Python列舉有例外：TypeError：試圖重用密鑰：

我試圖在列舉失敗時重新定義它，但隨后引發了錯誤。

我的代碼如下所示：

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

目標是3/0將引發例外然后重新定義foo。但是，當我按原樣運行它時，會顯示列印訊息，但會引發另一個錯誤，這對我來說沒有意義。這是輸出和堆疊跟蹤：

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'

擺脫此錯誤的唯一方法是在捕獲例外時洗掉例外的使用：

from enum import Enum

class FooEnum(Enum):
    try:
        foo = 3/0
    except:
        print('An error occurred')
        foo=0

然后它只是輸出： An error occurred

我正在使用 python 3.6.9

編輯 以下代碼更接近我的用例：

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

uj5u.com熱心網友回復：

發生這種情況的原因是：

• _EnumDict 跟蹤所有使用過的名字
• _EnumDict認為my_exception_instance應該是會員
• Pythonas在離開except子句 時清除變數
  • 通過分配None給my_exception_instance（然后洗掉變數）
  • 導致_EnumDict認為密鑰被重用

一種解決方法（從 Python 3.7 開始）是添加my_exception_instance到_ignore_¹屬性：

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

另一個解決方法是 makemy_exception_instance是一個全域的：

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

最后，如果您不想在列舉的主體中使用 try/except：

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

并在使用中：

>>> class FooEnum(FallbackEnum):
...     foo = '3/0', '0'
... 
3/0 error: division by zero
  using 0

>>> list(FooEnum)
[<FooEnum.foo: 0>]

¹aenum如果停留在 Python 3.6，您可以使用。

Disclosure: I am the author of the Python stdlib Enum, the enum34 backport, and the Advanced Enumeration (aenum) library.

標籤：Python 例外 枚举 python-3.6

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