python
diff --git a/‎Lib/configparser.py‎
Lines changed: 14 additions & 15 deletions b/‎Lib/configparser.py‎
Lines changed: 14 additions & 15 deletions
diff --git a/‎Lib/test/test_configparser.py‎
Lines changed: 4 additions & 3 deletions b/‎Lib/test/test_configparser.py‎
Lines changed: 4 additions & 3 deletions
diff --git a/‎Misc/NEWS‎
Lines changed: 3 additions & 0 deletions b/‎Misc/NEWS‎
Lines changed: 3 additions & 0 deletions
@@ -263,12 +263,9 @@ class InterpolationMissingOptionError(InterpolationError):
     """A string substitution required a setting which was not available."""
 
     def __init__(self, option, section, rawval, reference):
-        msg = ("Bad value substitution:\n"
-               "\tsection: [%s]\n"
-               "\toption : %s\n"
-               "\tkey    : %s\n"
-               "\trawval : %s\n"
-               % (section, option, reference, rawval))
+        msg = ("Bad value substitution: option {!r} in section {!r} contains "
+               "an interpolation key {!r} which is not a valid option name. "
+               "Raw value: {!r}".format(option, section, reference, rawval))
         InterpolationError.__init__(self, option, section, msg)
         self.reference = reference
         self.args = (option, section, rawval, reference)
@@ -286,11 +283,11 @@ class InterpolationDepthError(InterpolationError):
     """Raised when substitutions are nested too deeply."""
 
     def __init__(self, option, section, rawval):
-        msg = ("Value interpolation too deeply recursive:\n"
-               "\tsection: [%s]\n"
-               "\toption : %s\n"
-               "\trawval : %s\n"
-               % (section, option, rawval))
+        msg = ("Recursion limit exceeded in value substitution: option {!r} "
+               "in section {!r} contains an interpolation key which "
+               "cannot be substituted in {} steps. Raw value: {!r}"
+               "".format(option, section, MAX_INTERPOLATION_DEPTH,
+                         rawval))
         InterpolationError.__init__(self, option, section, msg)
         self.args = (option, section, rawval)
 
@@ -406,8 +403,9 @@ def before_set(self, parser, section, option, value):
 
     def _interpolate_some(self, parser, option, accum, rest, section, map,
                           depth):
+        rawval = parser.get(section, option, raw=True, fallback=rest)
         if depth > MAX_INTERPOLATION_DEPTH:
-            raise InterpolationDepthError(option, section, rest)
+            raise InterpolationDepthError(option, section, rawval)
         while rest:
             p = rest.find("%")
             if p < 0:
@@ -432,7 +430,7 @@ def _interpolate_some(self, parser, option, accum, rest, section, map,
                     v = map[var]
                 except KeyError:
                     raise InterpolationMissingOptionError(
-                        option, section, rest, var) from None
+                        option, section, rawval, var) from None
                 if "%" in v:
                     self._interpolate_some(parser, option, accum, v,
                                            section, map, depth + 1)
@@ -466,8 +464,9 @@ def before_set(self, parser, section, option, value):
 
     def _interpolate_some(self, parser, option, accum, rest, section, map,
                           depth):
+        rawval = parser.get(section, option, raw=True, fallback=rest)
         if depth > MAX_INTERPOLATION_DEPTH:
-            raise InterpolationDepthError(option, section, rest)
+            raise InterpolationDepthError(option, section, rawval)
         while rest:
             p = rest.find("$")
             if p < 0:
@@ -504,7 +503,7 @@ def _interpolate_some(self, parser, option, accum, rest, section, map,
                             "More than one ':' found: %r" % (rest,))
                 except (KeyError, NoSectionError, NoOptionError):
                     raise InterpolationMissingOptionError(
-                        option, section, rest, ":".join(path)) from None
+                        option, section, rawval, ":".join(path)) from None
                 if "$" in v:
                     self._interpolate_some(parser, opt, accum, v, sect,
                                            dict(parser.items(sect, raw=True)),
 
@@ -847,7 +847,8 @@ def test_interpolation(self):
            "something with lots of interpolation (10 steps)")
         e = self.get_error(cf, configparser.InterpolationDepthError, "Foo", "bar11")
         if self.interpolation == configparser._UNSET:
-            self.assertEqual(e.args, ("bar11", "Foo", "%(with1)s"))
+            self.assertEqual(e.args, ("bar11", "Foo",
+                "something %(with11)s lots of interpolation (11 steps)"))
         elif isinstance(self.interpolation, configparser.LegacyInterpolation):
             self.assertEqual(e.args, ("bar11", "Foo",
                 "something %(with11)s lots of interpolation (11 steps)"))
@@ -861,7 +862,7 @@ def test_interpolation_missing_value(self):
         self.assertEqual(e.option, "name")
         if self.interpolation == configparser._UNSET:
             self.assertEqual(e.args, ('name', 'Interpolation Error',
-                                    '', 'reference'))
+                                    '%(reference)s', 'reference'))
         elif isinstance(self.interpolation, configparser.LegacyInterpolation):
             self.assertEqual(e.args, ('name', 'Interpolation Error',
                                     '%(reference)s', 'reference'))
@@ -1177,7 +1178,7 @@ def test_strange_options(self):
         with self.assertRaises(exception_class) as cm:
             cf['interpolated']['$trying']
         self.assertEqual(cm.exception.reference, 'dollars:${sick')
-        self.assertEqual(cm.exception.args[2], '}') #rawval
+        self.assertEqual(cm.exception.args[2], '${dollars:${sick}}') #rawval
 
     def test_case_sensitivity_basic(self):
         ini = textwrap.dedent("""
 
@@ -16,6 +16,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #21159: Improve message in configparser.InterpolationMissingOptionError.
+  Patch from Łukasz Langa.
+
 - Issue #24847: Fixes tcltk installer layout of VC runtime DLL
 
 - Issue #24839: platform._syscmd_ver raises DeprecationWarning