On Mon, 31 Oct 2005 12:47:16 +0000, Alex Hunsley wrote:
Sorry, as I noted in another reply not long ago, I was having a 'braino'
and not saying what I actually meant!
What I was talking about was the accidental _setting_ of the wrong
attribute.
And the mixin class I'm looking for is one that could be told what were
valid attributes for the class,
Who decides what are valid attributes for a class? The class writer, or
the class user who may want to use it in ways the writer never imagined?
and would then catch the situation where
you mis-spelt an attribute name when setting an attrib.
If all you care about is preventing developers from adding any new
attributes at run time, you can do something like this:
# warning: untested
class Example:
def __init__(self, data):
self.__dict__['data'] = data
def __setattr__(self, name, value):
if self.__dict__.has_key(name):
self.__dict__[name] = value
else:
raise AttributeError
except that the developers will then simply bypass your code:
p = Example(None)
p.__dict__['surprise'] = 1
p.surprise
Trying to prevent setting new attributes is a pretty heavy-handed act just
to prevent a tiny subset of errors. Many people argue strongly that even
if you could do it, it would be pointless -- or at least, the cost is far
greater than whatever small benefit there is.
But, if you insist, something like this:
# Warning: untested.
class Declare:
def __init__(self, names):
"""names is a list of attribute names which are allowed.
Attributes are NOT initialised.
"""
self.__dict__['__ALLOWED'] = names
def __setattr__(self, name, value):
if name in self.__ALLOWED:
self.__dict__[name] = value
else:
raise AttributeError("No such attribute.")
If you want to initialise your attributes at the same time you declare
them, use:
# Warning: untested.
class DeclareInit:
def __init__(self, names):
"""names is a dictionary of attribute names/values which are
allowed.
"""
self.__dict__ = names
def __setattr__(self, name, value):
if self.__dict__.has_key(name):
self.__dict__[name] = value
else:
raise AttributeError("No such attribute.")
Of the two approaches, I would say the second is marginally less of a bad
idea.
--
Steven.