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
diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py
index 53083e4..0612e1f 100644
--- a/Lib/collections/__init__.py
+++ b/Lib/collections/__init__.py
@@ -821,9 +821,14 @@
__copy__ = copy
- def new_child(self): # like Django's Context.push()
- 'New ChainMap with a new dict followed by all previous maps.'
- return self.__class__({}, *self.maps)
+ def new_child(self, m=None): # like Django's Context.push()
+ '''
+ New ChainMap with a new map followed by all previous maps. If no
+ map is provided, an empty dict is used.
+ '''
+ if m is None:
+ m = {}
+ return self.__class__(m, *self.maps)
@property
def parents(self): # like Django's Context.pop()
diff --git a/Lib/test/test_collections.py b/Lib/test/test_collections.py
index 8850e8b..dfad78e 100644
--- a/Lib/test/test_collections.py
+++ b/Lib/test/test_collections.py
@@ -112,6 +112,38 @@
self.assertEqual(dict(d), dict(a=1, b=2, c=30))
self.assertEqual(dict(d.items()), dict(a=1, b=2, c=30))
+ def test_new_child(self):
+ 'Tests for changes for issue #16613.'
+ c = ChainMap()
+ c['a'] = 1
+ c['b'] = 2
+ m = {'b':20, 'c': 30}
+ d = c.new_child(m)
+ self.assertEqual(d.maps, [{'b':20, 'c':30}, {'a':1, 'b':2}]) # check internal state
+ self.assertIs(m, d.maps[0])
+
+ # Use a different map than a dict
+ class lowerdict(dict):
+ def __getitem__(self, key):
+ if isinstance(key, str):
+ key = key.lower()
+ return dict.__getitem__(self, key)
+ def __contains__(self, key):
+ if isinstance(key, str):
+ key = key.lower()
+ return dict.__contains__(self, key)
+
+ c = ChainMap()
+ c['a'] = 1
+ c['b'] = 2
+ m = lowerdict(b=20, c=30)
+ d = c.new_child(m)
+ self.assertIs(m, d.maps[0])
+ for key in 'abc': # check contains
+ self.assertIn(key, d)
+ for k, v in dict(a=1, B=20, C=30, z=100).items(): # check get
+ self.assertEqual(d.get(k, 100), v)
+
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### Named Tuples