Issue #21279: Flesh out str.translate docs

Initial patch by Kinga Farkas, Martin Panter, and John Posner.

Doc/library/stdtypes.rst

@@ -1976,21 +1976,22 @@ expression support in the :mod:`re` module).
         "They're Bill's Friends."
 
 
-.. method:: str.translate(map)
+.. method:: str.translate(table)
 
-   Return a copy of the *s* where all characters have been mapped through the
-   *map* which must be a dictionary of Unicode ordinals (integers) to Unicode
-   ordinals, strings or ``None``.  Unmapped characters are left untouched.
-   Characters mapped to ``None`` are deleted.
+   Return a copy of the string in which each character has been mapped through
+   the given translation table.  The table must be an object that implements
+   indexing via :meth:`__getitem__`, typically a :term:`mapping` or
+   :term:`sequence`.  When indexed by a Unicode ordinal (an integer), the
+   table object can do any of the following: return a Unicode ordinal or a
+   string, to map the character to one or more other characters; return
+   ``None``, to delete the character from the return string; or raise a
+   :exc:`LookupError` exception, to map the character to itself.
 
    You can use :meth:`str.maketrans` to create a translation map from
    character-to-character mappings in different formats.
 
-   .. note::
-
-      An even more flexible approach is to create a custom character mapping
-      codec using the :mod:`codecs` module (see :mod:`encodings.cp1251` for an
-      example).
+   See also the :mod:`codecs` module for a more flexible approach to custom
+   character mappings.
 
 
 .. method:: str.upper()

Objects/unicodeobject.c

@@ -13077,11 +13077,12 @@ unicode_maketrans_impl(PyObject *x, PyObject *y, PyObject *z)
 PyDoc_STRVAR(translate__doc__,
              "S.translate(table) -> str\n\
 \n\
-Return a copy of the string S, where all characters have been mapped\n\
-through the given translation table, which must be a mapping of\n\
-Unicode ordinals to Unicode ordinals, strings, or None.\n\
-Unmapped characters are left untouched. Characters mapped to None\n\
-are deleted.");
+Return a copy of the string S in which each character has been mapped\n\
+through the given translation table. The table must implement\n\
+lookup/indexing via __getitem__, for instance a dictionary or list,\n\
+mapping Unicode ordinals to Unicode ordinals, strings, or None. If\n\
+this operation raises LookupError, the character is left untouched.\n\
+Characters mapped to None are deleted.");
 
 static PyObject*
 unicode_translate(PyObject *self, PyObject *table)