diff options
| -rw-r--r-- | Doc/library/collections.rst | 13 | ||||
| -rw-r--r-- | Lib/collections/__init__.py | 11 | ||||
| -rw-r--r-- | Lib/test/test_collections.py | 32 |
3 files changed, 49 insertions, 7 deletions
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 @@ The class can be used to simulate nested scopes and is useful in templating. 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 @@ class ChainMap(MutableMapping): __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 @@ class TestChainMap(unittest.TestCase): 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) + ################################################################################ ### Named Tuples |