Add AutoEnum: automatically provides next value if missing. Issue 26988.

Lib/enum.py

@@ -8,7 +8,9 @@ except ImportError:
     from collections import OrderedDict
 
 
-__all__ = ['EnumMeta', 'Enum', 'IntEnum', 'unique']
+__all__ = [
+        'EnumMeta', 'Enum', 'IntEnum', 'AutoEnum', 'unique',
+        ]
 
 
 def _is_descriptor(obj):
@@ -52,7 +54,30 @@ class _EnumDict(dict):
     """
     def __init__(self):
         super().__init__()
+        # list of enum members
         self._member_names = []
+        # starting value
+        self._start = None
+        # last assigned value
+        self._last_value = None
+        # when the magic turns off
+        self._locked = True
+        # list of temporary names
+        self._ignore = []
+
+    def __getitem__(self, key):
+        if (
+                self._generate_next_value_ is None
+                or self._locked
+                or key in self
+                or key in self._ignore
+                or _is_sunder(key)
+                or _is_dunder(key)
+                ):
+            return super(_EnumDict, self).__getitem__(key)
+        next_value = self._generate_next_value_(key, self._start, len(self._member_names), self._last_value)
+        self[key] = next_value
+        return next_value
 
     def __setitem__(self, key, value):
         """Changes anything not dundered or not a descriptor.
@@ -64,19 +89,55 @@ class _EnumDict(dict):
 
         """
         if _is_sunder(key):
-            raise ValueError('_names_ are reserved for future Enum use')
+            if key not in ('_settings_', '_order_', '_ignore_', '_start_', '_generate_next_value_'):
+                raise ValueError('_names_ are reserved for future Enum use')
+            elif key == '_generate_next_value_':
+                if isinstance(value, staticmethod):
+                    value = value.__get__(None, self)
+                self._generate_next_value_ = value
+                self._locked = False
+            elif key == '_ignore_':
+                if isinstance(value, str):
+                    value = value.split()
+                else:
+                    value = list(value)
+                self._ignore = value
+                already = set(value) & set(self._member_names)
+                if already:
+                    raise ValueError(
+                            '_ignore_ cannot specify already set names: %r'
+                                % (already, ))
+            elif key == '_start_':
+                self._start = value
+                self._locked = False
         elif _is_dunder(key):
-            pass
+            if key == '__order__':
+                key = '_order_'
+            if _is_descriptor(value):
+                self._locked = True
         elif key in self._member_names:
             # descriptor overwriting an enum?
             raise TypeError('Attempted to reuse key: %r' % key)
+        elif key in self._ignore:
+            pass
         elif not _is_descriptor(value):
             if key in self:
                 # enum overwriting a descriptor?
-                raise TypeError('Key already defined as: %r' % self[key])
+                raise TypeError('%r already defined as: %r' % (key, self[key]))
             self._member_names.append(key)
+            if self._generate_next_value_ is not None:
+                self._last_value = value
+        else:
+            # not a new member, turn off the autoassign magic
+            self._locked = True
         super().__setitem__(key, value)
 
+    # for magic "auto values" an Enum class should specify a `_generate_next_value_`
+    # method; that method will be used to generate missing values, and is
+    # implicitly a staticmethod;
+    # the signature should be `def _generate_next_value_(name, last_value)`
+    # last_value will be the last value created and/or assigned, or None
+    _generate_next_value_ = None
 
 
 # Dummy value for Enum as EnumMeta explicitly checks for it, but of course
@@ -84,14 +145,31 @@ class _EnumDict(dict):
 # This is also why there are checks in EnumMeta like `if Enum is not None`
 Enum = None
 
-
+_ignore_sentinel = object()
 class EnumMeta(type):
     """Metaclass for Enum"""
     @classmethod
-    def __prepare__(metacls, cls, bases):
-        return _EnumDict()
-
-    def __new__(metacls, cls, bases, classdict):
+    def __prepare__(metacls, cls, bases, start=None, ignore=_ignore_sentinel):
+        # create the namespace dict
+        enum_dict = _EnumDict()
+        # inherit previous flags and _generate_next_value_ function
+        member_type, first_enum = metacls._get_mixins_(bases)
+        if first_enum is not None:
+            enum_dict['_generate_next_value_'] = getattr(first_enum, '_generate_next_value_', None)
+            if start is None:
+                start = getattr(first_enum, '_start_', None)
+        if ignore is _ignore_sentinel:
+            enum_dict['_ignore_'] = 'property classmethod staticmethod'.split()
+        elif ignore:
+            enum_dict['_ignore_'] = ignore
+        if start is not None:
+            enum_dict['_start_'] = start
+        return enum_dict
+
+    def __init__(cls, *args , **kwds):
+        super(EnumMeta, cls).__init__(*args)
+
+    def __new__(metacls, cls, bases, classdict, **kwds):
         # an Enum class is final once enumeration items have been defined; it
         # cannot be mixed with other types (int, float, etc.) if it has an
         # inherited __new__ unless a new __new__ is defined (or the resulting
@@ -102,12 +180,24 @@ class EnumMeta(type):
 
         # save enum items into separate mapping so they don't get baked into
         # the new class
-        members = {k: classdict[k] for k in classdict._member_names}
+        enum_members = {k: classdict[k] for k in classdict._member_names}
         for name in classdict._member_names:
             del classdict[name]
 
+        # adjust the sunders
+        _order_ = classdict.pop('_order_', None)
+        classdict.pop('_ignore_', None)
+
+        # py3 support for definition order (helps keep py2/py3 code in sync)
+        if _order_ is not None:
+            if isinstance(_order_, str):
+                _order_ = _order_.replace(',', ' ').split()
+            unique_members = [n for n in clsdict._member_names if n in _order_]
+            if _order_ != unique_members:
+                raise TypeError('member order does not match _order_')
+
         # check for illegal enum names (any others?)
-        invalid_names = set(members) & {'mro', }
+        invalid_names = set(enum_members) & {'mro', }
         if invalid_names:
             raise ValueError('Invalid enum member name: {0}'.format(
                 ','.join(invalid_names)))
@@ -151,7 +241,7 @@ class EnumMeta(type):
         # a custom __new__ is doing something funky with the values -- such as
         # auto-numbering ;)
         for member_name in classdict._member_names:
-            value = members[member_name]
+            value = enum_members[member_name]
             if not isinstance(value, tuple):
                 args = (value, )
             else:
@@ -165,7 +255,10 @@ class EnumMeta(type):
             else:
                 enum_member = __new__(enum_class, *args)
                 if not hasattr(enum_member, '_value_'):
-                    enum_member._value_ = member_type(*args)
+                    if member_type is object:
+                        enum_member._value_ = value
+                    else:
+                        enum_member._value_ = member_type(*args)
             value = enum_member._value_
             enum_member._name_ = member_name
             enum_member.__objclass__ = enum_class
@@ -572,6 +665,22 @@ class IntEnum(int, Enum):
 def _reduce_ex_by_name(self, proto):
     return self.name
 
+class AutoEnum(Enum):
+    """Enum where values are automatically assigned."""
+    def _generate_next_value_(name, start, count, last_value):
+        """
+        Generate the next value when not given.
+
+        name: the name of the member
+        start: the initital start value or None
+        count: the number of existing members
+        last_value: the last value assigned or None
+        """
+        # add one to the last assigned value
+        if not count:
+            return start if start is not None else 1
+        return last_value + 1
+
 def unique(enumeration):
     """Class decorator for enumerations ensuring unique member values."""
     duplicates = []

Misc/NEWS

@@ -78,6 +78,8 @@ Library
 - Issue 27512: Fix a segfault when os.fspath() called a an __fspath__() method
   that raised an exception. Patch by Xiang Zhang.
 
+- Issue 26988: Add AutoEnum.
+
 Tests
 -----