Merge pull request #5565 from y-p/PR_GH5147_fast_config_lookup

ENH: short-circuit config lookup when exact key given GH5147

Author: y-p

pandas/core/config.py

@@ -512,6 +512,11 @@ def _select_options(pat):
     if pat=="all", returns all registered options
     """
 
+    # short-circuit for exact key
+    if pat in _registered_options:
+        return [pat]
+
+    # else look through all of them
     keys = sorted(_registered_options.keys())
     if pat == 'all':  # reserved key
         return keys