Am 25.09.2012 16:08 schrieb Peter Otten:
Jayden wrote:
In the Python Tutorial, Section 9.4, it is said that
"Data attributes override method attributes with the same name."
The tutorial is wrong here. That should be
"Instance attributes override class attributes with the same name."
I jump in here:
THere is one point to consider: if you work with descriptors, it makes a
difference if they are "data descriptors" (define __set__ and/or
__delete__) or "non-data descriptors" (define neither).
As http://docs.python.org/reference/datamodel.html#invoking-descriptors
tells us, methods are non-data descriptors, so they can be overridden by
instances.
OTOH, properties are data descriptors which cannot be overridden by the
instance.
So, to stick to the original example:
class TestDesc(object):
def a(self): pass
@property
def b(self): print "trying to get value - return None"; return None
@b.setter
def b(self, v): print "value", v, "ignored."
@b.deleter
def b(self): print "delete called and ignored"
and now
>>> t=TestDesc()
>>> t.a
<bound method TestDesc.a of <__main__.TestDesc object at 0xb7387ccc>>
>>> t.b
trying to get value - return None
>>> t.a=12
>>> t.b=12
value 12 ignored.
>>> t.a
12
>>> t.b
trying to get value - return None
>>> del t.a
>>> del t.b
delete called and ignored
>>> t.a
<bound method TestDesc.a of <__main__.TestDesc object at 0xb7387ccc>>
>>> t.b
trying to get value - return None
Thomas
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