Special method (double bottom method)

1. init

   Instantiate object calls to encapsulate attributes for objects

   class A:
       def __init__(self,name):
           self.name = name
   a = A("Zhang San")
   

2. new

   Instantiate the object call, inherit the new generation of the object and return an object

   class A:
       def __init__(self):
           pass
       def __new__(self):
           print("__new__")
   a = A()
   

3. call

   Object bracketed trigger

   class A:
       def __init__(self):
           pass
       def __call__(self, *args, **kwargs):
           print("__call__")
   a = A()
   a()
   

4. item

   Called when a dictionary operation is performed on an object

   1. getitem

      class A:
          def __getitem__(self, item):
              print(item)
      a = A()
      a["name"]
      

   2. setitem

      class A:
          def __setitem__(self, key, value):
              print(key,value)
      a = A()
      a["name"] = "Zhang San"
      

   3. delitem

      It doesn't really delete attributes, it just calls methods, it can be deleted by inheriting delitem of object.

      class A:
          name = "Zhang San"
          def __delitem__(self, key):
              print(key)
      a = A()
      del a["name"]
      print(a.name)
      

5. len

   Len (Object) Time Triggered

   Only integers can be returned

   class A:
       def __len__(self):
           return 1
   a = A()
   print(len(a))
   

6. hash

   Triggered when hase (object)

   Only integers can be returned

   class A:
       def __hash__(self):
           return 1
   a = A()
   print(hash(a))
   

7. str

   Triggered when printing an object or str

   Only string type can be returned

   class A:
       def __str__(self):
           return "Zhang San"
   a = A()
   print(a)
   print(str(a))
   

8. repr

   Triggered when printing an object or str

   Only string type can be returned

   str priority is higher than repr, and only str is executed when it exists

   class A:
       def __repr__(self):
           return "Zhang San"
   a = A()
   print(a)
   print(str(a))
   

9. eq

   Triggered when Object== Object

   class A:
       def __init__(self,name):
           self.name = name
       def __eq__(self, other):
           return self.name == other.name
   a = A("Zhang San")
   b = A("Zhang San")
   print(a == b)
   

10. Context management

    Enter and exit must appear in pairs, and enter must return to self

    class A:
        def __enter__(self):
            print("enter") # 1
            return self
        def __exit__(self, exc_type, exc_val, exc_tb):
            print("exit") # 3
    with A() as f:
        print("1") # 2
    print("2") # 4