bpo-44242: [Enum] improve error messages (GH-26669)

(cherry picked from commit c956734d7af83ad31f847d31d0d26df087add9a4)

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>

1 files changed, 25 insertions, 12 deletions

diff --git a/Lib/enum.py b/Lib/enum.py
index 5263e51..bf3460c 100644
--- a/Lib/enum.py
+++ b/Lib/enum.py
@@ -1,5 +1,7 @@
 import sys
 from types import MappingProxyType, DynamicClassAttribute
+from operator import or_ as _or_
+from functools import reduce
 from builtins import property as _bltin_property, bin as _bltin_bin
 
 
@@ -97,6 +99,9 @@ def _iter_bits_lsb(num):
         yield b
         num ^= b
 
+def show_flag_values(value):
+    return list(_iter_bits_lsb(value))
+
 def bin(num, max_bits=None):
     """
     Like built-in bin(), except negative values are represented in
@@ -1601,14 +1606,16 @@ class verify:
                 else:
                     raise Exception('verify: unknown type %r' % enum_type)
                 if missing:
-                    raise ValueError('invalid %s %r: missing values %s' % (
+                    raise ValueError(('invalid %s %r: missing values %s' % (
                             enum_type, cls_name, ', '.join((str(m) for m in missing)))
-                            )
+                            )[:256])
+                            # limit max length to protect against DOS attacks
             elif check is NAMED_FLAGS:
                 # examine each alias and check for unnamed flags
                 member_names = enumeration._member_names_
                 member_values = [m.value for m in enumeration]
-                missing = []
+                missing_names = []
+                missing_value = 0
                 for name, alias in enumeration._member_map_.items():
                     if name in member_names:
                         # not an alias
@@ -1616,16 +1623,22 @@ class verify:
                     values = list(_iter_bits_lsb(alias.value))
                     missed = [v for v in values if v not in member_values]
                     if missed:
-                        plural = ('', 's')[len(missed) > 1]
-                        a =  ('a ', '')[len(missed) > 1]
-                        missing.append('%r is missing %snamed flag%s for value%s %s' % (
-                                name, a, plural, plural,
-                                ', '.join(str(v) for v in missed)
-                                ))
-                if missing:
+                        missing_names.append(name)
+                        missing_value |= reduce(_or_, missed)
+                if missing_names:
+                    if len(missing_names) == 1:
+                        alias = 'alias %s is missing' % missing_names[0]
+                    else:
+                        alias = 'aliases %s and %s are missing' % (
+                                ', '.join(missing_names[:-1]), missing_names[-1]
+                                )
+                    if _is_single_bit(missing_value):
+                        value = 'value 0x%x' % missing_value
+                    else:
+                        value = 'combined values of 0x%x' % missing_value
                     raise ValueError(
-                            'invalid Flag %r: %s'
-                            % (cls_name, '; '.join(missing))
+                            'invalid Flag %r: %s %s [use `enum.show_flag_values(value)` for details]'
+                            % (cls_name, alias, value)
                             )
         return enumeration