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gh-91456: [Enum] Deprecate default auto() behavior with mixed value t…
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…ypes (GH-91457)

When used with plain Enum, auto() returns the last numeric value assigned, skipping any incompatible member values (such as strings); starting in 3.13 the default auto() for plain Enums will require all the values to be of compatible types, and will return a new value that is 1 higher than any existing value.

Co-authored-by: Ethan Furman <ethan@stoneleaf.us>
(cherry picked from commit fb1e950)

Co-authored-by: Oscar R <89599049+oscar-LT@users.noreply.github.com>
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miss-islington and oscar-LT authored Jun 23, 2022
1 parent 00a25f8 commit 321acd4
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4 changes: 4 additions & 0 deletions Doc/library/enum.rst
Original file line number Diff line number Diff line change
Expand Up @@ -761,6 +761,10 @@ Utilities and Decorators
``_generate_next_value_`` can be overridden to customize the values used by
*auto*.

.. note:: in 3.13 the default ``"generate_next_value_`` will always return
the highest member value incremented by 1, and will fail if any
member is an incompatible type.

.. decorator:: property

A decorator similar to the built-in *property*, but specifically for
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34 changes: 26 additions & 8 deletions Lib/enum.py
Original file line number Diff line number Diff line change
Expand Up @@ -1218,21 +1218,39 @@ def __new__(cls, value):
def __init__(self, *args, **kwds):
pass

def _generate_next_value_(name, start, count, last_values):
def _generate_next_value_(name, start, count, last_value):
"""
Generate the next value when not given.
name: the name of the member
start: the initial start value or None
count: the number of existing members
last_value: the last value assigned or None
last_value: the list of values assigned
"""
for last_value in reversed(last_values):
try:
return last_value + 1
except TypeError:
pass
else:
if not last_value:
return start
try:
last = last_value[-1]
last_value.sort()
if last == last_value[-1]:
# no difference between old and new methods
return last + 1
else:
# trigger old method (with warning)
raise TypeError
except TypeError:
import warnings
warnings.warn(
"In 3.13 the default `auto()`/`_generate_next_value_` will require all values to be sortable and support adding +1\n"
"and the value returned will be the largest value in the enum incremented by 1",
DeprecationWarning,
stacklevel=3,
)
for v in last_value:
try:
return v + 1
except TypeError:
pass
return start

@classmethod
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65 changes: 56 additions & 9 deletions Lib/test/test_enum.py
Original file line number Diff line number Diff line change
Expand Up @@ -3972,23 +3972,54 @@ class Color(AutoNameEnum):
self.assertEqual(Color.blue.value, 'blue')
self.assertEqual(Color.green.value, 'green')

def test_auto_garbage(self):
class Color(Enum):
red = 'red'
blue = auto()
@unittest.skipIf(
python_version >= (3, 13),
'mixed types with auto() no longer supported',
)
def test_auto_garbage_ok(self):
with self.assertWarnsRegex(DeprecationWarning, 'will require all values to be sortable'):
class Color(Enum):
red = 'red'
blue = auto()
self.assertEqual(Color.blue.value, 1)

def test_auto_garbage_corrected(self):
class Color(Enum):
red = 'red'
blue = 2
green = auto()
@unittest.skipIf(
python_version >= (3, 13),
'mixed types with auto() no longer supported',
)
def test_auto_garbage_corrected_ok(self):
with self.assertWarnsRegex(DeprecationWarning, 'will require all values to be sortable'):
class Color(Enum):
red = 'red'
blue = 2
green = auto()

self.assertEqual(list(Color), [Color.red, Color.blue, Color.green])
self.assertEqual(Color.red.value, 'red')
self.assertEqual(Color.blue.value, 2)
self.assertEqual(Color.green.value, 3)

@unittest.skipIf(
python_version < (3, 13),
'mixed types with auto() will raise in 3.13',
)
def test_auto_garbage_fail(self):
with self.assertRaisesRegex(TypeError, 'will require all values to be sortable'):
class Color(Enum):
red = 'red'
blue = auto()

@unittest.skipIf(
python_version < (3, 13),
'mixed types with auto() will raise in 3.13',
)
def test_auto_garbage_corrected_fail(self):
with self.assertRaisesRegex(TypeError, 'will require all values to be sortable'):
class Color(Enum):
red = 'red'
blue = 2
green = auto()

def test_auto_order(self):
with self.assertRaises(TypeError):
class Color(Enum):
Expand All @@ -4010,6 +4041,22 @@ def _generate_next_value_(name, start, count, last):
self.assertEqual(Color.red.value, 'pathological case')
self.assertEqual(Color.blue.value, 'blue')

@unittest.skipIf(
python_version < (3, 13),
'auto() will return highest value + 1 in 3.13',
)
def test_auto_with_aliases(self):
class Color(Enum):
red = auto()
blue = auto()
oxford = blue
crimson = red
green = auto()
self.assertIs(Color.crimson, Color.red)
self.assertIs(Color.oxford, Color.blue)
self.assertIsNot(Color.green, Color.red)
self.assertIsNot(Color.green, Color.blue)

def test_duplicate_auto(self):
class Dupes(Enum):
first = primero = auto()
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Deprecate current default auto() behavior: In 3.13 the default will be for
for auto() to always return the largest member value incremented by
1, and to raise if incompatible value types are used.

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