New submission from Gerald Britton:
I was rereading the 2.7 docs about abstract base classes the other day. I
found this:
"This defines a read-only property; you can also define a read-write abstract
property using the ‘long’ form of property declaration:"
along with an example. so I copied the example and put in a little surrounding
code:
from abc import ABCMeta, abstractproperty
class C:
__metaclass__ = ABCMeta
def getx(self): pass
def setx(self, value): pass
x = abstractproperty(getx, setx)
class D(C):
@property
def x(self):self._x
d = D()
print(d)
When I ran this, I expected an exception, since I defined a read/write abstract
property but only implemented the read operation. However, the example runs
fine. That is the class D can be instantiated without error. Of course I
cannot set the property since I didn't implement that part.
Now, If I don't implement the property at all, I can' instantiate the class. I
get:
"TypeError: Can't instantiate abstract class D with abstract methods x"
which is what I would expect. What I don't understand is why I don't get a
similar error when I implement the read operation for the property but not the
write operation.
If this actually doesn't work (catching the non-implementation at instantiation
time), then why is it documented this way? To me at least the doc implies that
it *will* raise on the missing write property implementation.
If ABCs are working as intended, can the documentation be changed to reflect
that as per my experience above? If the documentation is correct, can the ABC
implementation be modified to function that way?
----------
assignee: docs@python
components: Documentation
messages: 285356
nosy: docs@python, gbritton
priority: normal
severity: normal
status: open
title: Possible error in discussion of Abstract Base Classes and abstract
properties
versions: Python 2.7
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Python tracker <[email protected]>
<http://bugs.python.org/issue29257>
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