8.1 property动态属性
from datetime import date, datetimeclass User: def __init__(self, name, birthday): self.name = name self.birthday = birthday self._age = 0 # #def get_age(self): #return datetime.now().year - self.birthday.year @property # 将函数变成属性的方式访问，可以通过 user.age 访问 def age(self): return datetime.now().year - self.birthday.year @age.setter # 设置属性 user.age = 24 def age(self, value): self._age = valueif __name__ == "__main__": user = User("bobby", date(year=1987, month=1, day=1)) user.age = 30 print (user._age) print(user.age)# ---3032 8.2 __getattr__、__getattribute__魔法函数
# __getattr__, __getattribute__# __getattr__ 就是在查找不到属性的时候调用class User: def __init__(self, info={}): self.info = info def __getattr__(self, item): # 属性不存在的时候，进入 return self.info[item] # return "not find " # def __getattribute__(self, item): # 无条件的进入，所有属性的访问入口 # return "lewen"if __name__ == "__main__": user = User(info={"company_name": "imooc", "name": "lewen"}) print(user.name) ---
lewen
8.3 属性描述符和属性查找过程
import numbersclass IntField: # 数据属性描述符，任意实现下面一个就是 def __get__(self, instance, owner): # return self.value def __set__(self, instance, value): if not isinstance(value, numbers.Integral): # 参数类型检查 raise ValueError("int value need") if value < 0: raise ValueError("positive value need") self.value = value def __delete__(self, instance): passclass User: age = IntField()if __name__ == "__main__": user = User() user.age = 30# user.__dict__["age"] = "abc" # print(user.age) 会报错，找不到value print(user.__dict__) # {} 并没有进入对象实例 print(user.age) print (getattr(user, 'age')) # ---{}3030# --- class NonDataIntField: # 非数据属性描述符 def __get__(self, instance, owner): return self.valueclass User: age = NonDataIntField()if __name__ == "__main__": user = User() user.age = 30 print(user.__dict__) # {'age': 30} print(user.age) print (getattr(user, 'age')) # ==={'age': 30}3030# ------ '''如果user是某个类的实例，那么user.age（以及等价的getattr(user,’age’)）首先调用__getattribute__。如果类定义了__getattr__方法，那么在__getattribute__抛出 AttributeError 的时候就会调用到__getattr__，而对于描述符(__get__）的调用，则是发生在__getattribute__内部的。user = User(), 那么user.age 顺序如下：（1）如果“age”是出现在User或其基类的__dict__中， 且age是data descriptor， 那么调用其__get__方法, 否则（2）如果“age”出现在user的__dict__中， 那么直接返回 obj.__dict__[‘age’]， 否则（3）如果“age”出现在User或其基类的__dict__中，如果age是non-data descriptor，那么调用其__get__方法， 否则返回 __dict__[‘age’]（4）如果User有__getattr__方法，调用__getattr__方法，否则（5）抛出AttributeError''' 8.4 __new__和__init__的区别
class User: def __new__(cls, *args, **kwargs): print (" in new ") return super().__new__(cls) def __init__(self, name): print (" in init") passa = int()#new 是用来控制对象的生成过程， 在对象生成之前#init是用来完善对象的#如果new方法不返回对象， 则不会调用init函数if __name__ == "__main__": user = User(name="bobby")--- in new in init 8.5 自定义元类
#类也是对象，type创建类的类def create_class(name): if name == "user": class User: def __str__(self): return "user" return User elif name == "company": class Company: def __str__(self): return "company" return Companyif __name__ == "__main__": MyClass = create_class("user") my_obj = MyClass() print(type(my_obj))--- .User'>#type动态创建类def say(self): # 定义的方法 return "i am user" # return self.nameclass BaseClass(): # 基类 def answer(self): return "i am baseclass"User = type("User", (BaseClass,), {"name":"lewen","say":say})# 类名 模板 属性my_obj = User()print(my_obj.name)print(my_obj.say()) # 调用方法print(my_obj.answer()) # 调用基类方法 # ---leweni am useri am baseclass---
#什么是元类， 元类是创建类的类 对象<-class(对象)<-typefrom collections.abc import *class MetaClass(type): # 控制实例化的过程 def __new__(cls, *args, **kwargs): # 作初始检查 return super().__new__(cls, *args, **kwargs)class User(metaclass=MetaClass): def __init__(self, name): self.name = name def __str__(self): return "user:%s"%(self.name)# python中类的实例化过程，会首先寻找metaclass，通过metaclass去创建user类.# 没有继承的metaclass找到 则最后由 # type去创建类对象，实例my_obj = User("lewen")print(my_obj) ---user:lewen
8.6 元类实现简单的orm
import numbersclass Field: passclass IntField(Field): # 数据描述符 def __init__(self, db_column, min_value=None, max_value=None): self._value = None self.min_value = min_value self.max_value = max_value self.db_column = db_column if min_value is not None: if not isinstance(min_value, numbers.Integral): raise ValueError("min_value must be int") elif min_value < 0: raise ValueError("min_value must be positive int") if max_value is not None: if not isinstance(max_value, numbers.Integral): raise ValueError("max_value must be int") elif max_value < 0: raise ValueError("max_value must be positive int") if min_value is not None and max_value is not None: if min_value > max_value: raise ValueError("min_value must be smaller than max_value") def __get__(self, instance, owner): return self._value def __set__(self, instance, value): if not isinstance(value, numbers.Integral): raise ValueError("int value need") if value < self.min_value or value > self.max_value: raise ValueError("value must between min_value and max_value") self._value = valueclass CharField(Field): def __init__(self, db_column, max_length=None): self._value = None self.db_column = db_column if max_length is None: raise ValueError("you must spcify max_lenth for charfiled") self.max_length = max_length def __get__(self, instance, owner): return self._value def __set__(self, instance, value): if not isinstance(value, str): raise ValueError("string value need") if len(value) > self.max_length: raise ValueError("value len excess len of max_length") self._value = valueclass ModelMetaClass(type): def __new__(cls, name, bases, attrs, **kwargs): if name == "BaseModel": # 反回 BaseModel 类 return super().__new__(cls, name, bases, attrs, **kwargs) # 子类的数据封装 fields = {} for key, value in attrs.items(): if isinstance(value, Field): # 封装属性字段值 fields[key] = value attrs_meta = attrs.get("Meta", None) # 获取User类中的 Meta 类对象 _meta = {} db_table = name.lower() # 默认表名 if attrs_meta is not None: # table = getattr(attrs_meta, "db_table", None) if table is not None: db_table = table _meta["db_table"] = db_table # 设置表名 attrs["_meta"] = _meta attrs["fields"] = fields del attrs["Meta"] return super().__new__(cls, name, bases, attrs, **kwargs)# 先__new__将创建类的数据进行封装并返回类对象 ,再__init__初始化class BaseModel(metaclass=ModelMetaClass): # 处理子类实例化时初始值 def __init__(self, *args, **kwargs): for key, value in kwargs.items(): setattr(self, key, value) return super().__init__() #？ # 反回__init__方法，子类中不需要定义 def save(self): fields = [] values = [] for key, value in self.fields.items(): db_column = value.db_column if db_column is None: db_column = key.lower() fields.append(db_column) value = getattr(self, key) values.append(str(value)) sql = "insert {db_table}({fields}) value({values})".format(db_table=self._meta["db_table"], fields=",".join(fields), values=",".join(values)) #passclass User(BaseModel): # 创建类触发__new__： 类名User, bases 为BaseModel，下面的是属性 attrs name = CharField(db_column="name", max_length=10) age = IntField(db_column="age", min_value=1, max_value=100) class Meta: db_table = "user"if __name__ == "__main__": user = User(name="lewen", age=28) # 实例化，触发__init__ # user.name = "bobby" # user.age = 28 user.save()