ABCMeta.register is great.  It adds the cls argument to the _abc_registry of the ABC.  However, the class that was passed in does not get touched.  If you then want to find out to which classes a class has been registered, you can&#39;t find out from that class.  Whereas your can find out from an abstract base class which classes have been registered to it.<div>
<br></div><div>I propose having ABCMeta.register add/update a special method __implements__ to the class that is getting registered.  This would not be done to builtin/extension types.  It adds the ABC to the __implements__ of the subclass that is getting registered.  Something along these lines, right before the final return in the method:</div>
<div><br></div><div><div>        if not hasattr(subclass, &quot;__implements__&quot;):</div><div>            try:</div><div>                subclass.__implements__ = {cls}</div><div>            except TypeError:</div><div>
                pass</div><div>        else:</div><div>            subclass.__implements__.add(cls)</div></div><div><br></div><div>This is a small addition, but I realize it [potentially] adds another special method to classes, so it&#39;s not trivial.</div>
<div><br></div><div>The use case is that I want to be able to validate that a class implements all of the abstract methods of all the classes to which it has been registered.  I don&#39;t have a programmatic way of discovering that set without asking every class out there.  This is an easy way to accomplish this (for non-extension/non-builtin types).  An alternative is to subclass ABCMeta and tack this on, but that only works for my ABCs.  Another is to use a class decorator to do this any place I do a register (or even to do the register too), but again, only for the places that I do the registration.</div>
<div><br></div><div>Anyway, if it&#39;s useful to me then it may be useful to others, so I wanted to put this out there.  I expect this has come up before, particularly during discussions about PEP 3119.  However, I wasn&#39;t able to track down anything specifically about doing this sort of &quot;reverse registration&quot;.  And, of course, I may be overestimating the value of this functionality.  If this does not seem that valuable to anyone else, then no big deal.  :)</div>
<div><br></div><div>-eric</div>