--- a/mercurial/error.py	Tue Oct 06 20:45:52 2020 -0700
+++ b/mercurial/error.py	Tue Oct 06 21:06:18 2020 -0700
@@ -111,6 +111,11 @@
 class UnknownCommand(Exception):
     """Exception raised if command is not in the command table."""
 
+    def __init__(self, command, all_commands=None):
+        self.command = command
+        self.all_commands = all_commands
+        super(UnknownCommand, self).__init__()
+
     __bytes__ = _tobytes