Add defaultdict to collections

Lib/collections/__init__.py

@@ -1277,3 +1277,9 @@ class UserString(_collections_abc.Sequence):
         return self.__class__(self.data.translate(*args))
     def upper(self): return self.__class__(self.data.upper())
     def zfill(self, width): return self.__class__(self.data.zfill(width))
+
+# FIXME: try to implement defaultdict in collections.rs rather than in Python
+# I (coolreader18) couldn't figure out some class stuff with __new__ and
+# __init__ and __missing__ and subclassing built-in types from Rust, so I went
+# with this instead.
+from ._defaultdict import *

Lib/collections/_defaultdict.py

@@ -0,0 +1,20 @@
+class defaultdict(dict):
+    def __new__(cls, *args, **kwargs):
+        if len(args) >= 1:
+            default_factory = args[0]
+            args = args[1:]
+        else:
+            default_factory = None
+        self = dict.__new__(cls, *args, **kwargs)
+        self.default_factory = default_factory
+        return self
+
+    def __missing__(self, key):
+        if self.default_factory:
+            return self.default_factory()
+        else:
+            raise KeyError(key)
+
+    def __repr__(self):
+        return f"defaultdict({self.default_factory}, {dict.__repr__(self)})"
+