异常:
try:
<语句>#可能发生异常的代码
except<名字>:
<语句>#如果在try部份引发了'name'异常
except<名字> as <异常参数>:
<语句>#如果引发了‘name’异常,获得附加的异常对象
else:
<语句>#如果没有异常发生
finally:
    <语句>#始终都会执行的语句

嵌套异常捕获
try:
    try:
        1/0
    except IOError:
        print("IOError ")
except Exception as e:
print(e)

except不携带任何异常类型
try:
    int("2.3")
except:
    print("error")
else:
print("no error")

except携带多种异常类型
需要用括号
try:
    int("2.3")
except (ValueError,TypeError) as e:
    print(e)
else:
print("no error")

多个except
try:
    int("2.3")
except ValueError as e:
    print(e)

except TypeError as e:
    print(e)
except Exception as e:
    print(e)

else:
print("no error")

try-finally
try:
    fp = open("e:\\1e.txt")
    try:
        content = fp.read()
        print(content)
    finally:
        print("关闭文件")
        fp.close()
except IOError:
    print("没有找到文件")

文件存在

文件不存在

try-else
try:
    fp = open("e:\\e.txt")
    try:
        content = fp.read()
        print(content)
    finally:
        print("关闭文件")
        fp.close()

except IOError:
    print("没有找到文件")

else:
    print("找到了文件")

try-except-else-finally
finally需要放到else的后面
try:
    fp = open("e:\\e.txt")
    try:
        content = fp.read()
        print(content)
    finally:
        print("关闭文件")
        fp.close()

except IOError:
    print("没有找到文件")

else:
    print("找到了文件")

finally:
    print("finally  始终都会执行")

异常可携带参数e
import traceback

try:
    fp = open("e:\\eeeee.txt")

except IOError as e:
    print(e)
    print(traceback.print_exc())

else:
print("找到了文件")

主动抛出异常raise
#encoding=utf-8
def div(a,b):
    if b == 0:
        raise Exception("除数为0")
    else:
        return a/b

if __name__ == "__main__":
    try:
        res = div(2,0)
    except Exception as e:
        print(e)
    else:
        print(res)

自定义异常
#encoding=utf-8

class MyException(Exception):
    def __init__(self,value):
        self.value = value

if __name__ == "__main__":
    try:
        raise MyException("自定义异常信息")
    except MyException as e:
        print("捕获自定义异常",e.value)

#coding=utf-8
class ShortInputException(Exception):
  '''A user-defined exception class.'''
  def __init__(self, length, atleast):
    Exception.__init__(self)
    self.length = length
    self.atleast = atleast
try:
  s = input('Enter something --> ')
  if len(s) < 3:
    #如果输入的内容长度小于3,触发异常
    raise ShortInputException(len(s), 3)
except EOFError:
  print ('\nWhy did you do an EOF on me?')
except ShortInputException as x:
  print ('ShortInputException: The input was of length %d,\
  was expecting at least %d' % (x.length, x.atleast))
else:
  print ('No exception was raised.')

异常抛出机制:
1、如果在运行时发生异常,解释器会查找相应的处理语句(称为handler)。
2、要是在当前函数里没有找到的话,它会将异常传递给上层的调用函数,看看 
那里能不能处理。
3、如果在最外层(全局“main”)还是没有找到的话,解释器就会退出,同时打印
出traceback以便让用户找到错误产生的原因。
注意:
虽然大多数错误会导致异常,但一个异常不一定代表错误,有时候它们只是一个
警告,有时候它们可能是一个终止信号,比如退出循环等