python的异常处理

可能触发异常产生的代码会放到try语句块里，而处理异常的代码会在except语句块里实现。例如：

try:
    file = open('test.txt', 'rb')
except IOError as e:
    print('An IOError occurred. {}'.format(e.args[-1]))

我们可以使用三种方法来处理多个异常。

第一种方法需要把所有可能发生的异常放到一个元组里。像这样：

try:
    file = open('test.txt', 'rb')
except (IOError, EOFError) as e:
    print("An error occurred. {}".format(e.args[-1]))

另外一种方式是对每个单独的异常在单独的except语句块中处理。我们想要多少个except语句块都可以：

try:
    file = open('test.txt', 'rb')
except EOFError as e:
    print("An EOF error occurred.")
    raise e
except IOError as e:
    print("An error occurred.")
    raise e

最后一种方式会捕获所有异常：

try:
    file = open('test.txt', 'rb')
except Exception as e:
    # Some logging if you want
    raise e

注意，捕获所有异常可能会造成意外的结果，比如，通常我们使用CTRL+C来终止程序，但如果程序中捕获了所有异常，CTRL+C就无法终止程序了。

包裹到finally从句中的代码不管异常是否触发都将会被执行。这可以被用来在脚本执行之后做清理工作：

try:
    file = open('test.txt', 'rb')
except IOError as e:
    print('An IOError occurred. {}'.format(e.args[-1]))
finally:
    print("This would be printed whether or not an exception occurred!")

# Output: An IOError occurred. No such file or directory
# This would be printed whether or not an exception occurred!

如果想在没有触发异常的时候执行一些代码，可以使用else从句。

有人也许问了：如果你只是想让一些代码在没有触发异常的情况下执行，为啥你不直接把代码放在try里面呢？回答是，那样的话这段代码中的任意异常都还是会被try捕获，而你并不一定想要那样。

try:
    print('I am sure no exception is going to occur!')
except Exception:
    print('exception')
else:
    # any code that should only run if no exception occurs in the try,
    # but for which exceptions should NOT be caught
    print('This would only run if no exception occurs. And an error here '
          'would NOT be caught.')
finally:
    print('This would be printed in every case.')

# Output: I am sure no exception is going to occur!
# This would only run if no exception occurs. And an error here would NOT be caught
# This would be printed in every case.

else从句只会在没有异常的情况下执行，而且它会在finally语句之前执行。