[Python-Dev] decorators: If you go for it, go all the way!!! :)

alex_nanou at cs.msu.su alex_nanou at cs.msu.su
Thu Aug 26 12:58:30 CEST 2004

the inability to decorate anything but a function, is quite a limitation,
though I must agree that decorating classes is not such a common task, but
using the syntax to *decorate* attributes would indeed prove useful (in
both the readability department and the uniformity of code)..

here is an example we use quite often:
import time
import sys

def intonly(obj=0, doc=None):
    intonly(int) -> descriptor

    the resulting descriptor will prevent both the deletion of the
    attribute and assignments of any value other than ints.
    if type(obj) is not int:
        raise TypeError, 'value must be int (got: %s)' % obj

    f_locals = sys._getframe(1).f_locals

    # name the attr (I know this is ugly ^_^ )
    while True:
        name = '_priv_' + str(int(time.time()*10**8))
        if name not in f_locals:
            f_locals[name] = obj

    # the descriptor reader function...
    def read(self):
        return getattr(self, name)

    # the descriptor writer...
    def write(self, val):
        if type(val) is not int:
            raise TypeError, 'value must be int (got: %s)' % val
        setattr(self, name, val)

    # now create the descriptor and return it...
    return property(read, write, doc=doc)


and here are a couple of usage examples:
the desired decorator syntax:
class A(object):
    i = 123


the current usage syntax:
class A(object):
    i = intonly(123)


// sorry if I bring this up agen! :)

                                           Best Regards...

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