1.2.8. Python 中的异常处理

如果您键入了本教程中之前的所有命令，则很可能已经引发了异常。例如，如果您输入的命令有错别字，则可能会引发异常。

异常是由执行 Python 代码时出现的各种错误引发的。在您自己的代码中，您也可以捕获错误或定义自定义错误类型。在查找正确的异常类型时，您可能需要查看内置异常的描述。

1.2.8.1. 异常

异常是由 Python 中的错误引发的

In [1]: 1/0
---------------------------------------------------------------------------
ZeroDivisionError                         Traceback (most recent call last)
Cell In[1], line 1
----> 1 1/0

ZeroDivisionError: division by zero

In [2]: 1 + 'e'
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[2], line 1
----> 1 1 + 'e'

TypeError: unsupported operand type(s) for +: 'int' and 'str'

In [3]: d = {1:1, 2:2}

In [4]: d[3]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
Cell In[4], line 1
----> 1 d[3]

KeyError: 3

In [5]: l = [1, 2, 3]

In [6]: l[4]
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[6], line 1
----> 1 l[4]

IndexError: list index out of range

In [7]: l.foobar
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[7], line 1
----> 1 l.foobar

AttributeError: 'list' object has no attribute 'foobar'

如您所见，针对不同的错误，存在**不同类型**的异常。

1.2.8.2. 捕获异常

try/except

In [8]: while True:
   ...:     try:
   ...:         x = int(input('Please enter a number: '))
   ...:         break
   ...:     except ValueError:
   ...:         print('That was no valid number.  Try again...')
   ...: 
Please enter a number: a
That was no valid number.  Try again...
Please enter a number: 1

In [9]: x
Out[9]: 1

try/finally

In [10]: try:
   ....:     x = int(input('Please enter a number: '))
   ....: finally:
   ....:     print('Thank you for your input')
   ....: 
Please enter a number: a
Thank you for your input
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[10], line 2
      1 try:
----> 2     x = int(input('Please enter a number: '))
      3 finally:
      4     print('Thank you for your input')
ValueError: invalid literal for int() with base 10: 'a'

对于资源管理（例如关闭文件）非常重要

寻求宽恕比寻求许可更容易

In [11]: def print_sorted(collection):
   ....:     try:
   ....:         collection.sort()
   ....:     except AttributeError:
   ....:         pass # The pass statement does nothing
   ....:     print(collection)
   ....: 

In [12]: print_sorted([1, 3, 2])
[1, 2, 3]

In [13]: print_sorted(set((1, 3, 2)))
{1, 2, 3}

In [14]: print_sorted('132')
132

1.2.8.3. 引发异常

• 捕获和重新引发异常

  In [15]: def filter_name(name):
     ....:    try:
     ....:        name = name.encode('ascii')
     ....:    except UnicodeError as e:
     ....:        if name == 'Gaël':
     ....:            print('OK, Gaël')
     ....:        else:
     ....:            raise e
     ....:    return name
     ....: 
  
  In [16]: filter_name('Gaël')
  OK, Gaël
  Out[16]: 'Gaël'
  
  In [17]: filter_name('Stéfan')
  ---------------------------------------------------------------------------
  UnicodeEncodeError                        Traceback (most recent call last)
  Cell In[17], line 1
  ----> 1 filter_name('Stéfan')
  
  Cell In[15], line 8, in filter_name(name)
        6         print('OK, Gaël')
        7     else:
  ----> 8         raise e
        9 return name
  
  Cell In[15], line 3, in filter_name(name)
        1 def filter_name(name):
        2    try:
  ----> 3        name = name.encode('ascii')
        4    except UnicodeError as e:
        5        if name == 'Gaël':
  
  UnicodeEncodeError: 'ascii' codec can't encode character '\xe9' in position 2: ordinal not in range(128)
  

• 异常用于在代码的不同部分之间传递消息

  In [18]: def achilles_arrow(x):
     ....:    if abs(x - 1) < 1e-3:
     ....:        raise StopIteration
     ....:    x = 1 - (1-x)/2.
     ....:    return x
     ....: 
  
  In [19]: x = 0
  
  In [20]: while True:
     ....:     try:
     ....:         x = achilles_arrow(x)
     ....:     except StopIteration:
     ....:         break
     ....: 
     ....: 
  
  In [21]: x
  Out[21]: 0.9990234375
  

使用异常来通知某些条件是否满足（例如 StopIteration）或不满足（例如自定义错误引发）