Closes #16613: Added optional mapping argument to ChainMap.new_child.
diff --git a/Doc/library/collections.rst b/Doc/library/collections.rst
index 6ef114e..ea4a311 100644
--- a/Doc/library/collections.rst
+++ b/Doc/library/collections.rst
@@ -76,14 +76,19 @@
be modified to change which mappings are searched. The list should
always contain at least one mapping.
- .. method:: new_child()
+ .. method:: new_child(m=None)
- Returns a new :class:`ChainMap` containing a new :class:`dict` followed by
- all of the maps in the current instance. A call to ``d.new_child()`` is
- equivalent to: ``ChainMap({}, *d.maps)``. This method is used for
+ Returns a new :class:`ChainMap` containing a new map followed by
+ all of the maps in the current instance. If ``m`` is specified,
+ it becomes the new map at the front of the list of mappings; if not
+ specified, an empty dict is used, so that a call to ``d.new_child()``
+ is equivalent to: ``ChainMap({}, *d.maps)``. This method is used for
creating subcontexts that can be updated without altering values in any
of the parent mappings.
+ .. versionchanged:: 3.4
+ The optional ``m`` parameter was added.
+
.. attribute:: parents
Property returning a new :class:`ChainMap` containing all of the maps in