标签：python

在编程的过程中，总是无法避免处理异常的情况，这些异常也有可能是错误，也有可能是一些偶现的异常，Python中提供了处理这些情况的有效机制。

语法错误

语法错误，即解析错误，也许会是你学习Python的过程中遇到最多的错误：

>>> while True print('Hello world')
  File "<stdin>", line 1, in ?
    while True print('Hello world')
                   ^
SyntaxError: invalid syntax

异常

即使语句在语法上是正确的，当执行它的时候任然可能引起一个错误。执行期间的错误叫做异常，后面将讲述在Python中怎么处理异常。下面展示了一些异常的信息：

>>> 10 * (1/0)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
ZeroDivisionError: division by zero
>>> 4 + spam*3
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: name 'spam' is not defined
>>> '2' + 2
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: Can't convert 'int' object to str implicitly

处理异常

可以在代码中对指定的异常做出处理，看下面的例子，请求用户输入直到获取到一个有效的整数，但允许用户中断程序（Control-C或者其它操作系统支持的），用户中断程序通常会导致KeyboardInterrupt异常：

>>> while True:
	try:
		x = int(input("Please enter a number: "))
		break
	except ValueError:
		print("Oops!  That was no valid number.  Try again...")

import sys

try:
    f = open('myfile.txt')
    s = f.readline()
    i = int(s.strip())
except OSError as err:
    print("OS error: {0}".format(err))
except ValueError:
    print("Could not convert data to an integer.")
except:
    print("Unexpected error:", sys.exc_info()[0])
    raise

for arg in sys.argv[1:]:
    try:
        f = open(arg, 'r')
    except IOError:
        print('cannot open', arg)
    else:
        print(arg, 'has', len(f.readlines()), 'lines')
        f.close()

>>> try:
		raise Exception('spam', 'eggs')
	except Exception as inst:
		print(type(inst))    # the exception instance
		print(inst.args)     # arguments stored in .args
		print(inst)          # __str__ allows args to be printed directly,
							 # but may be overridden in exception subclasses
		x, y = inst.args     # unpack args
		print('x =', x)

<class 'Exception'>
('spam', 'eggs')
('spam', 'eggs')
x = spam

>>> def this_fails():
        x = 1/0
   
>>> try:
        this_fails()
    except ZeroDivisionError as err:
        print('Handling run-time error:', err)
   
Handling run-time error: int division or modulo by zero

raise子句

使用raise可以抛出一个指定的异常，例如：

>>> raise NameError('HiThere')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
NameError: HiThere

>>> try:
        raise NameError('HiThere')
    except NameError:
        print('An exception flew by!')
        raise
   
An exception flew by!
Traceback (most recent call last):
  File "<stdin>", line 2, in ?
NameError: HiThere

用户自定义异常

如果内建的异常无法满足你的要求，则需要创建你自己的异常类。自定义的异常类应该是Exception的子类（直接或者间接）。例如

>>> class MyError(Exception):
        def __init__(self, value):
            self.value = value
        def __str__(self):
            return repr(self.value)
   
>>> try:
        raise MyError(2*2)
    except MyError as e:
        print('My exception occurred, value:', e.value)
 
My exception occurred, value: 4
>>> raise MyError('oops!')
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
__main__.MyError: 'oops!'

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

有另一个可选的子句是finally子句，用于定义在任何情况下都会执行的代码，通常用于做清理操作，例如：

>>> try:
        raise KeyboardInterrupt
    finally:
        print('Goodbye, world!')
   
Goodbye, world!
KeyboardInterrupt

>>> def divide(x, y):
        try:
            result = x / y
        except ZeroDivisionError:
            print("division by zero!")
        else:
            print("result is", result)
        finally:
            print("executing finally clause")
 
>>> divide(2, 1)
result is 2.0
executing finally clause
>>> divide(2, 0)
division by zero!
executing finally clause
>>> divide("2", "1")
executing finally clause
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "<stdin>", line 3, in divide
TypeError: unsupported operand type(s) for /: 'str' and 'str'

预定义清理行为

一些对象定义标准的清理行为，当对象不再被需要时调用，不关心使用对象的操作是否成功。看下面的例子，尝试打开一个文件并打印它的内容：

for line in open("myfile.txt"):
    print(line, end="")

with open("myfile.txt") as f:
    for line in f:
        print(line, end="")

python14：错误和异常

标签：python

原文地址：http://blog.csdn.net/tomato__/article/details/45557583