I'm trying to redefine an enum when it fails, but then an error is raised.
My code looks like the following:
from enum import Enum
class FooEnum(Enum):
try:
foo = 3/0
except Exception as my_exception_instance:
print('An error occurred:', my_exception_instance)
foo=0
The goal is that 3/0
will raise an exception and then re-define foo
.
However, when I run it as is, the print message is shown but another error is thrown, which does not make sense for me. Here goes the output and stack trace:
An error occurred: division by zero
---------------------------------------------------------------------------
TypeError Traceback (most recent call last)
<ipython-input-10-489d2391f28b> in <module>
1 from enum import Enum
2
----> 3 class FooEnum(Enum):
4 try:
5 foo = 3/0
<ipython-input-10-489d2391f28b> in FooEnum()
6 except Exception as my_exception_instance:
7 print('An error occurred:', my_exception_instance)
----> 8 foo=0
/usr/lib/python3.6/enum.py in __setitem__(self, key, value)
90 elif key in self._member_names:
91 # descriptor overwriting an enum?
---> 92 raise TypeError('Attempted to reuse key: %r' % key)
93 elif not _is_descriptor(value):
94 if key in self:
TypeError: Attempted to reuse key: 'my_exception_instance'
The only way to get rid of this error, is by removing the usage of the exception when catching it:
from enum import Enum
class FooEnum(Enum):
try:
foo = 3/0
except:
print('An error occurred')
foo=0
Then it just outputs: An error occurred
I'm using python 3.6.9
EDIT The following code is closer to my use case:
import tensorflow as tf
from enum import Enum
class VisualModels(Enum):
try:
MobileNet = tf.keras.applications.MobileNetV2
except Exception as e:
print(f'MobileNetV2 Not found, using MobileNet instead. Error: {e}.')
MobileNet = tf.keras.applications.MobileNet
# more models are defined similarly
CodePudding user response:
The reason that is happening is:
_EnumDict
tracks all used names_EnumDict
thinksmy_exception_instance
should be a member- Python clears the
as
variable when leaving theexcept
clause- by assigning
None
tomy_exception_instance
(and then deleting the variable) - causing
_EnumDict
to think a key is being reused
- by assigning
One workaround (as of Python 3.7) is to add my_exception_instance
to an _ignore_
1 attribute:
class FooEnum(Enum):
_ignore_ = 'my_exception_instance'
try:
foo = 3/0
except Exception as my_exception_instance:
print('An error occurred:', my_exception_instance)
foo=0
The other workaround is make my_exception_instance
be a global:
class FooEnum(Enum):
global my_exception_instance
try:
foo = 3/0
except Exception as my_exception_instance:
print('An error occurred:', my_exception_instance)
foo=0
Finally, if you don't want the try/except in the body of your enum:
class FallbackEnum(Enum):
def __new__(cls, *values):
# first actual value wins
member = object.__new__(cls)
fallback = False
for v in values:
try:
member._value_ = eval(v)
break
except Exception as e:
print('%s error: %s' % (v, e))
fallback = True
continue
else:
# never found a value
raise ValueError('no valid value found')
# if we get here, we found a value
if fallback:
# if the first value didn't work, print the one we did use
print(' using %s' % v)
return member
and in use:
>>> class FooEnum(FallbackEnum):
... foo = '3/0', '0'
...
3/0 error: division by zero
using 0
>>> list(FooEnum)
[<FooEnum.foo: 0>]
1 You can use aenum
if stuck at Python 3.6.
Disclosure: I am the author of the Python stdlib Enum
, the enum34
backport, and the Advanced Enumeration (aenum
) library.