Add freezable dict

importlib_metadata/_collections.py

@@ -0,0 +1,21 @@
+import collections
+
+
+class freezable_defaultdict(collections.defaultdict):
+    """
+    Mix-in to freeze a defaultdict.
+
+    >>> dd = freezable_defaultdict(list)
+    >>> dd[0].append('1')
+    >>> dd.freeze()
+    >>> dd[1]
+    []
+    >>> len(dd)
+    1
+    """
+
+    def __missing__(self, key):
+        return getattr(self, '_frozen', super().__missing__)(key)
+
+    def freeze(self):
+        self._frozen = lambda key: self.default_factory()