Steven D'Aprano wrote:
py> class MyDict(dict): ... @classmethod ... def fromkeys(cls, func): ... # Expects a callback function that gets called with no arguments ... # and returns two items, a list of keys and a default value. ... return super(MyDict, cls).fromkeys(*func())
Here you've overridden a method with one having a different signature. That's not something you'd normally do, because, being a method, it's likely to get invoked polymorphically. Constructors, on the other hand, are usually *not* invoked polymorphically. Most of the time we know exactly which constructor we're calling, because we write the class name explicitly at the point of call. Consequently, we have a different attitude when it comes to constructors. We choose not to require LSP for constructors, because it turns out to be very useful not to be bound by that constraint. Practicality beats purity here. The reason IPython gets into trouble is that it tries to make a polymorphic call to something that nobody expects to need to be polymorphic. -- Greg -- http://mail.python.org/mailman/listinfo/python-list
