介绍一个自己造的轮子，Python2异常链。
需求
习惯了用java撸码，虽说胶水代码多，但能比较好的用代码表达思路；而Python则简洁到了简陋的地步——各种鸡肋的语法糖，各种不完善的机制。比如错误处理。
Python2没有异常链，让问题排查变得非常困难：
# coding=utf-8import sysclass UnexpectedError(StandardError):
passdef divide(division, divided):
if division == 0: raise ValueError("illegal input: %s, %s" % (division, divided))
ans = division / divided return ans
a = 0b = 0try: print divide(a, b)except ValueError as e: # m_counter.inc("err", 1)
raise UnexpectedError("illegal input: %s, %s" % (a, b))except ZeroDivisionError as e: # m_counter.inc("err", 1)
raise UnexpectedError("divide by zero")except StandardError as e: # m_counter.inc("err", 1)
raise UnexpectedError("other error...")打印异常如下：
Traceback (most recent call last):
File "/Users/monkeysayhi/PycharmProjects/Wheel/utils/tmp/tmp.py", line 22, in <module>
raise UnexpectedError("illegal input: %s, %s" % (a, b))
__main__.UnexpectedError: illegal input: 0, 0不考虑代码风格，是标准的Python2异常处理方式：分别捕获异常，再统一成一个异常，只有msg不同，重新抛出。这种写法又丑又冗余，顶多可以改成这样：
try: print divide(a, b)except StandardError as e: # m_counter.inc("err", 1)
raise UnexpectedError(e.message)即便如此，也无法解决一个最严重的问题：明明是11行抛出的异常，但打印出来的异常栈却只能追踪到22行重新抛出异常的raise语句。重点在于没有记录cause，使我们追踪到22行之后，不知道为什么会抛出cause，也就无法定位到实际发生问题的代码。
异常链
最理想的方式，还是在异常栈中打印异常链：
try: print divide(a, b)except StandardError as cause: # m_counter.inc("err", 1)
raise UnexpectedError("some msg", cause)就像Java的异常栈，区分“要抛出的异常UnexpectedError和引起该异常的原因cause”：
java.lang.RuntimeException: level 2 exception at com.msh.demo.exceptionStack.Test.fun2(Test.java:17) at com.msh.demo.exceptionStack.Test.main(Test.java:24) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)Caused by: java.io.IOException: level 1 exception at com.msh.demo.exceptionStack.Test.fun1(Test.java:10) at com.msh.demo.exceptionStack.Test.fun2(Test.java:15) ... 6 more
上述异常栈表示，RuntimeException由IOException导致；1行与9行下是各异常的调用路径trace。不熟悉Java异常栈的可参考你真的会阅读Java的异常信息吗？。
轮子
调研让我们拒绝重复造轮子
Python3已经支持了异常链，通过from关键字即可记录cause。
Python2 future包提供的所谓异常链raise_from我是完全没明白到哪里打印了cause:
from future.utils import raise_fromclass DatabaseError(Exception):
passclass FileDatabase:
def __init__(self, filename):
try:
self.file = open(filename) except IOError as exc:
raise_from(DatabaseError('failed to open'), exc)# testfd = FileDatabase('non_existent_file.txt')那么，11行抛出的IOError呢？？？似乎仅仅多了一句无效信息（future包里的raise e）。
Traceback (most recent call last):
File "/Users/mobkeysayhi/PycharmProjects/Wheel/utils/tmp/tmp.py", line 17, in <module>
fd = FileDatabase('non_existent_file.txt')
File "/Users/mobkeysayhi/PycharmProjects/Wheel/utils/tmp/tmp.py", line 13, in __init__
raise_from(DatabaseError('failed to open'), exc)
File "/Library/Python/2.7/site-packages/future/utils/__init__.py", line 454, in raise_from
raise e
__main__.DatabaseError: failed to open有知道正确姿势的求点破。
没找到重复轮子真是极好的
非常简单：
import tracebackclass TracedError(BaseException):
def __init__(self, msg="", cause=None):
trace_msg = msg if cause is not None:
_spfile = SimpleFile()
traceback.print_exc(file=_spfile)
_cause_tm = _spfile.read()
trace_msg += "\n" \
+ "\nCaused by:\n\n" \
+ _cause_tm
super(TracedError, self).__init__(trace_msg)class ErrorWrapper(TracedError):
def __init__(self, cause):
super(ErrorWrapper, self).__init__("Just wrapping cause", cause)class SimpleFile(object):
def __init__(self, ):
super(SimpleFile, self).__init__()
self.buffer = ""
def write(self, str):
self.buffer += str def read(self):
return self.buffer目前只支持单线程模型，github上有doc和测试用例，戳我戳我。
一个测试输出如下：
Traceback (most recent call last):
File "/Users/monkeysayhi/PycharmProjects/Wheel/utils/exception_chain/traced_error.py", line 68, in <module>
__test()
File "/Users/monkeysayhi/PycharmProjects/Wheel/utils/exception_chain/traced_error.py", line 64, in __test
raise MyError("test MyError", e)
__main__.MyError: test MyError
Caused by:Traceback (most recent call last):
File "/Users/monkeysayhi/PycharmProjects/Wheel/utils/exception_chain/traced_error.py", line 62, in __test
zero_division()
File "/Users/monkeysayhi/PycharmProjects/Wheel/utils/exception_chain/traced_error.py", line 58, in zero_division
a = 1 / 0ZeroDivisionError: integer division or modulo by zero另外，为方便处理后重新抛出某些异常，还提供了ErrorWrapper，仅接收一个cause作为参数。用法如下：
for pid in pids: # process might have died before getting to this line
# so wrap to avoid OSError: no such process
try:
os.kill(pid, signal.SIGKILL) except OSError as os_e: if os.path.isdir("/proc/%d" % int(pid)):
logging.warn("Timeout but fail to kill process, still exist: %d, " % int(pid)) raise ErrorWrapper(os_e)
logging.debug("Timeout but no need to kill process, already no such process: %d" % int(pid))参考：
作者：猴子007
链接：https://www.jianshu.com/p/2b291f69b1d4
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