David S. wrote:
Steven Bethard <steven.bethard <at> gmail.com> writes:
David S. wrote:
I am looking for a way to implement the same simple validation on many
instance attributes and I thought descriptors
(http://users.rcn.com/python/download/Descriptor.htm) looked like the
right tool.
Looks like you're trying to reinvent the property descriptor. Try using
the builtin property instead:
py> def getchar(self):
... if not hasattr(self, '_char'):
... self._char = None
... return self._char
...
py> def setchar(self, value):
... if not len(value) == 1:
... raise ValueError
... self._char = value
...
py> singlechar = property(getchar, setchar)
py> class Flags(object):
... a = singlechar
... b = singlechar
...
py> f = Flags()
py> f.a = "a"
py> f.b = "bb"
Traceback (most recent call last):
File "<interactive input>", line 1, in ?
File "<interactive input>", line 3, in setchar
ValueError
This still fails to work for instances variables of the class. That is
if I use your property in the following:
py> ...class Flags(object):
... def __init__(self):
... a = singlechar
...
py> f = Flags()
py> f.a = "a"
Now f.a.__class__.__name__ returns 'str'. So the property was not
used at all.
You want assignment to a method-local variable to turn an attribute into
a property? That's programming with a magic wand ...
Also, it seems that using a property, I can not do the other useful
things I can do with a proper class, like provide an __init__, __str__,
or __repr__.
That will depend on the value returned by property access, surely?
I suspect you are a little confused about properties and descriptors.
regards
Steve
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