traceback：让你更加灵活地处理python的异常

异常

异常在python中是屡见不鲜了，程序在执行到某一行代码时，发现有问题，比如数组索引越界，变量没有定义啊等等，此时就会抛出异常

---

捕获异常

在python，一般都是使用try···except来对异常进行捕获

try:
    1 / 0
except Exception as e:
    print(e)  # division by zero

然而仅仅只有这些也看不出什么东西来，我们需要知道在哪一行代码引发的异常。

大家在程序报错的时候，会经常看到报错信息如下

Traceback (most recent call last):

这个Traceback是什么鬼？实际上，这是python关于程序报错的回溯信息，来自于一个叫做traceback object的对象，而这个traceback object对象是通过sys.exc_info()来获取的

---

traceback对象

import sys

try:
    1 / 0
except Exception as e:
    print(e)  # division by zero
    exc_type, exc_value, exc_tb = sys.exc_info()
    print(exc_type)  # <class 'ZeroDivisionError'>
    print(exc_value)  # division by zero
    print(exc_tb)  # <traceback object at 0x0000000009F484C8>

可以看到，sys.exc_info()获取了当前处理的exception的相关信息，并返回一个元组。元组的第一个元素是异常的类型，第二个元素是异常的value值，第三个异常信息则是traceback object。print(e)打印的是异常的值。

有了traceback object我们则可以打印和格式化traceback的相关信息

---

traceback模块

---

print_tb

接收一个tracebackobject

import sys
import traceback

try:
    1 / 0
except NameError as e:
    exc_type, exc_value, exc_tb = sys.exc_info()
    traceback.print_tb(exc_tb)
"""
File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 6, in <module>
    1 / 0
"""

# 如果我们不捕获异常看看输出啥？
"""
Traceback (most recent call last):
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 6, in <module>
    1 / 0
ZeroDivisionError: division by zero
"""
# 可以看到最后一行的ZeroDivisionError则是异常类型，division by zero则是异常值。中间的则是我们的traceback object

然而除了traceback object，print_tb还可以接收两个参数

• limit

  比如我们在调用C函数出现了异常，但我们是先调用A函数，在A函数里面调用B函数，在B函数里面调用C函数，limit参数则是限制stack trace的层级的，如果为None也就是不指定，那么会打印所有层级

  import sys
  import traceback
  
  def C():
      1 / 0
  
  def B():
      C()
  
  def A():
      B()
  
  try:
      A()
  except Exception as e:
      exc_type, exc_value, exc_tb = sys.exc_info()
      traceback.print_tb(exc_tb)
      """
        File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 16, in <module>
          A()
        File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 12, in A
          B()
        File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 9, in B
          C()
        File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 6, in C
          1 / 0
      """
  
      traceback.print_tb(exc_tb, limit=2)
      """
        File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 16, in <module>
          A()
        File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 12, in A
          B()
      """
  

• file

  可以指定file，输出到某个文件里，默认是sys.stderr

  ---

print_exception

与print_tb相比多了两个参数，需要传入exc_type,exc_value,exc_tb，也就是sys.exc_info返回的三个值。与print_tb相比，打印信息多了开头的Traceback (most recent call last):，以及最后一行的异常类型和value信息。还有一个不同是当异常为SyntaxError时，会有"^"来指示语法错误的位置

import sys
import traceback

def C():
    1 / 0

def B():
    C()

def A():
    B()

try:
    A()
except Exception as e:
    exc_type, exc_value, exc_tb = sys.exc_info()
    traceback.print_exception(exc_type, exc_value, exc_tb)
"""
Traceback (most recent call last):
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 16, in <module>
    A()
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 12, in A
    B()
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 9, in B
    C()
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 6, in C
    1 / 0
ZeroDivisionError: division by zero
"""

可以看到，打印的结果和报错的信息是一样的。

---

print_exc

和print_exception类似，只不过不需要我们手动的传入sys.exc_info返回的三个值，而是会自动帮我们调用

import sys
import traceback

def C():
    1 / 0

def B():
    C()

def A():
    B()

try:
    A()
except Exception as e:
    traceback.print_exc()
"""
Traceback (most recent call last):
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 16, in <module>
    A()
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 12, in A
    B()
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 9, in B
    C()
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 6, in C
    1 / 0
ZeroDivisionError: division by zero
"""

---

format_exc

和print_exc一样，只不过是以字符串的形式返回，需要我们自己手动打印

import sys
import traceback

def C():
    1 / 0

def B():
    C()

def A():
    B()

try:
    A()
except Exception as e:
    tb_info = traceback.format_exc()
    print(tb_info)
"""
Traceback (most recent call last):
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 16, in <module>
    A()
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 12, in A
    B()
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 9, in B
    C()
  File "C:/Users/EDZ/Desktop/satori/wtfpython/1.py", line 6, in C
    1 / 0
ZeroDivisionError: division by zero
"""