PEP 416: Add links to existing implementations

pep-0416.txt

@@ -125,6 +125,38 @@ However dictproxy can be useful in some cases, where its mutable property is
 not an issue, to avoid a copy of the dictionary.
 
 
+Existing implementations
+========================
+
+Whitelist approach.
+
+ * `Implementing an Immutable Dictionary (Python recipe 498072)
+   <http://code.activestate.com/recipes/498072/>`_ by Aristotelis Mikropoulos.
+   Similar to frozendict except that it is not truly read-only: it is possible
+   to access to this private internal dict.  It does not implement __hash__ and
+   has an implementation issue: it is possible to call again __init__() to
+   modify the mapping.
+ * PyWebmail contains an ImmutableDict type: `webmail.utils.ImmutableDict
+   <http://pywebmail.cvs.sourceforge.net/viewvc/pywebmail/webmail/webmail/utils/ImmutableDict.py?view=markup>`_.
+   It is hashable if keys and values are hashable. It is not truly read-only:
+   its internal dict is a public attribute.
+
+Blacklist approach: inherit from dict and override write methods to raise an
+exception. It is not truly read-only: it is still possible to call dict methods
+on such "frozen dictionary" to modify it.
+
+ * brownie: `brownie.datastructures.ImmuatableDict
+   <https://github.com/DasIch/brownie/blob/HEAD/brownie/datastructures/mappings.py>`_.
+   It is hashable if keys and values are hashable. werkzeug project has the
+   same code: `werkzeug.datastructures.ImmutableDict
+   <https://github.com/mitsuhiko/werkzeug/blob/master/werkzeug/datastructures.py>`_.
+ * SQLAchemy project: `sqlachemy.util.immutabledict
+   <http://hg.sqlalchemy.org/sqlalchemy/file/tip/lib/sqlalchemy/util/_collections.py>`_.
+   It is not hashable and has an extra method: union().
+ * `Frozen dictionaries (Python recipe 414283) <http://code.activestate.com/recipes/414283/>`_
+   by Oren Tirosh. It is hashable if keys and values are hashable.
+
+
 Links
 =====
 
@@ -133,16 +165,8 @@ Links
  * PEP 412: Key-Sharing Dictionary
    (`issue #13903 <http://bugs.python.org/issue13903>`_)
  * PEP 351: The freeze protocol
- * `The case for immutable dictionaries; and the central misunderstanding of PEP 351 <http://www.cs.toronto.edu/~tijmen/programming/immutableDictionaries.html>`_
+ * `The case for immutable dictionaries; and the central misunderstanding of
- * `Frozen dictionaries (Python recipe 414283) <http://code.activestate.com/recipes/414283/>`_
+   PEP 351 <http://www.cs.toronto.edu/~tijmen/programming/immutableDictionaries.html>`_
-   by Oren Tirosh. Blacklist approach: inherit from dict and override write
-   methods to raise an exception. It is not truly read-only: it is still
-   possible to call dict methods on such "frozen dictionary" to modify it.
- * `Implementing an Immutable Dictionary (Python recipe 498072) <http://code.activestate.com/recipes/498072/>`_
-   by Aristotelis Mikropoulos. Similar to frozendict except that it is not
-   truly read-only. It is possible to access to this private internal dict.
-   It does not implement __hash__ and has an implementation issue: it is
-   possible to call again __init__() to modify the mapping.
  * `make dictproxy object via ctypes.pythonapi and type() (Python recipe
    576540) <http://code.activestate.com/recipes/576540/>`_ by Ikkei Shimomura.
  * Python security modules implementing read-only object proxies using a C