bpo-44242: [Enum] remove missing bits test from Flag creation (GH-26586)

Move the check for missing named flags in flag aliases from Flag creation
to a new *verify* decorator.

Lib/enum.py

@@ -6,10 +6,10 @@ from builtins import property as _bltin_property, bin as _bltin_bin
 __all__ = [
         'EnumType', 'EnumMeta',
         'Enum', 'IntEnum', 'StrEnum', 'Flag', 'IntFlag',
-        'auto', 'unique',
-        'property',
+        'auto', 'unique', 'property', 'verify',
         'FlagBoundary', 'STRICT', 'CONFORM', 'EJECT', 'KEEP',
         'global_flag_repr', 'global_enum_repr', 'global_enum',
+        'EnumCheck', 'CONTINUOUS', 'NAMED_FLAGS', 'UNIQUE',
         ]
 
 
@@ -89,6 +89,9 @@ def _make_class_unpicklable(obj):
         setattr(obj, '__module__', '<unknown>')
 
 def _iter_bits_lsb(num):
+    # num must be an integer
+    if isinstance(num, Enum):
+        num = num.value
     while num:
         b = num & (~num + 1)
         yield b
@@ -538,13 +541,6 @@ class EnumType(type):
                 else:
                     # multi-bit flags are considered aliases
                     multi_bit_total |= flag_value
-            if enum_class._boundary_ is not KEEP:
-                missed = list(_iter_bits_lsb(multi_bit_total & ~single_bit_total))
-                if missed:
-                    raise TypeError(
-                            'invalid Flag %r -- missing values: %s'
-                            % (cls, ', '.join((str(i) for i in missed)))
-                            )
             enum_class._flag_mask_ = single_bit_total
             #
             # set correct __iter__
@@ -688,7 +684,10 @@ class EnumType(type):
         return MappingProxyType(cls._member_map_)
 
     def __repr__(cls):
-        return "<enum %r>" % cls.__name__
+        if Flag is not None and issubclass(cls, Flag):
+            return "<flag %r>" % cls.__name__
+        else:
+            return "<enum %r>" % cls.__name__
 
     def __reversed__(cls):
         """
@@ -1303,7 +1302,8 @@ class Flag(Enum, boundary=STRICT):
             else:
                 # calculate flags not in this member
                 self._inverted_ = self.__class__(self._flag_mask_ ^ self._value_)
-            self._inverted_._inverted_ = self
+            if isinstance(self._inverted_, self.__class__):
+                self._inverted_._inverted_ = self
         return self._inverted_
 
 
@@ -1561,6 +1561,91 @@ def _simple_enum(etype=Enum, *, boundary=None, use_args=None):
         return enum_class
     return convert_class
 
+@_simple_enum(StrEnum)
+class EnumCheck:
+    """
+    various conditions to check an enumeration for
+    """
+    CONTINUOUS = "no skipped integer values"
+    NAMED_FLAGS = "multi-flag aliases may not contain unnamed flags"
+    UNIQUE = "one name per value"
+CONTINUOUS, NAMED_FLAGS, UNIQUE = EnumCheck
+
+
+class verify:
+    """
+    Check an enumeration for various constraints. (see EnumCheck)
+    """
+    def __init__(self, *checks):
+        self.checks = checks
+    def __call__(self, enumeration):
+        checks = self.checks
+        cls_name = enumeration.__name__
+        if Flag is not None and issubclass(enumeration, Flag):
+            enum_type = 'flag'
+        elif issubclass(enumeration, Enum):
+            enum_type = 'enum'
+        else:
+            raise TypeError("the 'verify' decorator only works with Enum and Flag")
+        for check in checks:
+            if check is UNIQUE:
+                # check for duplicate names
+                duplicates = []
+                for name, member in enumeration.__members__.items():
+                    if name != member.name:
+                        duplicates.append((name, member.name))
+                if duplicates:
+                    alias_details = ', '.join(
+                            ["%s -> %s" % (alias, name) for (alias, name) in duplicates])
+                    raise ValueError('aliases found in %r: %s' %
+                            (enumeration, alias_details))
+            elif check is CONTINUOUS:
+                values = set(e.value for e in enumeration)
+                if len(values) < 2:
+                    continue
+                low, high = min(values), max(values)
+                missing = []
+                if enum_type == 'flag':
+                    # check for powers of two
+                    for i in range(_high_bit(low)+1, _high_bit(high)):
+                        if 2**i not in values:
+                            missing.append(2**i)
+                elif enum_type == 'enum':
+                    # check for powers of one
+                    for i in range(low+1, high):
+                        if i not in values:
+                            missing.append(i)
+                else:
+                    raise Exception('verify: unknown type %r' % enum_type)
+                if missing:
+                    raise ValueError('invalid %s %r: missing values %s' % (
+                            enum_type, cls_name, ', '.join((str(m) for m in missing)))
+                            )
+            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 = []
+                for name, alias in enumeration._member_map_.items():
+                    if name in member_names:
+                        # not an alias
+                        continue
+                    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:
+                    raise ValueError(
+                            'invalid Flag %r: %s'
+                            % (cls_name, '; '.join(missing))
+                            )
+        return enumeration
+
 def _test_simple_enum(checked_enum, simple_enum):
     """
     A function that can be used to test an enum created with :func:`_simple_enum`