provide a better error message for "pip config get index-url"

src/pip/_internal/configuration.py

@@ -147,6 +147,9 @@ class Configuration:
         try:
             return self._dictionary[key]
         except KeyError:
+            # disassembling triggers a more useful error message than simply
+            # "No such key" in the case that the key isn't in the form command.option
+            _disassemble_key(key)
             raise ConfigurationError(f"No such key - {orig_key}")
 
     def set_value(self, key: str, value: Any) -> None:

tests/unit/test_configuration.py

@@ -1,6 +1,7 @@
 """Tests for all things related to the configuration
 """
 
+import re
 from unittest.mock import MagicMock
 
 import pytest
@@ -87,6 +88,25 @@ class TestConfigurationLoading(ConfigurationMixin):
             err.value
         )
 
+    def test_no_such_key_error_message_no_command(self) -> None:
+        self.configuration.load_only = kinds.GLOBAL
+        self.configuration.load()
+        expected_msg = (
+            "Key does not contain dot separated section and key. "
+            "Perhaps you wanted to use 'global.index-url' instead?"
+        )
+        pat = f"^{re.escape(expected_msg)}$"
+        with pytest.raises(ConfigurationError, match=pat):
+            self.configuration.get_value("index-url")
+
+    def test_no_such_key_error_message_missing_option(self) -> None:
+        self.configuration.load_only = kinds.GLOBAL
+        self.configuration.load()
+        expected_msg = "No such key - global.index-url"
+        pat = f"^{re.escape(expected_msg)}$"
+        with pytest.raises(ConfigurationError, match=pat):
+            self.configuration.get_value("global.index-url")
+
 
 class TestConfigurationPrecedence(ConfigurationMixin):
     # Tests for methods to that determine the order of precedence of