基于元类实现ORM

"""
基于元类，实现ORM--模拟django
"""

# Field定义字段基类,包括字段名字和字段类型
class Field(object):
    def __init__(self, name, column_type):
        self.name = name
        self.column_type = column_type
    # 定义可读性更好的__str__()
    def __str__(self):
        return f'<{self.__class__.__name__}: {self.name}>'

# 定义charField类
class CharField(Field):
    def __init__(self, name, max_length):
        super().__init__(name, f'varchar({max_length})')

# 定义Integer类
class IntegerField(Field):
    def __init__(self, name):
        super().__init__(name, 'int')

# 定义模型基类
class ModelMeta(type):
    # attrs 是字典对象
    def __new__(cls, name, bases, attrs):# 查看这个attr的键值对
        if name == 'Model':
            return type.__new__(cls, name, bases, attrs)

        print('Model name : ', name)# 会自动传送类名过来
        mappings = dict()
        for k, v in attrs.items():
            if isinstance(v, Field):
                print('Field Mapping',(k, v))
                mappings[k] = v
        # 便利mappings 将字典attrs中的键删掉
        for k in mappings:
            attrs.pop(k)

        attrs['__mappings__'] = mappings
        attrs['__table__'] = name

        return type.__new__(cls, name, bases, attrs)


class Model(dict, metaclass=ModelMeta):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)

    def __getattr__(self, key):
        try:
            return self[key]
        except:
            raise AttributeError(f'Model not {key} key')

    def __setattr__(self, key, value):
        self[key] = value

    def save(self):
        fields = []
        params = []
        args = []
        # __mappings__是一个字典
        for k, v in self.__mappings__.items():
            fields.append(v.name)
            params.append('?')
            args.append(getattr(self, k, None))

        sql = 'insert into %s(%s) VALUES (%s)' 
        % (self.__table__, ','.join(fields), ','.join(params))
        print(sql, args)


class Person(Model):
    id = IntegerField(name='pid')
    name = CharField('pname', max_length=20)
    age = IntegerField('age')


if __name__ == '__main__':
    p = Person(id=1, name='disen', age=20)
    p.save()

Q&A

1. args.append(getattr(self, k, None))为啥能够获得用户输入的值,而不是类变量
2. for k, v in self.mappings.items():此处字典__mappings__中的键值对则是变成如下图中的Field Mapping括号中所示
   
3. 对1,2 两点的猜测,涉及到类变量和实例变量getattr()得到的是当前实例变量的属性,而底层(父类)得到的则是类变量