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()