Data and Introspection__ dict__ Attributes and__ slots__ attribute

Variables defined in a class are called class attributes , Methods defined in a class are called class methods ; Class attribute classification ：

1. Public attribute ： General attribute

2. Private property ： With “_” or "__" The first attribute is called a private attribute , Private properties can also be inherited and accessed .

for example ：

class Info(object):
    _name = ' Springfield '
    __age = 4


if __name__ == '__main__': 
    i = Info()
    print(i._name)           #  Output ： Springfield 
    print(Info._name)        #  Output ： Springfield 
    print(Info._Info__age)   #  Output ：4
    print(i.__age)           #  Output ：AttributeError: 'Info' object has no attribute '__age'
    print(Info.__age)        #  Output ：AttributeError: 'Info' object has no attribute '__age'

_name attribute ： Outside through “Info._name" or “ object ._name" visit .

__age attribute ： External access only “Info._Info__age" visit .

3.__dict__ Method ：

3.1 Class calls the class itself __dict__ attribute , What is returned is the dictionary of the properties and methods of a class .

3.2 The instance object calls the... Of the object itself __dict__ attribute , What is returned is a dictionary of the properties and methods of an instance object .

class Info(object):
    _name = ' Springfield '
    __age = 4

    def run(self):
        print('run')


if __name__ == '__main__':
    i = Info()
    i.gender = 'male'
    #  Call the object's __dict__ Method ：{'gender': 'male'}
    print(i.__dict__)
    #  The calling class __dict__ Method ：
    # {'__module__': '__main__', '_name': ' Springfield ', '_Info__age': 4,
    # 'run': <function Info.run at 0x7f84a00a0d30>,
    # '__dict__': <attribute '__dict__' of 'Info' objects>,
    # '__weakref__': <attribute '__weakref__' of 'Info' objects>, '__doc__': None}
    print(Info.__dict__)

It can be seen that ： When the class is created ,Python Will automatically add {'__dict__': <attribute '__dict__' of 'Info' objects>, '__weakref__': <attribute '__weakref__' of 'Info' objects>, '__doc__': None} These attributes , But we don't need these attributes , If this class is instantiated with many objects , Then a lot of memory space will be wasted to store these attributes we don't need , Increased performance overhead . How to solve ？

Through built-in properties ：__slots__ To limit the properties that a class can be defined , Solve the problem that the class is automatically added with attributes .

__slots__ attribute ：slots Properties cannot be inherited

        1. Specify the properties that class objects can bind ;

        2. Limiting attributes ;

        3. To save memory ： Defined __slots__ After attribute , Then the object will not be generated automatically __dict__ attribute .

         By defining... In a class __slots__ Property to override the default __dict__ Behavior ;__slots__ Declare to receive a sequence of instance variables , And only enough space is reserved for each variable value in each instance , Therefore, it will not be created for each instance __dict__, So save space .

class Test(object):
    #  adopt __slots__ Restrict the attributes that a class can exist ,slots Properties cannot be inherited 
    __slots__ = ['name', 'age']

    def __init__(self, name, age, gender):
        self.name = name
        self.age = age
        # self.gender = gender  # __slots__ This is not included in gender attribute , So there's an error 


class Case(object):
    #  The test platform creates test cases 
    __slots__ = ['case_id', "title", "url", "data", "expected"]
    
    def __init__(self):
        self.case_id = None
        self.title = None
        self.url = None
        self.data = None
        self.expected = None
        

if __name__ == '__main__':
    t = Test(' Springfield ', "4", 'male')
    #  View the of the class __dict__ Method , The properties and methods of the discovery class are no longer in __dict__ Content. .
    print(Test.__dict__)   # {'__module__': '__main__', '__slots__': ['name', 'age'], '__init__': <function Test.__init__ at 0x7f81200a0d30>, 'age': <member 'age' of 'Test' objects>, 'name': <member 'name' of 'Test' objects>, '__doc__': None}

    print(t.__dict__)   # AttributeError: 'Test' object has no attribute '__dict__'
    print(t.age)
    print(t.name)
    print(t.gender)
    #  Output ：AttributeError: 'Test' object has no attribute 'gender'