Python进阶03 异常

编写计算机程序时，总会碰到一些异常事件。

如果在每个可能发生这些事件的地方都使用条件语句，不仅效率低下、缺乏灵活性，还可能导致程序的可读性比较差。

好在Python提供了功能强大的异常处理机制。

一、异常是什么

Python使用异常对象来表示异常状态，并在遇到错误时引发异常。异常对象未被处理或捕获时，程序将终止并显示一条错误信息。

>>> 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero

二、捕获异常

异常比较有趣的地方是可对其进行处理，通常称之为捕获异常。

1、使用try/except语句捕获异常

try:
    1/0
except ZeroDivisionError:
    print('ZeroDivisionError happened.')

结果：
ZeroDivisionError happened.

try:
    x = int(input('Enter the first number: '))
    y = int(input('Enter the second number: '))
    print(x / y)
except ZeroDivisionError:
    print("The second number can't be zero!")

示例：
Enter the first number: 1
Enter the second number: 0
The second number can't be zero

如果在这里并没有捕获异常，异常将向程序的最顶层传播。

2、多个except子句

针对不同的异常类型，可以使用多个except子句进行捕获。

try:
    x = int(input('Enter the first number: '))
    y = int(input('Enter the second number: '))
    print(x / y)
except ZeroDivisionError:
    print("The second number can't be zero!")
except ValueError:
    print("That wasn't a number, was it?")

结果：
Enter the first number: 1
Enter the second number: 0
The second number can't be zero!

Enter the first number: 1
Enter the second number: a
That wasn't a number, was it?

也可在一个元组中指定这些异常：

try:
    x = int(input('Enter the first number: '))
    y = int(input('Enter the second number: '))
    print(x / y)
except (ZeroDivisionError, ValueError):
    print("Yous numbers wrong!")

3、捕获对象

可以在except字句中访问异常对象本身，同时向用户显示具体错误信息。

try:
    x = int(input('Enter the first number: '))
    y = int(input('Enter the second number: '))
    print(x / y)
except (ZeroDivisionError, ValueError) as e:
    print(e)

4、捕获所有异常

如果要使用一段代码捕获所有的异常，只需在except字句中不指定任何异常类即可。

try:
    x = int(input('Enter the first number: '))
    y = int(input('Enter the second number: '))
    print(x / y)
except:
    print('Something wrong happened......')

像这样捕获所有的异常很危险，因为很有可能会隐藏你没有考虑过的错误。

更好的选择是使用except Exception as e并对异常对象进行检查。这样做将忽略不是从Exception派生而来的为数不多的异常，如SystemExit和KeyboardInterrupt，因为他们是从BaseException(Exception的父类)派生而来的。

5、else子句

try/except语句可以添加一个else子句，用来在没有出现异常时执行一个代码块。

try:
    print('A simple task')
except:
    print('What? Something went wrong?')
else:
    print('Ah ... It went as planned.')

结果：
A simple task
Ah ... It went as planned

使用else子句，可实现特定条件下的循环操作：

while True:
    try:
        x = int(input('Enter the first number: '))
        y = int(input('Enter the second number: '))
        print(x / y)
    except Exception as e:
        print('Invalid input: ', e)
        print('Please try again......')
    else:
        break

结果：
Enter the first number: 1
Enter the second number: 0
Invalid input:  division by zero
Please try again......
Enter the first number: 1
Enter the second number: a
Invalid input:  invalid literal for int() with base 10: 'a'
Please try again......
Enter the first number: 1
Enter the second number: 2
0.5

6、finally子句

finally子句可用于执行清理工作，无论是否发生异常。

while True:
    try:
        x = int(input('Enter the first number: '))
        y = int(input('Enter the second number: '))
        print(x / y)
    except Exception as e:
        print('Invalid input: ', e)
        print('Please try again......')
    else:
        break
    finally:
        print('Cleaning up ...')

结果：
Enter the first number: 1
Enter the second number: 0
Invalid input:  division by zero
Please try again......
Cleaning up ...
Enter the first number: 1
Enter the second number: a
Invalid input:  invalid literal for int() with base 10: 'a'
Please try again......
Cleaning up ...
Enter the first number: 1
Enter the second number: 2
0.5
Cleaning up ..

7、raise

捕获异常后，如果要继续向上传播，可调用内置函数raise且不提供任何参数。

try:
    1/0
except ZeroDivisionError:
    raise ValueError

结果：
ZeroDivisionError: division by zero

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Volumes/DATA/python/python_test/test.py", line 371, in <module>
    raise ValueError
ValueError

可使用raise A from B 语句来提供自己的异常上下文，也可使用None来禁用上下文。

try:
    1/0
except ZeroDivisionError as e:
    raise ValueError from e

结果：
ZeroDivisionError: division by zero

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/Volumes/DATA/python/python_test/test.py", line 424, in <module>
    raise ValueError from e
ValueError

try:
    1/0
except ZeroDivisionError:
    raise ValueError from None

结果：
    raise ValueError from None
ValueError

三、警告信息

如果你只是想发出警告而不是异常信息，可使用模块warnings中的函数warn。

>>> from warnings import warn
>>> warn('Something will be wrong.')
__main__:1: UserWarning: Something will be wrong.

警告只会显示一次。