1、列舉

• 可讀性更強

from enum import Enum

class VIP(Enum):
    YELLOW = 1
    GREEN  = 2
    BALCK  = 3
    RED    = 4

print(VIP.BALCK)
print(type(VIP.BALCK))
# [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
# VIP.BALCK
# <enum 'VIP'>

按照普通類來分析，類名.類變數 ，應該輸出的是3，但是其實卻是VIP.BALCK。

先不解釋為什麼。先說說，其實這種列印結果應該對呼叫該類的業務方來說，是更希望看到VIP.BALCK的吧，而不是一個數字3，也不知道這個意義是啥。

這就是說，列舉類不能按照普通類來分析。

接下來說說列舉類的特點和優勢

• 列舉類中的value不可變:Cannot reassign members；業務需求中，往往也需要定義的常量型別，是不允許被修改的。

• from enum import Enum
  
  class VIP(Enum):
      YELLOW = 1
      GREEN  = 1
      BALCK  = 3
      RED    = 4
  
  print(VIP.GREEN)
  VIP.GREEN 
   
  = 5
  # [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
  # VIP.GREEN
  # Traceback (most recent call last):
  #   File "/Users/anson/Documents/Project/python_ToolCodes/test17.py", line 10, in <module>
  #     VIP.GREEN = 5
  #   File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/enum/__init__.py", line 419, in __setattr__
   
  
  #     raise AttributeError('Cannot reassign members.')
  # AttributeError: Cannot reassign members.
  
  用普通類代替列舉類，被輕易修改
  

  class Common():
      GREEN = 1
  Common.GREEN = 3
  print(Common.GREEN)
  # [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
  # 3

  dict來替代列舉類，被輕易修改

  a = {"GREEN":1,"RED":4}
  a["GREEN"] = 3
  print(a)
  # [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
  # {'GREEN': 3, 'RED': 4}

• 列舉類中不能存在相同的標籤名

• from enum import Enum
  
  class Animal(Enum):
      CAT = 1
      CAT = 2
  # anson@ansonwandeMacBook-Pro python_ToolCodes % python3 test19.py 
  # Traceback (most recent call last):
  #   File "test19.py", line 3, in <module>
  #     class Animal(Enum):
  #   File "test19.py", line 5, in Animal
  #     CAT = 2
  #   File "/usr/local/Cellar/[email protected]/3.8.3_2/Frameworks/Python.framework/Versions/3.8/lib/python3.8/enum.py", line 95, in __setitem__
  #     raise TypeError('Attempted to reuse key: %r' % key)
  # TypeError: Attempted to reuse key: 'CAT'

  需要注意的是，python2.7版本是允許有相同名稱的標籤的，但是目前所學的python3.8時不允許有相同名稱的標籤的

• 獲取列舉的標籤和數值，該怎麼操作VIP.YELLOW.name,VIP.YELLOW.value

• from enum import Enum
  #coding=utf-8
  
  class VIP(Enum):
      YELLOW = 1
      GRAY   = 2
      BALCK  = 3
      RED    = 4
  print(VIP.YELLOW)
  print(VIP.YELLOW.name)
  print(VIP.YELLOW.value)
  # [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
  # VIP.YELLOW
  # YELLOW
  # 1

• 列舉遍歷

  from enum import Enum
  #coding=utf-8
  
  class VIP(Enum):
      YELLOW = 1
      GRAY   = 2
      BALCK  = 3
      RED    = 4
  for v in VIP:
      print(v)
  # [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
  # VIP.YELLOW
  # VIP.GRAY
  # VIP.BALCK
  # VIP.RED

• 列舉比較

  • “==”比較：允許
    

    from enum import Enum
    #coding=utf-8
    
    class VIP(Enum):
        YELLOW = 1
        GRAY   = 2
        BALCK  = 3
        RED    = 4
    print(VIP.BALCK == VIP.GRAY)
    # [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
    # False

    
    >,<比較：不允許！！！
    

    from enum import Enum
    #coding=utf-8
    
    class VIP(Enum):
        YELLOW = 1
        GRAY   = 2
        BALCK  = 3
        RED    = 4
    print(VIP.BALCK > VIP.GRAY)
    # [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
    # Traceback (most recent call last):
    #   File "/Users/anson/Documents/Project/python_ToolCodes/test17.py", line 9, in <module>
    #     print(VIP.BALCK > VIP.GRAY)
    #   File "/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/lib/python/enum/__init__.py", line 740, in __gt__
    #     raise TypeError("unorderable types: %s() > %s()" % (self.__class__.__name__, other.__class__.__name__))
    # TypeError: unorderable types: VIP() > VIP()

    
    is 比較：允許

    from enum import Enum
    #coding=utf-8
    
    class VIP(Enum):
        YELLOW = 1
        GRAY   = 2
        BALCK  = 3
        RED    = 4
    print(VIP.BALCK is VIP.BALCK)
    # [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
    # True

    列舉類之間比較：允許
    

    from enum import Enum
    #coding=utf-8
    
    class VIP1(Enum):
        YELLOW = 1
        GRAY   = 2
        BALCK  = 3
        RED    = 4
    
    class VIP2(Enum):
        YELLOW = 1
        GRAY   = 2
        BALCK  = 3
        RED    = 4
    print(VIP1.GRAY is VIP2.GRAY)
    # [Running] python -u "/Users/anson/Documents/Project/python_ToolCodes/test17.py"
    # False