具有默认枚举值的枚举类方法失败

Question

我很清楚，如果您有一个使用枚举的类名进行类型提示的类方法，那么有一个技巧可以让它在 Python 3.6 及更低版本中工作。

而不是...

class Release(Enum):
   ...
   @classmethod
   def get(cls, release: Release):
      ...

你需要像这样使用字符串值...

class Release(Enum):
   ...
   @classmethod
   def get(cls, release: "Release"):
      ...

我相信在 Python 3.7 及更高版本中，您不必使用引号就可以绕过这种“hack”。原因类似于“在首先完成所有方法和变量之前，该类还不存在”。由于该类尚不存在，因此我还不能使用类名，并且必须使用带引号的字符串作为破解。

但是，我试图更进一步并使用默认值。那是行不通的。 Python 3.6 是否有一种不是 hack 的 Pythonic 方法？ 另外，python 3.7 及更高版本是否有修复？

代码

from enum import Enum

class Release(Enum):
    Canary = (1, [])
    Beta = (2, [1])
    RC = (3, [2, 1])
    Stable = (4, [3, 2, 1])

    def __new__(cls, value, cascade):
        obj = object.__new__(cls)
        obj._value_ = value
        obj.current = ["Release" * value] # This would technically be a list of all releasese in this enum. This is just to emulate different values
        obj.cascade = cascade
        return obj

    @classmethod
    def get_all_releases(cls, release: "Release" = Canary):  # Default Value = Release.Canary
        return release.current


print(Release.get_all_releases(Release.Canary))
print(Release.get_all_releases(Release.Beta))
print(Release.get_all_releases(Release.RC))
print(Release.get_all_releases(Release.Stable))

# Error. Even with default value
# print(Release.get_all_releases())

使用此代码，我收到以下错误消息

AttributeError: 'tuple' object has no attribute 'current'

那是因为它返回的是 Canary 的元组，而不是实际的值。

Answer 1

虽然这绝对是一种解决方法，但这似乎对我来说效果很好：

@classmethod
def get_all_releases(cls, release: "Release" = Canary):  # Default Value = Release.Canary
    if release == (Release.Canary.value,):
        return Release.Canary.current
    return release.current

它适用于您分配给Canary 的任何值。因此，只要这是您的默认设置，我相信它会起作用。

---

为了更通用，您只需调整类定义中的默认值而不是每个函数，您可以按如下方式进行：

class Release(Enum):
    Canary = 6,
    Beta = 2,
    RC = 3,
    Stable = 4
    default = Canary

    ...

    @classmethod
    def get_all_releases(cls, release: "Release" = default):
        if release == (Release.Canary.value,):
            return Release.Canary.current
        return release.current

Answer 2

从@ufoxDan 的回答中得到了提示，但试图使其不那么变通，更自然。

基本上，我首先在returning 之前检查type(release)，并注意到我得到了...的结果。

<enum 'Release'>
<enum 'Release'>
<enum 'Release'>
<enum 'Release'>
<class 'tuple'>

我注意到，如果类型是 Release，那么我可以执行代码，但是如果它是 anything else，比如 None 而不是未创建的 Canary 类型，那么我可以假设它要求Canary。所以我做了以下...

@classmethod
def get_all_releases(cls, release: "Release" = None):
   if type(release) is Release:
       return release.current
   return Release.Canary.current

# Now these all work
print(Release.get_all_releases())
print(Release.get_all_releases(Release.Canary))
print(Release.get_all_releases(Release.Stable))

这似乎是实现结果的最 Pythonic 方式。这似乎也是阅读代码且无需重复代码的最佳方式。似乎任何人都应该能够实现类似的东西。

Answer 3

您可以在Release Enum 中做一些事情来让生活更轻松，第一个是here 显示的技术：

    def __new__(cls, value, cascade):
        obj = object.__new__(cls)
        obj._value_ = value
        obj.current = ["Release" * value]      # not sure what this should actually be

        # if always the previous versions (don't need cascade defined)
        obj.cascade = sorted(list(cls), reverse=True)

        # if some already defined subset (need cascade defined)
        obj.cascade = [cls._value2member_map_(c) for c in cascade]

        return obj

第二种技术可以有两种方式——您的默认值始终是第一个Enum 成员：

    @classmethod
    def get_all_releases(cls):
        return list(cls[0]).current

或者，如果默认值可以是任何成员，那么类似于 this answer 的东西应该可以工作：

class add_default:
    """
    add DEFAULT psuedo-member to enumeration; use first member if none specified
    (default should be name of member)
    """
    def __init__(self, default=''):
        self._default = default
    def __call__(self, enumeration):
        if self._default:
            member = enumeration[self._default]
        else:
            member = enumeration[enumeration._member_names_[0]]
        enumeration._member_map_['DEFAULT'] = member
        return enumeration

你最终的Enum 看起来像（假设cascade 是所有以前的成员并使用装饰器方法）：

@add_default('Canary')
class Release(Enum):
    Canary = 1
    Beta = 2
    RC = 3
    Stable = 4
    def __new__(cls, value):
        obj = object.__new__(cls)
        obj._value_ = value
        obj.current = ["Release" * value]      # not sure what this should actually be or how it's calculated
        obj.cascade = list(cls)[::-1]
        return obj
    @classmethod
    def get_all_releases(cls, release: "Release" = None):
        if release is None:
            release = cls.DEFAULT
        return release.current

并在使用中：

>>> Release.DEFAULT
<Release.Canary: 1>

>>> Release.get_all_releases()
['Release']

>>> Release.get_all_releases(Release.RC)
['ReleaseReleaseRelease']

---

原答案

您的代码有问题：

class Release(Enum):
    Canary = 1,

通过包含额外的逗号，您使Canary 的值变为(1, )。删除该逗号以摆脱 tuple 异常。