[Python-Dev] read-only properties

[Jeff Epler]

On Tue, Oct 08, 2002 at 11:37:46AM -0400, David Abrahams wrote:
[...]
>     AttributeError: can't set attribute
> 
> This struck me as a very poor error message, given that mechanism in
> Boost.Python v1 used to say:
[...]
>     AttributeError: 'l' attribute is read-only
> 
> Is it worth doing something about this?

The following comes close, in Python, but requires a setter object for
each attribute:

    class SetROProperty:
        def __init__(self, name):
            self.name = name
        def __call__(self, obj, val):
            raise AttributeError, "%r attribute is read-only" % self.name

    class X(object):
        def get(self): return 'x'
        l = property(get, SetROProperty('l'))

    >>> x = X()
    >>> x.l
    'x'
    >>> x.l = 1
    Traceback (most recent call last):
      File "<stdin>", line 1, in ?
      File "srop.py", line 5, in __call__
        raise AttributeError, "%r attribute is read-only" % self.name
    AttributeError: 'l' attribute is read-only

The key seems to be that property() doesn't know the name the property
has.

Jeff