Is isinstance always "considered harmful"?

[Leif K-Brooks]

Jordan Rastrick wrote:
> Say you're writing, in Python, the extend() method for a Linked List
> version of python's builtin list. Its really important to know what
> kind of iterable youre being passed in - because if its another
> Linked list, and you know it, you can connect the two in 0(1) time;
> whereas any other arbitrary iterable is going to take 0(n), as you're
> just going to have to append the items one by one. Is this a case
> where use of isinstance, to see exactly what kind of Iterable you
> have, can be justified?
>
> def extend(self, elems):
>    if isinstance(elems, LinkedList):
>       # Point last to first
>    else:
>       for elem in elems: self.append(elem)

Regardless of the various issues surrounding isinstance(), you have a
difference in functionality. Since you're just storing a reference in
the case of another LinkedList instead of copying it, mutating the
LinkedList will be different from mutating another iterable type which
has been passed to extend:

>>> linkedlist1 = LinkedList()
>>> list1 = [1, 2, 3]
>>> linkedlist2 = LinkedList([4, 5, 6])
>>> linkedlist1.extend(list1)
>>> linkedlist1.extend(linkedlist2)
>>> linkedlist1
LinkedList([1, 2, 3, 4, 5, 6])
>>> list1.append(4)
>>> linkedlist1 # Notice how there's still only one 4
LinkedList([1, 2, 3, 4, 5, 6])
>>> linkedlist2.append(7)
>>> linkedlist1 # Notice how there's now a 7
LinkedList([1, 2, 3, 4, 5, 6, 7])