Two python issues

L.S.,
Python seem to suffer from a few poor design decisions regarding strings and lists that affect the elegance of the language.
(a) An error-prone "feature" is returning -1 if a substring is not found by "find", since -1 currently refers to the last item. An example:
 >>> s = 'qwertyuiop'
 >>> s[s.find('r')]
'r'
 >>> s[s.find('p')]
'p'
 >>> s[s.find('a')]
'p'
 >>>
If "find" is unsuccessful, an error message is the only clean option.
Moreover, using index -1 for the last item is a bad choice: it should be len(s) - 1 (no laziness!).
Negative indices should be reserved for elements preceding the element with index 0 (currently not implemented, but a must for orthogonal design supporting general sequences).
(b) When using assignment for slices, only lists with the same length as the slice should be acceptable, otherwise an error should be given.  Anything that re-indexes items not covered by the slice is against the essential idea of assignment. For changes that imply re-indexing (e.g., inserting a list longer than the slice), Python offers cleaner solutions.
Comments are welcome.
With best regards,
Raymond