python第七周：反射、异常

1.反射：

1.1定义：通过字符串映射或修改程序运行时的状态、属性、方法

1.2有以下四个方法：

（1）hasattr(object,str) 判断object对象中是否有对应的方法或属性，返回值：True·、False

class People(object):
    '''this is the description of People'''
    def __init__(self,name,sex,age):
        self.name = name
        self.sex = sex
        self.age = age
    def eat(self):
        print("%s is eating...."%self.name)
    def piao(self):
        print("%s is piaoing..."%self.name)
    def __call__(self, *args, **kwargs):
        print("My name is Mr Wu")
    def __str__(self):
        return "hello world"

man = People("dog","male",19)
if hasattr(man,"name") and hasattr(man,"eat"):
    print("hello world!")
#output:hello world!

（2）func = getattr(object,str)根据str去获取object对象中的对应的方法的内存地址或属性的值(调用：func(argument))

class People(object):
    '''this is the description of People'''
    def __init__(self,name,sex,age):
        self.name = name
        self.sex = sex
        self.age = age
    def eat(self):
        print("%s is eating...."%self.name)
    def piao(self):
        print("%s is piaoing..."%self.name)
    def __call__(self, *args, **kwargs):
        print("My name is Mr Wu")
    def __str__(self):
        return "hello world"

man = People("dog","male",19)
func = getattr(man,"eat")
func() #dog is eating....
name = getattr(man,"name")
print(name) #dog

（3）delattr(object,str) 删除object对象中的str对应的方法或属性

class People(object):
    '''this is the description of People'''
    def __init__(self,name,sex,age):
        self.name = name
        self.sex = sex
        self.age = age
    def eat(self):
        print("%s is eating...."%self.name)
    def piao(self):
        print("%s is piaoing..."%self.name)
    def __call__(self, *args, **kwargs):
        print("My name is Mr Wu")
    def __str__(self):
        return "hello world"

man = People("dog","male",19)
delattr(man,"name")
print(man.name) #AttributeError: 'People' object has no attribute 'name'
delattr(man,"eat")
man.eat()#AttributeError: eat

（4）setattr(x,y,v)相当于x.y = v，设置新的属性或方法（可修改原有的属性和方法）

class People(object):
    '''this is the description of People'''
    def __init__(self,name,sex,age):
        self.name = name
        self.sex = sex
        self.age = age
    def eat(self):
        print("%s is eating...."%self.name)
    def piao(self):
        print("%s is piaoing..."%self.name)

def sleep(self):
    print("%s is sleeping...."%(self.name))
name = "Mr Wu"

man = People("dog","male",19)
setattr(man,"sleep",sleep)
man.sleep(man) #dog is sleeping....
setattr(man,"name",name)
print(man.name) #Mr Wu

2.异常处理

2.1 概述：

Python的异常处理能力是很强大的，可向用户准确反馈出错信息。在Python中，异常也是对象，可对它进行操作。所有异常都是基类Exception的成员。所有异常都从基类Exception继承，而且都在exceptions模块中定义。Python自动将所有异常名称放在内建命名空间中，所以程序不必导入exceptions模块即可使用异常。一旦引发而且没有捕捉SystemExit异常，程序执行就会终止。如果交互式会话遇到一个未被捕捉的SystemExit异常，会话就会终止。

2.2 python中所有的标准异常类

2.2 语法格式

try:

    代码块

except 异常名称 as e:

    print(e) #输出异常的内容

except 异常名称 as e:

    print(e) #输出异常处理

...............

特别地：

except Exception as e:

    #如果不知道异常具体是什么类型，可以使用Exception

else:

    #如果没有出现前面的异常则执行这条语句

finally:
    #不管有没有出现前面的异常都会执行这条语句

注：如果已经捕获到了try下的一条异常，那么就不会再捕获其他异常了

代码实例：

names = ["Mr Wu","Ms Li"]
dict_names = {"name":"Mr Wu","age":19}
try:
    print(names[3])
    dict_names["Mr Wang"]
except IndexError as e:
    print("出错了:",e)
except KeyError as e:
    print("出错了:",e)
except Exception as e:
    print("未知错误:",e)
#output：出错了: list index out of range

names = ["Mr Wu","Ms Li"]
dict_names = {"name":"Mr Wu","age":19}
try:
    print(names[3])
    dict_names["Mr Wang"]
#except IndexError as e:
#    print("出错了:",e)
#except KeyError as e:
#    print("出错了:",e)
except Exception as e:
    print("未知错误:",e)
#output：未知错误: list index out of range

2.3 自定义异常

我们可以自己定义异常类，并且触发执行这个异常。

注：自定义的异常必须继承异常类基类Exception，并通过raise语句实例化这个类

class AlexException(Exception):
    def __init__(self,msg):
        self.message = msg
try:
    raise AlexException("\033[1;41m数据库连接失败\033[0m")
except AlexException as e:
    print(e)
#output：数据库连接失败