Improve extended slicing support in builtin types and classes. Specifically:

 - Specialcase extended slices that amount to a shallow copy the same way as
   is done for simple slices, in the tuple, string and unicode case.

 - Specialcase step-1 extended slices to optimize the common case for all
   involved types.

 - For lists, allow extended slice assignment of differing lengths as long
   as the step is 1. (Previously, 'l[:2:1] = []' failed even though
   'l[:2] = []' and 'l[:2:None] = []' do not.)

 - Implement extended slicing for buffer, array, structseq, mmap and
   UserString.UserString.

 - Implement slice-object support (but not non-step-1 slice assignment) for
   UserString.MutableString.

 - Add tests for all new functionality.

1 files changed, 6 insertions, 0 deletions

diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index e3c5d04..db1e43d 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -7385,6 +7385,12 @@ unicode_subscript(PyUnicodeObject* self, PyObject* item)
 
         if (slicelength <= 0) {
             return PyUnicode_FromUnicode(NULL, 0);
+        } else if (start == 0 && step == 1 && slicelength == self->length &&
+                   PyUnicode_CheckExact(self)) {
+            Py_INCREF(self);
+            return (PyObject *)self;
+        } else if (step == 1) {
+            return PyUnicode_FromUnicode(self->str + start, slicelength);
         } else {
             source_buf = PyUnicode_AS_UNICODE((PyObject*)self);
             result_buf = (Py_UNICODE *)PyMem_MALLOC(slicelength*