8000 bpo-39728: enum with invalid value results in ValueError twice by dorosch · Pull Request #18641 · python/cpython · GitHub
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bpo-39728: enum with invalid value results in ValueError twice #18641

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bpo-39728: enum with invalid value results in ValueError twice #18641

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d15d1cc
Merge pull request #1 from python/master
dorosch Feb 23, 2020
ca86aff
bpo-39728: enum with invalid value results in ValueError twice
Feb 24, 2020
72eb2c4
bpo-39728: Fixed after review
Feb 24, 2020
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2 changes: 1 addition & 1 deletion Lib/enum.py
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Original file line number Diff line number Diff line change
Expand Up @@ -585,7 +585,7 @@ def __new__(cls, value):
'error in %s._missing_: returned %r instead of None or a valid member'
% (cls.__name__, result)
)
exc.__context__ = ve_exc
exc.__context__ = ve_exc
raise exc

def _generate_next_value_(name, start, count, last_values):
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5 changes: 3 additions & 2 deletions Lib/test/test_enum.py
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Expand Up @@ -1776,7 +1776,8 @@ def _missing_(cls, item):
# trigger not found
return None
self.assertIs(Color('three'), Color.blue)
self.assertRaises(ValueError, Color, 7)
with self.assertRaises(ValueError):
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This statement passes on the old code as well. Is it possible to specifically assert that a ValueError has been raised, but without the second ValueError being raised in the process?

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Yes, that can (and should) be tested, with something like this:

        with self.assertRaises(ValueError) as cm:
            Color(7)
        self.assertIsNone(cm.exception.__context__)

Color(7)
try:
Color('bad return')
except TypeError as exc:
Expand All @@ -1786,7 +1787,7 @@ def _missing_(cls, item):
try:
Color('error out')
except ZeroDivisionError as exc:
self.assertTrue(isinstance(exc.__context__, ValueError))
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Hm, I hadn't noticed but this test would (still) be affected by the fix. And it seems reasonable to keep : \

Instead of changing when context is added to the exception, could the default implementation of _missing be changed to return None? Or would that break something?

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I think that would break logic because None might be as value of enum:

from enum import Enum

class Color(Enum):
    BLUE = 'blue'
    RED = 'red'
    UNDEFINED = None

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@jonasmalacofilho jonasmalacofilho Feb 24, 2020 •
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But in that example _missing_ would need to return cls.UNDEFINED instead of None, because it has to return an instance of cls.

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Maybe you're right) I will try it

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The original verification here should be kept. It's checking that if using _missing_ explodes when searching for a value, the final result is the ValueError (of the bad value) and the ZeroDivisionError (of the internal problem).

self.assertTrue(not exc.__context__)
else:
raise Exception('Exception not raised.')

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1 change: 1 addition & 0 deletions Misc/NEWS.d/next/Library/2020-02-24-10-58-34.bpo-39728.kOOaHn.rst
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@@ -0,0 +1 @@
Instantiating enum with invalid value results in ValueError twice
0