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Title: Possible error in discussion of Abstract Base Classes and abstract properties
Type: Stage: resolved
Components: Documentation Versions: Python 3.10, Python 3.9, Python 3.8, Python 3.7, Python 3.6
Status: closed Resolution: out of date
Dependencies: Superseder:
Assigned To: docs@python Nosy List: docs@python, gbritton, iritkatriel
Priority: normal Keywords:

Created on 2017-01-13 00:48 by gbritton, last changed 2022-04-11 14:58 by admin. This issue is now closed.

Messages (2)
msg285356 - (view) Author: Gerald Britton (gbritton) Date: 2017-01-13 00:48
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):
    def x(self):self._x
d = 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?
msg410656 - (view) Author: Irit Katriel (iritkatriel) * (Python committer) Date: 2022-01-15 19:37
I think this is out of date, the example is different now:
Date User Action Args
2022-04-11 14:58:41adminsetgithub: 73443
2022-01-22 08:16:45iritkatrielsetstatus: pending -> closed
stage: resolved
2022-01-15 19:37:29iritkatrielsetstatus: open -> pending

nosy: + iritkatriel
messages: + msg410656

resolution: out of date
2020-05-31 13:26:40serhiy.storchakasetversions: + Python 3.6, Python 3.7, Python 3.8, Python 3.9, Python 3.10, - Python 2.7
2017-01-13 00:48:36gbrittoncreate