Python代码调试之异常回溯

当发生异常时，Python会回溯异常，给出大量的提示，可能会给程序员的定位和纠错带来一定的困难，这时可以使用sys模块的exc_info()函数来回溯最近一次异常。

sys.exc_info( )的返回值tuple是一个三元组(type,  value,  traceback)，其中：

• type —— 异常的类型
• value —— 异常的信息或者参数
• traceback —— 包含调用栈信息的对象

例如：

>>> 1/0
Traceback (most recent call last):
  File "<pyshell#25>", line 1, in <module>
    1/0
ZeroDivisionError: integer division or modulo by zero
>>> import sys
>>> try:
  1/0
except:
  r = sys.exc_info()
  print(r)
(<class 'ZeroDivisionError'>, ZeroDivisionError('division by zero',), <traceback object at 0x000000000375C788>)

sys.exc_info()可以直接定位最终引发异常的原因，结果也比较简洁，但是缺点是难以直接确定引发异常的代码位置。假设有如下函数定义：

>>> def A():1/0 
>>> def B():A() 
>>> def C():B()

直接调用函数，抛出异常：

>>> C()
Traceback (most recent call last):
  File "<pyshell#35>", line 1, in <module>
    C()
  File "<pyshell#34>", line 2, in C
    B()
  File "<pyshell#31>", line 2, in B
    A()
  File "<pyshell#28>", line 2, in A
    1/0
ZeroDivisionError: integer division or modulo by zero

使用sys.exc_info()查看异常信息时并不是非常直观：

>>> try:
         C()
except:
         r = sys.exc_info()
         print(r)
 
(<type 'exceptions.ZeroDivisionError'>, ZeroDivisionError('integer division or modulo by zero',), <traceback object at 0x0134C990>)

如果需要的话，可以使用traceback模块来查看详细信息：

>>> import traceback
>>> import sys
>>> def A():1/0
>>> def B():A()
>>> def C():B()
>>> try:
    C()
except:
    excType, excValue, excTraceback = sys.exc_info()
    traceback.print_exception(excType, excValue,
                              excTraceback, limit=3)
    print(excValue)
    traceback.print_tb(excTraceback)
Traceback (most recent call last):
  File "<pyshell#44>", line 2, in <module>
  File "<pyshell#42>", line 1, in C
  File "<pyshell#40>", line 1, in B
ZeroDivisionError: division by zero
division by zero
  File "<pyshell#44>", line 2, in <module>
  File "<pyshell#42>", line 1, in C
  File "<pyshell#40>", line 1, in B
  File "<pyshell#38>", line 1, in A