元类编程

简单的动态获取类的方法

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 Company


if __name__ == "__main__":
    MyClass = create_class("user")
    my_obj = MyClass()
    print(type(my_obj)) # <class '__main__.create_class.<locals>.User'>

使用type动态创建类

一

# type动态创建类

def say(self):
    return "i am saying"

# 第一个参数是类名，第二个参数是基类，第三个参数是类的属性
User = type("User", (), {"name":"wanghw","say":say})

my_obj = User()
print(my_obj) # <__main__.User object at 0x109ec30f0>
print(my_obj.name) # wanghw
print(my_obj.say()) # i am saying

二

def say(self):
    return "i am user"
    # return self.name


class BaseClass():
    def answer(self):
        return "i am baseclass"


if __name__ == "__main__":
    # BaseClass后面的逗号必须有
    User = type("User", (BaseClass, ), {"name":"user", "say":say})
    my_obj = User()
    print(my_obj) # <__main__.User object at 0x109fd7d68>
    print(my_obj.answer()) # i am baseclass

使用元类创建类 —— 实际中的用法 ***

##### python中类的实例化过程：会首先寻找metaclass，通过metaclass去创建user类 去创建类对象，实例

### 继承type
class MetaClass(type):
    def __new__(cls, *args, **kwargs):
        return super().__new__(cls, *args, **kwargs)

# 元类是创建类的类 对象<-class(对象)<-type
class User(metaclass=MetaClass):
    def __init__(self, name):
        self.name = name

    def __str__(self):
        return self.name


if __name__ == "__main__":
    user = User("wanghw")
    print(user) # wanghw

使用元类实现自定义的ORM

# 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


class ModelMetaClass(type):
    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__()

    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))
        pass

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="whw", age=28)
    # user.name = "whw"
    # user.age = 28
    user.save()

～～～