On 26/06/20 4:01 am, Steele Farnsworth wrote:
My point was only that, as far as I know, all the methods for built in container types that serve only to change what is contained return None, and that this was an intentional design choice, so changing it in one case would have to evoke a larger discussion about what those sorts of methods should return.
The reason for that convention is so that there is no confusion about which methods return new objects and which modify the object in place.
However, achieving that goal only requires that mutating methods don't return 'self'. It doesn't mean they can't return some other value that might be useful.
A wider argument against that might be that methods should be classifiable into "procedures" (that have side effects but not return values) and "functions" (that return values but don't have side effects). I'm not sure whether that's considered part of the Python design philosophy -- I don't remember seeing much discussion about it.
In this particular case, it might be better to add a new method rather than redefining the existing 'add' method, because if code assuming the new behaviour were run on an older version of Python, it would fail in an obscure way.