This patch originated from an idea by Martin v. Loewis who submitted a

patch for sharing single character Unicode objects.

Martin's patch had to be reworked in a number of ways to take Unicode
resizing into consideration as well. Here's what the updated patch
implements:

* Single character Unicode strings in the Latin-1 range are shared
  (not only ASCII chars as in Martin's original patch).

* The ASCII and Latin-1 codecs make use of this optimization,
  providing a noticable speedup for single character strings. Most
  Unicode methods can use the optimization as well (by virtue
  of using PyUnicode_FromUnicode()).

* Some code cleanup was done (replacing memcpy with Py_UNICODE_COPY)

* The PyUnicode_Resize() can now also handle the case of resizing
  unicode_empty which previously resulted in an error.

* Modified the internal API _PyUnicode_Resize() and
  the public PyUnicode_Resize() API to handle references to
  shared objects correctly. The _PyUnicode_Resize() signature
  changed due to this.

* Callers of PyUnicode_FromUnicode() may now only modify the Unicode
  object contents of the returned object in case they called the API
  with NULL as content template.

Note that even though this patch passes the regression tests, there
may still be subtle bugs in the sharing code.

Doc/api/api.tex

@@ -2457,7 +2457,9 @@ use these APIs:
 Create a Unicode Object from the Py_UNICODE buffer \var{u} of the
 given size. \var{u} may be \NULL{} which causes the contents to be
 undefined. It is the user's responsibility to fill in the needed data.
-The buffer is copied into the new object.
+The buffer is copied into the new object. If the buffer is not \NULL{},
+the return value might be a shared object. Therefore, modification of
+the resulting Unicode Object is only allowed when \var{u} is \NULL{}.
 \end{cfuncdesc}
 
 \begin{cfuncdesc}{Py_UNICODE*}{PyUnicode_AsUnicode}{PyObject *unicode}

Include/unicodeobject.h

@@ -239,8 +239,12 @@ extern DL_IMPORT(PyTypeObject) PyUnicode_Type;
 /* --- Plain Py_UNICODE --------------------------------------------------- */
 
 /* Create a Unicode Object from the Py_UNICODE buffer u of the given
-   size. u may be NULL which causes the contents to be undefined. It
-   is the user's responsibility to fill in the needed data.
+   size. 
+
+   u may be NULL which causes the contents to be undefined. It is the
+   user's responsibility to fill in the needed data afterwards. Note
+   that modifying the Unicode object contents after construction is
+   only allowed if u was set to NULL.
 
    The buffer is copied into the new object. */