[New-bugs-announce] [issue29257] Possible error in discussion of Abstract Base Classes and abstract properties

Gerald Britton report at bugs.python.org
Thu Jan 12 19:48:36 EST 2017

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):
    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?

assignee: docs at python
components: Documentation
messages: 285356
nosy: docs at 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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