元类应用ORM实现

首先看下一个简单的例子

# 需求
import numbers


class Field:
    pass

class 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 = value class 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 = value

上述为属性描述符

元类的应用

class ModelMetaClass(type):
    def __new__(cls, *args, **kwargs):
        return super().__new__(cls, *args, **kwargs)

class BaseModel(metaclass=ModelMetaClass):
    def __init__(self,*args,**kwargs):
        for key,value in kwargs.items(): setattr(self,key,value) # return super().__init__() class User(BaseModel): 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="bobby", age=28) # user.name = "bobby" # user.age = 28 # user.save() print(user.name) print(user.age) print(user.Meta)

执行后结果：

bobby
28
<class '__main__.User.Meta'>

进一步改进代码：

class ModelMetaClass(type):
    """
    name 类名
    bases 基类（元组）
    attrs:属性
    还记得下面type生成类的使用方法吗？
    User = type("User", (BaseClass, ), {"name":"user", "say":say})
    """
    def __new__(cls, name, bases, attrs, **kwargs):
        if name == "BaseModel":
            return super().__new__(cls, name, bases, attrs, **kwargs) fields = {} for key,value in attrs.items(): if isinstance(value,Field): fields[key] = value attrs_meta = attrs.get('Meta',None) _meta = {} db_table = name.lower() 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 attrs['_meta'] = _meta attrs['fields'] = fields del attrs["Meta"] return super().__new__(cls, name, bases, attrs, **kwargs) class BaseModel(metaclass=ModelMetaClass): def __init__(self,*args,**kwargs): for key,value in kwargs.items(): setattr(self,key,value) # return super().__init__() class User(BaseModel): 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="bobby", age=28) # user.name = "bobby" # user.age = 28 # user.save() print(user.name) print(user.age) print(user._meta) print(user.fields)

执行结果：

bobby
28
{'db_table': 'user'}
{'age': <__main__.IntField object at 0x0000000000C1F438>, 'name': <__main__.CharField object at 0x0000000000C1F3C8>}