Python--operator庫中的attrgetter()和itemgetter()

• operator.attrgetter()

1. 傳回：傳回從其操作數擷取attr的可調用對象。如果請求多個屬性，則傳回屬性元組。
2. attrgetter()實作代碼：官方标準庫3.7

   def attrgetter(*items):
       if any(not isinstance(item, str) for item in items):
           raise TypeError('attribute name must be a string')
       if len(items) == 1:
           attr = items[0]
           def g(obj):
               return resolve_attr(obj, attr)
       else:
           def g(obj):
               return tuple(resolve_attr(obj, attr) for attr in items)
       return g
   
   def resolve_attr(obj, attr):
       for name in attr.split("."):
           obj = getattr(obj, name)
       return obj
              

3. 示例：

   class Teacher():
       def __init__(self, name, salary, age):
           self.name = name
           self.age  = age
           self.salary = salary
       def __repr__(self):
           return  repr((self.name,self.age,self.salary))
   
   teachers = [
   Teacher("A",1200,30),
   Teacher("B",1200,31),
   Teacher("C",1300,30)
   ]
   
   from operator import attrgetter
   print(sorted(teachers,key=attrgetter("age")))  # 根據age排序
   print(sorted(teachers,key=attrgetter("salary","age")))   # 根據salary和age排序
   
   結果：
   [('A', 30, 1200), ('C', 30, 1300), ('B', 31, 1200)]
   [('A', 30, 1200), ('B', 31, 1200), ('C', 30, 1300)]
              

•  operator.itemgetter()

1. 傳回一個可調用對象，該對象使用操作數的__getitem __（）方法從其操作數中擷取項。如果指定了多個項，則傳回一個查找值元組。      

2. itemgetter()實作代碼：官方标準庫3.7

   def itemgetter(*items):
       if len(items) == 1:
           item = items[0]
           def g(obj):
               return obj[item]
       else:
           def g(obj):
               return tuple(obj[item] for item in items)
       return g
              

3.  示例：

   >>> itemgetter(1)('ABCDEFG')
   'B'
   >>> itemgetter(1,3,5)('ABCDEFG')
   ('B', 'D', 'F')
   >>> itemgetter(slice(2,None))('ABCDEFG')
   'CDEFG'
   >>> soldier = dict(rank='captain', name='dotterbart')
   >>> itemgetter('rank')(soldier)
   'captain'
   
   >>> inventory = [('apple', 3), ('banana', 2), ('pear', 5), ('orange', 1)]
   >>> getcount = itemgetter(1)
   >>> list(map(getcount, inventory))
   [3, 2, 5, 1]
   >>> sorted(inventory, key=getcount)
   [('orange', 1), ('banana', 2), ('apple', 3), ('pear', 5)]