bpo-39264: Fix UserDict.get() to account for __missing__() (GH-17910)

Here's the patch according to the discussion at the [Python-Dev mailing list](https://mail.python.org/archives/list/python-dev@python.org/thread/SDXOEMAEM6KQ3CQCJVBVRT5QNSPAVU6X/).
UserDict.get() will match dict's behavior and not call `__missing__`.

Automerge-Triggered-By: GH:rhettinger

1 files changed, 8 insertions, 1 deletions

diff --git a/Lib/collections/__init__.py b/Lib/collections/__init__.py
index 7af8dcd..5860787 100644
--- a/Lib/collections/__init__.py
+++ b/Lib/collections/__init__.py
@@ -1132,10 +1132,17 @@ class UserDict(_collections_abc.MutableMapping):
     def __iter__(self):
         return iter(self.data)
 
-    # Modify __contains__ to work correctly when __missing__ is present
+    # Modify __contains__ and get() to work like dict
+    # does when __missing__ is present.
     def __contains__(self, key):
         return key in self.data
 
+    def get(self, key, default=None):
+        if key in self:
+            return self[key]
+        return default
+
+
     # Now, add the methods in dicts but not in MutableMapping
     def __repr__(self):
         return repr(self.data)