : str }
foo = Foo()
foo.a = 1
foo.a = 1.0
> TypeError
foo.b = '1'
foo.b = 1
> TypeError
foo.c = 1
type(foo.c) == '1'
-Kevin
--
--
Kevin Jacobs
The OPAL Group - Enterprise Systems Architect
Voice: (216) 986-0710 x 19 E-mail: jacobs@theopalgroup.com
Fax: (216) 986-0714 WWW: http://www.theopalgroup.com
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