TraitListObject misformats items_changed events on delete with extended slice

[k2bd]

TraitListObject.__delitem__ handles slices with steps !=1 specially:

traits/traits/trait_list_object.py

Lines 188 to 190 in d8d851e

	else:
	index = key
	removed = [removed]

This produces e.g. [[9, 5]] instead of [9, 5] for the removed part of the event on these lines:

traits/traits/tests/test_traits.py

Lines 1114 to 1115 in d8d851e

	del self.obj.alist[1:4:2]
	self.assertLastTraitListEventEqual(slice(1, 4, 2), [[9, 5]], [])

This doesn't match the behaviour found when e.g. list items are changed:

traits/traits/tests/test_traits.py

Lines 1104 to 1105 in d8d851e

	self.obj.alist[0:2:1] = [8, 9]
	self.assertLastTraitListEventEqual(0, [4, 5], [8, 9])

Discussed offline, the current behaviour is intentional to reproduce legacy results

[mdickinson]

If we're going to change this, 6.0 might be a good release to do it in. It's technically a backwards-compatibility break, but it's one that's unlikely to affect much real code (deleting slices with a step != 1 is rare).

Note that this is not just about __delitem__, but about slice assignment, too:

>>> from traits.api import *
>>> class A(HasTraits):
...     foo = List(Int)
...     def _foo_items_changed(self, list_event):
...         print(f"Event: index={list_event.index}, added={list_event.added}, removed={list_event.removed}")
... 
>>> a = A()
>>> a.foo 
[]
>>> a.foo.extend([1, 2, 3])
Event: index=0, added=[1, 2, 3], removed=[]
>>> a.foo[::2] = [4, 5]
Event: index=slice(None, None, 2), added=[[4, 5]], removed=[[1, 3]]
>>> del a.foo[::]
Event: index=0, added=[], removed=[4, 2, 5]

I'm +1 on changing this for 6.0.

There are some other oddities/inconsistencies, too: note the difference between added and removed in the following:

>>> del a.foo[::-1]
Event: index=slice(None, None, -1), added=[], removed=[[]]