Add tests for the _ChainMap helper class.

Lib/collections/__init__.py

@@ -695,6 +695,15 @@ 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)
+
+    @property
+    def parents(self):                          # like Django's Context.pop()
+        'New ChainMap from maps[1:].'
+        return self.__class__(*self.maps[1:])
+
     def __setitem__(self, key, value):
         self.maps[0][key] = value
 

Lib/test/test_collections.py

@@ -11,6 +11,7 @@ import keyword
 import re
 import sys
 from collections import UserDict
+from collections import _ChainMap as ChainMap
 from collections.abc import Hashable, Iterable, Iterator
 from collections.abc import Sized, Container, Callable
 from collections.abc import Set, MutableSet
@@ -18,6 +19,97 @@ from collections.abc import Mapping, MutableMapping, KeysView, ItemsView
 from collections.abc import Sequence, MutableSequence
 from collections.abc import ByteString
 
+
+################################################################################
+### _ChainMap (helper class for configparser and the string module)
+################################################################################
+
+class TestChainMap(unittest.TestCase):
+
+    def test_basics(self):
+        c = ChainMap()
+        c['a'] = 1
+        c['b'] = 2
+        d = c.new_child()
+        d['b'] = 20
+        d['c'] = 30
+        self.assertEqual(d.maps, [{'b':20, 'c':30}, {'a':1, 'b':2}])  # check internal state
+        self.assertEqual(d.items(), dict(a=1, b=20, c=30).items())    # check items/iter/getitem
+        self.assertEqual(len(d), 3)                                   # check len
+        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)
+
+        del d['b']                                                    # unmask a value
+        self.assertEqual(d.maps, [{'c':30}, {'a':1, 'b':2}])          # check internal state
+        self.assertEqual(d.items(), dict(a=1, b=2, c=30).items())     # check items/iter/getitem
+        self.assertEqual(len(d), 3)                                   # check len
+        for key in 'abc':                                             # check contains
+            self.assertIn(key, d)
+        for k, v in dict(a=1, b=2, c=30, z=100).items():              # check get
+            self.assertEqual(d.get(k, 100), v)
+        self.assertIn(repr(d), [                                      # check repr
+            type(d).__name__ + "({'c': 30}, {'a': 1, 'b': 2})",
+            type(d).__name__ + "({'c': 30}, {'b': 2, 'a': 1})"
+        ])
+
+        for e in d.copy(), copy.copy(d):                               # check shallow copies
+            self.assertEqual(d, e)
+            self.assertEqual(d.maps, e.maps)
+            self.assertIsNot(d, e)
+            self.assertIsNot(d.maps[0], e.maps[0])
+            for m1, m2 in zip(d.maps[1:], e.maps[1:]):
+                self.assertIs(m1, m2)
+
+        for e in [pickle.loads(pickle.dumps(d)),
+                  copy.deepcopy(d),
+                  eval(repr(d))
+                ]:                                                    # check deep copies
+            self.assertEqual(d, e)
+            self.assertEqual(d.maps, e.maps)
+            self.assertIsNot(d, e)
+            for m1, m2 in zip(d.maps, e.maps):
+                self.assertIsNot(m1, m2, e)
+
+        d.new_child()
+        d['b'] = 5
+        self.assertEqual(d.maps, [{'b': 5}, {'c':30}, {'a':1, 'b':2}])
+        self.assertEqual(d.parents.maps, [{'c':30}, {'a':1, 'b':2}])   # check parents
+        self.assertEqual(d['b'], 5)                                    # find first in chain
+        self.assertEqual(d.parents['b'], 2)                            # look beyond maps[0]
+
+    def test_contructor(self):
+        self.assertEqual(ChainedContext().maps, [{}])                  # no-args --> one new dict
+        self.assertEqual(ChainMap({1:2}).maps, [{1:2}])                # 1 arg --> list
+
+    def test_missing(self):
+        class DefaultChainMap(ChainMap):
+            def __missing__(self, key):
+                return 999
+        d = DefaultChainMap(dict(a=1, b=2), dict(b=20, c=30))
+        for k, v in dict(a=1, b=2, c=30, d=999).items():
+            self.assertEqual(d[k], v)                                  # check __getitem__ w/missing
+        for k, v in dict(a=1, b=2, c=30, d=77).items():
+            self.assertEqual(d.get(k, 77), v)                          # check get() w/ missing
+        for k, v in dict(a=True, b=True, c=True, d=False).items():
+            self.assertEqual(k in d, v)                                # check __contains__ w/missing
+        self.assertEqual(d.pop('a', 1001), 1, d)
+        self.assertEqual(d.pop('a', 1002), 1002)                       # check pop() w/missing
+        self.assertEqual(d.popitem(), ('b', 2))                        # check popitem() w/missing
+        with self.assertRaises(KeyError):
+            d.popitem()
+
+    def test_dict_coercion(self):
+        d = ChainMap(dict(a=1, b=2), dict(b=20, c=30))
+        self.assertEqual(dict(d), dict(a=1, b=2, c=30))
+        self.assertEqual(dict(d.items()), dict(a=1, b=2, c=30))
+
+
+################################################################################
+### Named Tuples
+################################################################################
+
 TestNT = namedtuple('TestNT', 'x y z')    # type used for pickle tests
 
 class TestNamedTuple(unittest.TestCase):
@@ -229,6 +321,10 @@ class TestNamedTuple(unittest.TestCase):
         self.assertEqual(repr(B(1)), 'B(x=1)')
 
 
+################################################################################
+### Abstract Base Classes
+################################################################################
+
 class ABCTestCase(unittest.TestCase):
 
     def validate_abstract_methods(self, abc, *names):
@@ -626,6 +722,11 @@ class TestCollectionABCs(ABCTestCase):
         self.validate_abstract_methods(MutableSequence, '__contains__', '__iter__',
             '__len__', '__getitem__', '__setitem__', '__delitem__', 'insert')
 
+
+################################################################################
+### Counter
+################################################################################
+
 class TestCounter(unittest.TestCase):
 
     def test_basics(self):
@@ -789,6 +890,11 @@ class TestCounter(unittest.TestCase):
         self.assertEqual(m,
              OrderedDict([('a', 5), ('b', 2), ('r', 2), ('c', 1), ('d', 1)]))
 
+
+################################################################################
+### OrderedDict
+################################################################################
+
 class TestOrderedDict(unittest.TestCase):
 
     def test_init(self):
@@ -1067,6 +1173,10 @@ class SubclassMappingTests(mapping_tests.BasicTestMappingProtocol):
         self.assertRaises(KeyError, d.popitem)
 
 
+################################################################################
+### Run tests
+################################################################################
+
 import doctest, collections
 
 def test_main(verbose=None):