python快速学习6

语法错误和异常

异常处理

try语句

try和except组合使用

>>> try:
...     f = open('E:\test_for_filefun1.txt','rb+') 
...     s = f.readline()
... except IOError as e:
...     print "I/O error({0}):{1}".format(e.errno, e.strerror)
... except ValueError:
...     print "could not convert data to int"
... except :
...     print "Unexpected error:", sys.exc_info()[0]
...     
I/O error(2):No such file or directory

else
try except 和else的组合使用
else必须放在所有的except之后.当try语句中没有抛出异常时,就会执行else中的代码

>>> try:
...     f = open('E:\test_for_filefun.txt','rb+') 
... except IOError as e:
...     print "I/O error({0}):{1}".format(e.errno, e.strerror)
... except :
...     print "Unexpected error:", sys.exc_info()[0]
... else:
...     f.write(str(ip))
...     f.close()
...     

raise引发的异常

raise为引发一个异常
常用 raise ValueError

>>> raise NameError('song')
Traceback (most recent call last):
  File "D:PyCharm Community Edition 2017.1.3helperspydev\_pydevd_bundlepydevd_exec.py", line 3, in Exec
    exec exp in global_vars, local_vars
  File "<input>", line 1, in <module>
NameError: song

自定义的异常Exception

可以通过派生Exception类来创建自定义的异常类型

>>> class MyError(Exception):
...     def __init__(self,value):
...         self.value = value
...     def __str__(self):
...         return repr(self.value)
...     
>>> raise MyError('good')
Traceback (most recent call last):
  File "D:PyCharm Community Edition 2017.1.3helperspydev\_pydevd_bundlepydevd_exec.py", line 3, in Exec
    exec exp in global_vars, local_vars
  File "<input>", line 1, in <module>
MyError: 'good'

标准的做法

• 为Exception定义一个异常基类
• 然后根据这个基类Error派生出对应的子类

class Error(Exception):
    """Base class for exceptions in this module."""
    pass

class InputError(Error):
    """Exception raised for errors in the input.

    Attributes:
        expression -- input expression in which the error occurred
        message -- explanation of the error
    """

    def __init__(self, expression, message):
        self.expression = expression
        self.message = message

class TransitionError(Error):
    """Raised when an operation attempts a state transition that's not
    allowed.

    Attributes:
        previous -- state at beginning of transition
        next -- attempted new state
        message -- explanation of why the specific transition is not allowed
    """

    def __init__(self, previous, next, message):
        self.previous = previous
        self.next = next
        self.message = message

finally语句

无论什么情况下都会执行
常用语释放外部的资源(文件,网络连接...)

>>> def test_for_error(x, y):
...     try:
...         result = x/y
...     except ZeroDivisionError:
...         print 'y is 0 ,division zero error'
...     else:
...         print result
...     finally:
...         print 'excute the finally clause'
...         
>>> test_for_error(3,3)
1
excute the finally clause
>>> test_for_error(3,0)
y is 0 ,division zero error
excute the finally clause