第七章:Python高级编程-元类编程

第七章:Python高级编程-元类编程

Python3高级核心技术97讲 笔记

目录
  • 第七章:Python高级编程-元类编程
    • 7.1 property动态属性
    • 7.2 __getattr__、__getattribute__魔法函数
    • 7.3 属性描述符和属性查找过程
    • 7.4 __new__和__init__的区别
    • 7.5 自定义元类
    • 7.6 自定义元类

7.1 property动态属性

from datetime import data, datetime

class 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
    def age(self):
        return datetime.now().year - self.birthday.year
    
    
    @age.setter
    def age(self, value):
        self._age = value
        
if __name__ == "__main__":
    user = User("bobby", date(year=1987, month=1, day=1))
    print(user.get_age())
    user.age = 30
    print(self._age)

7.2 __getattr__、__getattribute__魔法函数

#__getattr__, __getattribute__
#__getattr__ 就是在查找不到属性的时候调用
from datetime import date
class User:
    def __init__(self,info={}):
        self.info = info

    def __getattr__(self, item):  # 查找不到属性的时候调用
        return self.info[item]

    # def __getattribute__(self, item):  # 查找属性时调用
    #     return "bobby"

if __name__ == "__main__":
    user = User(info={"company_name":"imooc", "name":"bobby"})
    print(user.test)

7.3 属性描述符和属性查找过程

from datetime import date, datetime
import numbers

class IntField:
    #数据描述符
    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 < 0:
            raise ValueError("positive value need")
        self.value = value
    def __delete__(self, instance):
        pass


class NonDataIntField:
    #非数据属性描述符
    def __get__(self, instance, owner):
        return self.value

class User:
    age = IntField()
    # age = NonDataIntField()

'''
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

(3.1)如果age是non-data descriptor,那么调用其__get__方法, 否则

(3.2)返回 __dict__[‘age’]

(4)如果User有__getattr__方法,调用__getattr__方法,否则

(5)抛出AttributeError

'''

# class User:
#
#     def __init__(self, name, email, birthday):
#         self.name = name
#         self.email = email
#         self.birthday = birthday
#         self._age = 0
#
#     # def get_age(self):
#     #     return datetime.now().year - self.birthday.year
#
#     @property
#     def age(self):
#         return datetime.now().year - self.birthday.year
#
#     @age.setter
#     def age(self, value):
#         #检查是否是字符串类型
#         self._age = value

if __name__ == "__main__":
    user = User()
    user.__dict__["age"] = "abc"
    print (user.__dict__)
    print (user.age)
    # print (getattr(user, 'age'))
    # user = User("bobby", date(year=1987, month=1, day=1))
    # user.age = 30
    # print (user._age)
    # print(user.age)


7.4 __new__和__init__的区别

class User:
    def __new__(cls, *args, **kwargs):  # args 位置参数  kwargs 有名参数
        print (" in new ")
        return super().__new__(cls)
    def __init__(self, name):
        print (" in init")
        pass
a = 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")

7.5 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 Company

#type动态创建类
# User = type("User", (), {})

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


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


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

from collections.abc import *

#什么是元类, 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 "user"
#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

if __name__ == "__main__":
    # MyClass = create_class("user")
    # my_obj = MyClass()
    # print(type(my_obj))

    # User = type("User", (BaseClass, ), {"name":"user", "say":say})
    my_obj = User(name="bobby")
    print(my_obj)

7.6 自定义元类

# 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(v alue, 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="bobby", age=28)
    # user.name = "bobby"
    # user.age = 28
    user.save()

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