Python：从 multiprocessing.Process 获取回溯

Question

我正在尝试从 multiprocessing.Process 中获取回溯对象。 不幸的是，通过管道传递异常信息不起作用，因为无法腌制回溯对象：

def foo(pipe_to_parent):
    try:
        raise Exception('xxx')
    except:
        pipe_to_parent.send(sys.exc_info())

to_child, to_self = multiprocessing.Pipe()
process = multiprocessing.Process(target = foo, args = (to_self,))
process.start()
exc_info = to_child.recv()
process.join()
print traceback.format_exception(*exc_info)
to_child.close()
to_self.close()

追溯：

Traceback (most recent call last):
  File "/usr/lib/python2.6/multiprocessing/process.py", line 231, in _bootstrap
    self.run()
  File "/usr/lib/python2.6/multiprocessing/process.py", line 88, in run
    self._target(*self._args, **self._kwargs)
  File "foo", line 7, in foo
    to_parent.send(sys.exc_info())
PicklingError: Can't pickle <type 'traceback'>: attribute lookup __builtin__.traceback failed

还有其他方法可以访问异常信息吗？我想避免传递格式化的字符串。

Answer 1

似乎很难使回溯对象可腌制。 但是您只能发送sys.exc_info() 的前2 项，以及使用traceback.extract_tb 方法预先格式化的回溯信息：

import multiprocessing
import sys
import traceback

def foo(pipe_to_parent):
    try:
        raise Exception('xxx')
    except:
        except_type, except_class, tb = sys.exc_info()
        pipe_to_parent.send((except_type, except_class, traceback.extract_tb(tb)))

to_child, to_self = multiprocessing.Pipe()
process = multiprocessing.Process(target = foo, args = (to_self,))
process.start()
exc_info = to_child.recv()
process.join()
print exc_info
to_child.close()
to_self.close()

给你：

(<type 'exceptions.Exception'>, Exception('xxx',), [('test_tb.py', 7, 'foo', "raise Exception('xxx')")])

然后，您将能够获取有关异常原因的更多信息（文件名、引发异常的行号、方法名称和引发异常的语句）

Answer 2

由于multiprocessing 确实打印了子进程中引发的异常的字符串内容，因此您可以将所有子进程代码包装在一个 try-except 中，以捕获任何异常，格式化相关的堆栈跟踪，并引发一个新的 Exception在其字符串中包含所有相关信息：

我与multiprocessing.map 一起使用的函数示例：

def run_functor(functor):
    """
    Given a no-argument functor, run it and return its result. We can 
    use this with multiprocessing.map and map it over a list of job 
    functors to do them.

    Handles getting more than multiprocessing's pitiful exception output
    """

    try:
        # This is where you do your actual work
        return functor()
    except:
        # Put all exception text into an exception and raise that
        raise Exception("".join(traceback.format_exception(*sys.exc_info())))

您得到的是一个堆栈跟踪，其中包含另一个格式化的堆栈跟踪作为错误消息，这有助于调试。

Answer 3

使用tblib，您可以传递包装的异常并在以后重新引发它们：

import tblib.pickling_support
tblib.pickling_support.install()

from multiprocessing import Pool
import sys


class ExceptionWrapper(object):

    def __init__(self, ee):
        self.ee = ee
        __, __, self.tb = sys.exc_info()

    def re_raise(self):
        raise self.ee.with_traceback(self.tb)
        # for Python 2 replace the previous line by:
        # raise self.ee, None, self.tb


# example of how to use ExceptionWrapper

def inverse(i):
    """ will fail for i == 0 """
    try:
        return 1.0 / i
    except Exception as e:
        return ExceptionWrapper(e)


def main():
    p = Pool(1)
    results = p.map(inverse, [0, 1, 2, 3])
    for result in results:
        if isinstance(result, ExceptionWrapper):
            result.re_raise()


if __name__ == "__main__":
    main()

因此，如果您在远程进程中捕获异常，请使用ExceptionWrapper 将其包装起来，然后将其传回。在主进程中调用re_raise() 即可完成工作。

Answer 4

这是this excellent answer 的变体。两者都依赖tblib 来存储回溯。

但是，worker 函数可以保持原样，只需包装在 try/except 中以存储异常对象，而不是返回异常对象（按照 OP 的要求） -加注。

import tblib.pickling_support
tblib.pickling_support.install()

import sys

class DelayedException(Exception):

    def __init__(self, ee):
        self.ee = ee
        __,  __, self.tb = sys.exc_info()
        super(DelayedException, self).__init__(str(ee))

    def re_raise(self):
        raise self.ee, None, self.tb

示例

def worker():
    try:
        raise ValueError('Something went wrong.')
    except Exception as e:
        raise DelayedException(e)


if __name__ == '__main__':

    import multiprocessing

    pool = multiprocessing.Pool()
    try:
        pool.imap(worker, [1, 2, 3])
    except DelayedException as e:
        e.re_raise()

Answer 5

Python 3

在 Python 3 中，现在 multiprocessing.pool.Async 的 get 方法返回完整的回溯，请参阅 http://bugs.python.org/issue13831。

Python 2

使用traceback.format_exc（表示格式化的expetion）来获取回溯字符串。 制作如下装饰器会更方便。

def full_traceback(func):
    import traceback, functools
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        try:
            return func(*args, **kwargs)
        except Exception as e:
            msg = "{}\n\nOriginal {}".format(e, traceback.format_exc())
            raise type(e)(msg)
    return wrapper

例子：

def func0():
    raise NameError("func0 exception")

def func1():
    return func0()

# Key is here!
@full_traceback
def main(i):
    return func1()

if __name__ == '__main__':
    from multiprocessing import Pool
    pool = Pool(4)
    try:
        results = pool.map_async(main, range(5)).get(1e5)
    finally:
        pool.close()
        pool.join()

带有装饰器的回溯：

Traceback (most recent call last):
  File "bt.py", line 34, in <module>
    results = pool.map_async(main, range(5)).get(1e5)
  File "/opt/anaconda/lib/python2.7/multiprocessing/pool.py", line 567, in get
    raise self._value
NameError: Exception in func0

Original Traceback (most recent call last):
  File "bt.py", line 13, in wrapper
    return func(*args, **kwargs)
  File "bt.py", line 27, in main
    return func1()
  File "bt.py", line 23, in func1
    return func0()
  File "bt.py", line 20, in func0
    raise NameError("Exception in func0")
NameError: Exception in func0

没有装饰器的回溯：

Traceback (most recent call last):
  File "bt.py", line 34, in <module>
    results = pool.map_async(main, range(5)).get(1e5)
  File "/opt/anaconda/lib/python2.7/multiprocessing/pool.py", line 567, in get
    raise self._value
NameError: Exception in func0

Answer 6

与@Syrtis Major 和@interfect 相同的解决方案，但使用Python 3.6 测试：

import sys
import traceback
import functools

def catch_remote_exceptions(wrapped_function):
    """ https://stackoverflow.com/questions/6126007/python-getting-a-traceback """

    @functools.wraps(wrapped_function)
    def new_function(*args, **kwargs):
        try:
            return wrapped_function(*args, **kwargs)

        except:
            raise Exception( "".join(traceback.format_exception(*sys.exc_info())) )

    return new_function

用法：

class ProcessLocker(object):
    @catch_remote_exceptions
    def __init__(self):
        super().__init__()

    @catch_remote_exceptions
    def create_process_locks(self, total_processes):
        self.process_locks = []
        # ...