8.1 property动态属性
from datetime import date, 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 # 将函数变成属性的方式访问，可以通过 user.age 访问
def age(self):
return datetime.now().year - self.birthday.year
@age.setter # 设置属性 user.age = 24
def age(self, value):
self._age = value
if __name__ == "__main__":
user = User("bobby", date(year=1987, month=1, day=1))
user.age = 30
print (user._age)
print(user.age)
# ---
30
32
8.2 __getattr__、__getattribute__魔法函数
# __getattr__, __getattribute__
# __getattr__ 就是在查找不到属性的时候调用
class User:
def __init__(self, info={}):
self.info = info
def __getattr__(self, item): # 属性不存在的时候，进入
return self.info[item]
# return "not find "
# def __getattribute__(self, item): # 无条件的进入，所有属性的访问入口
# return "lewen"
if __name__ == "__main__":
user = User(info={"company_name": "imooc", "name": "lewen"})
print(user.name)
---
lewen
8.3 属性描述符和属性查找过程
import numbers
class IntField:
# 数据属性描述符，任意实现下面一个就是
def __get__(self, instance, owner): #
return self.value
def __set__(self, instance, value):
if not isinstance(value, numbers.Integral): # 参数类型检查
raise ValueError("int value need")
if value < 0:
raise ValueError("positive value need")
self.value = value
def __delete__(self, instance):
pass
class User:
age = IntField()
if __name__ == "__main__":
user = User()
user.age = 30
# user.__dict__["age"] = "abc" # print(user.age) 会报错，找不到value
print(user.__dict__) # {} 并没有进入对象实例
print(user.age)
print (getattr(user, 'age'))
# ---
{}
30
30
# ---
class NonDataIntField:
# 非数据属性描述符
def __get__(self, instance, owner):
return self.value
class User:
age = NonDataIntField()
if __name__ == "__main__":
user = User()
user.age = 30
print(user.__dict__) # {'age': 30}
print(user.age)
print (getattr(user, 'age'))
# ===
{'age': 30}
30
30
# ------
'''
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，如果age是non-data descriptor，那么调用其__get__方法， 否则返回 __dict__[‘age’]
（4）如果User有__getattr__方法，调用__getattr__方法，否则
（5）抛出AttributeError
'''
8.4 __new__和__init__的区别
class User:
def __new__(cls, *args, **kwargs):
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")
---
in new
in init
8.5 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
if __name__ == "__main__":
MyClass = create_class("user")
my_obj = MyClass()
print(type(my_obj))
---
<class '__main__.create_class.<locals>.User'>
#type动态创建类
def say(self): # 定义的方法
return "i am user"
# return self.name
class BaseClass(): # 基类
def answer(self):
return "i am baseclass"
User = type("User", (BaseClass,), {"name":"lewen","say":say})
# 类名 模板 属性
my_obj = User()
print(my_obj.name)
print(my_obj.say()) # 调用方法
print(my_obj.answer()) # 调用基类方法
# ---
lewen
i am user
i am baseclass
---
#什么是元类， 元类是创建类的类 对象<-class(对象)<-type
from collections.abc import *
class MetaClass(type): # 控制实例化的过程
def __new__(cls, *args, **kwargs): # 作初始检查
return super().__new__(cls, *args, **kwargs)
class User(metaclass=MetaClass):
def __init__(self, name):
self.name = name
def __str__(self):
return "user:%s"%(self.name)
# python中类的实例化过程，会首先寻找metaclass，通过metaclass去创建user类.
# 没有继承的metaclass找到 则最后由
# type去创建类对象，实例
my_obj = User("lewen")
print(my_obj)
---
user:lewen
8.6 元类实现简单的orm
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": # 反回 BaseModel 类
return super().__new__(cls, name, bases, attrs, **kwargs)
# 子类的数据封装
fields = {}
for key, value in attrs.items():
if isinstance(value, Field): # 封装属性字段值
fields[key] = value
attrs_meta = attrs.get("Meta", None) # 获取User类中的 Meta 类对象
_meta = {}
db_table = name.lower() # 默认表名
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 # 设置表名
attrs["_meta"] = _meta
attrs["fields"] = fields
del attrs["Meta"]
return super().__new__(cls, name, bases, attrs, **kwargs)
# 先__new__将创建类的数据进行封装并返回类对象 ,再__init__初始化
class BaseModel(metaclass=ModelMetaClass):
# 处理子类实例化时初始值
def __init__(self, *args, **kwargs):
for key, value in kwargs.items():
setattr(self, key, value)
return super().__init__() #？
# 反回__init__方法，子类中不需要定义
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): # 创建类触发__new__： 类名User, bases 为BaseModel，下面的是属性 attrs
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="lewen", age=28) # 实例化，触发__init__
# user.name = "bobby"
# user.age = 28
user.save()
