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Python thread class wont exit

I want to exit the crawler script which is using threading, but it doesn't detect Ctrl+c. I tried to try and except inside the main thread and in newly created threads, but it doesn't raise the exception, signal in main thread, but main thread isn't active, and it doesn't detect it.
import threading
import signal
class Crawler(threading.Thread):
def __init__(self, num):
threading.Thread.__init__(self)
# Other stuff
def run(self, ):
try:
self.crawl()
except KeyboardInterrupt:
self.exit()
def crawl(self):
for url in self.links_list:
try:
# Getting the website and all links on it and adding it to links_list
pass
except (KeyboardInterrupt, SystemExit):
self.exit()
except AssertionError:
self.exit()
def exit(self):
# Saving progress
raise SystemExit()
def handler(signum, frame):
exit()
if __name__ == "__main__":
signal.signal(signal.SIGINT, handler)
for i in range(8):
crawler = Crawler(i)
crawler.start()

Instead of trying to do stuff with signals, use a threading.Event() to signal all of the other threads to stop:
import threading
import time
stop_signal = threading.Event()
class Crawler(threading.Thread):
def __init__(self, num):
threading.Thread.__init__(self)
# Other stuff
def run(self):
try:
for url in self.links_list:
# Between every url, see if we should stop
if stop_signal.is_set():
break
# Getting the website and all links on it and adding it to links_list
finally:
self.save_progress()
def save_progress(self):
pass
def main():
crawlers = [Crawler(x) for x in range(8)]
for crawler in crawlers:
crawler.start()
# Do nothing for a whole long while in the main loop
# (you could exit the loop when all jobs are done, though)
while True:
try:
time.sleep(1)
except KeyboardInterrupt:
stop_signal.set()
break
for crawler in crawlers:
crawler.join()
if __name__ == "__main__":
main()

Can an exception or error in the worker thread be caught in a function in the main thread in PyQT5? [duplicate]

I'm very new to Python and multithreaded programming in general. Basically, I have a script that will copy files to another location. I would like this to be placed in another thread so I can output .... to indicate that the script is still running.
The problem that I am having is that if the files cannot be copied it will throw an exception. This is OK if running in the main thread; however, having the following code does not work:
try:
threadClass = TheThread(param1, param2, etc.)
threadClass.start() ##### **Exception takes place here**
except:
print "Caught an exception"
In the thread class itself, I tried to re-throw the exception, but it does not work. I have seen people on here ask similar questions, but they all seem to be doing something more specific than what I am trying to do (and I don't quite understand the solutions offered). I have seen people mention the usage of sys.exc_info(), however I do not know where or how to use it.
Edit: The code for the thread class is below:
class TheThread(threading.Thread):
def __init__(self, sourceFolder, destFolder):
threading.Thread.__init__(self)
self.sourceFolder = sourceFolder
self.destFolder = destFolder
def run(self):
try:
shul.copytree(self.sourceFolder, self.destFolder)
except:
raise

The problem is that thread_obj.start() returns immediately. The child thread that you spawned executes in its own context, with its own stack. Any exception that occurs there is in the context of the child thread, and it is in its own stack. One way I can think of right now to communicate this information to the parent thread is by using some sort of message passing, so you might look into that.
Try this on for size:
import sys
import threading
import queue
class ExcThread(threading.Thread):
def __init__(self, bucket):
threading.Thread.__init__(self)
self.bucket = bucket
def run(self):
try:
raise Exception('An error occured here.')
except Exception:
self.bucket.put(sys.exc_info())
def main():
bucket = queue.Queue()
thread_obj = ExcThread(bucket)
thread_obj.start()
while True:
try:
exc = bucket.get(block=False)
except queue.Empty:
pass
else:
exc_type, exc_obj, exc_trace = exc
# deal with the exception
print exc_type, exc_obj
print exc_trace
thread_obj.join(0.1)
if thread_obj.isAlive():
continue
else:
break
if __name__ == '__main__':
main()

There are a lot of really weirdly complicated answers to this question. Am I oversimplifying this, because this seems sufficient for most things to me.
from threading import Thread
class PropagatingThread(Thread):
def run(self):
self.exc = None
try:
if hasattr(self, '_Thread__target'):
# Thread uses name mangling prior to Python 3.
self.ret = self._Thread__target(*self._Thread__args, **self._Thread__kwargs)
else:
self.ret = self._target(*self._args, **self._kwargs)
except BaseException as e:
self.exc = e
def join(self, timeout=None):
super(PropagatingThread, self).join(timeout)
if self.exc:
raise self.exc
return self.ret
If you're certain you'll only ever be running on one or the other version of Python, you could reduce the run() method down to just the mangled version (if you'll only be running on versions of Python before 3), or just the clean version (if you'll only be running on versions of Python starting with 3).
Example usage:
def f(*args, **kwargs):
print(args)
print(kwargs)
raise Exception('I suck at this')
t = PropagatingThread(target=f, args=(5,), kwargs={'hello':'world'})
t.start()
t.join()
And you'll see the exception raised on the other thread when you join.
If you are using six or on Python 3 only, you can improve the stack trace information you get when the exception is re-raised. Instead of only the stack at the point of the join, you can wrap the inner exception in a new outer exception, and get both stack traces with
six.raise_from(RuntimeError('Exception in thread'),self.exc)
or
raise RuntimeError('Exception in thread') from self.exc

The concurrent.futures module makes it simple to do work in separate threads (or processes) and handle any resulting exceptions:
import concurrent.futures
import shutil
def copytree_with_dots(src_path, dst_path):
with concurrent.futures.ThreadPoolExecutor(max_workers=1) as executor:
# Execute the copy on a separate thread,
# creating a future object to track progress.
future = executor.submit(shutil.copytree, src_path, dst_path)
while future.running():
# Print pretty dots here.
pass
# Return the value returned by shutil.copytree(), None.
# Raise any exceptions raised during the copy process.
return future.result()
concurrent.futures is included with Python 3.2, and is available as the backported futures module for earlier versions.

Although it is not possible to directly catch an exception thrown in a different thread, here's a code to quite transparently obtain something very close to this functionality. Your child thread must subclass the ExThread class instead of threading.Thread and the parent thread must call the child_thread.join_with_exception() method instead of child_thread.join() when waiting for the thread to finish its job.
Technical details of this implementation: when the child thread throws an exception, it is passed to the parent through a Queue and thrown again in the parent thread. Notice that there's no busy waiting in this approach .
#!/usr/bin/env python
import sys
import threading
import Queue
class ExThread(threading.Thread):
def __init__(self):
threading.Thread.__init__(self)
self.__status_queue = Queue.Queue()
def run_with_exception(self):
"""This method should be overriden."""
raise NotImplementedError
def run(self):
"""This method should NOT be overriden."""
try:
self.run_with_exception()
except BaseException:
self.__status_queue.put(sys.exc_info())
self.__status_queue.put(None)
def wait_for_exc_info(self):
return self.__status_queue.get()
def join_with_exception(self):
ex_info = self.wait_for_exc_info()
if ex_info is None:
return
else:
raise ex_info[1]
class MyException(Exception):
pass
class MyThread(ExThread):
def __init__(self):
ExThread.__init__(self)
def run_with_exception(self):
thread_name = threading.current_thread().name
raise MyException("An error in thread '{}'.".format(thread_name))
def main():
t = MyThread()
t.start()
try:
t.join_with_exception()
except MyException as ex:
thread_name = threading.current_thread().name
print "Caught a MyException in thread '{}': {}".format(thread_name, ex)
if __name__ == '__main__':
main()

If an exception occurs in a thread, the best way is to re-raise it in the caller thread during join. You can get information about the exception currently being handled using the sys.exc_info() function. This information can simply be stored as a property of the thread object until join is called, at which point it can be re-raised.
Note that a Queue.Queue (as suggested in other answers) is not necessary in this simple case where the thread throws at most 1 exception and completes right after throwing an exception. We avoid race conditions by simply waiting for the thread to complete.
For example, extend ExcThread (below), overriding excRun (instead of run).
Python 2.x:
import threading
class ExcThread(threading.Thread):
def excRun(self):
pass
def run(self):
self.exc = None
try:
# Possibly throws an exception
self.excRun()
except:
import sys
self.exc = sys.exc_info()
# Save details of the exception thrown but don't rethrow,
# just complete the function
def join(self):
threading.Thread.join(self)
if self.exc:
msg = "Thread '%s' threw an exception: %s" % (self.getName(), self.exc[1])
new_exc = Exception(msg)
raise new_exc.__class__, new_exc, self.exc[2]
Python 3.x:
The 3 argument form for raise is gone in Python 3, so change the last line to:
raise new_exc.with_traceback(self.exc[2])

concurrent.futures.as_completed
https://docs.python.org/3.7/library/concurrent.futures.html#concurrent.futures.as_completed
The following solution:
returns to the main thread immediately when an exception is called
requires no extra user defined classes because it does not need:
an explicit Queue
to add an except else around your work thread
Source:
#!/usr/bin/env python3
import concurrent.futures
import time
def func_that_raises(do_raise):
for i in range(3):
print(i)
time.sleep(0.1)
if do_raise:
raise Exception()
for i in range(3):
print(i)
time.sleep(0.1)
with concurrent.futures.ThreadPoolExecutor(max_workers=2) as executor:
futures = []
futures.append(executor.submit(func_that_raises, False))
futures.append(executor.submit(func_that_raises, True))
for future in concurrent.futures.as_completed(futures):
print(repr(future.exception()))
Possible output:
0
0
1
1
2
2
0
Exception()
1
2
None
It is unfortunately not possible to kill futures to cancel the others as one fails:
concurrent.futures; Python: concurrent.futures How to make it cancelable?
threading: Is there any way to kill a Thread?
C pthreads: Kill Thread in Pthread Library
If you do something like:
for future in concurrent.futures.as_completed(futures):
if future.exception() is not None:
raise future.exception()
then the with catches it, and waits for the second thread to finish before continuing. The following behaves similarly:
for future in concurrent.futures.as_completed(futures):
future.result()
since future.result() re-raises the exception if one occurred.
If you want to quit the entire Python process, you might get away with os._exit(0), but this likely means you need a refactor.
Custom class with perfect exception semantics
I ended up coding the perfect interface for myself at: The right way to limit maximum number of threads running at once? section "Queue example with error handling". That class aims to be both convenient, and give you total control over submission and result / error handling.
Tested on Python 3.6.7, Ubuntu 18.04.

In Python 3.8, we can use threading.excepthook to hook the uncaught exceptions in all the child threads! For example,
threading.excepthook = thread_exception_handler
Referer: https://stackoverflow.com/a/60002752/5093308

What im doing is, simple overriding join and run method of the Thread:
class RaisingThread(threading.Thread):
def run(self):
self._exc = None
try:
super().run()
except Exception as e:
self._exc = e
def join(self, timeout=None):
super().join(timeout=timeout)
if self._exc:
raise self._exc
Used as followed:
def foo():
time.sleep(2)
print('hi, from foo!')
raise Exception('exception from foo')
t = RaisingThread(target=foo)
t.start()
try:
t.join()
except Exception as e:
print(e)
Result:
hi, from foo!
exception from foo!

This was a nasty little problem, and I'd like to throw my solution in. Some other solutions I found (async.io for example) looked promising but also presented a bit of a black box. The queue / event loop approach sort of ties you to a certain implementation. The concurrent futures source code, however, is around only 1000 lines, and easy to comprehend. It allowed me to easily solve my problem: create ad-hoc worker threads without much setup, and to be able to catch exceptions in the main thread.
My solution uses the concurrent futures API and threading API. It allows you to create a worker which gives you both the thread and the future. That way, you can join the thread to wait for the result:
worker = Worker(test)
thread = worker.start()
thread.join()
print(worker.future.result())
...or you can let the worker just send a callback when done:
worker = Worker(test)
thread = worker.start(lambda x: print('callback', x))
...or you can loop until the event completes:
worker = Worker(test)
thread = worker.start()
while True:
print("waiting")
if worker.future.done():
exc = worker.future.exception()
print('exception?', exc)
result = worker.future.result()
print('result', result)
break
time.sleep(0.25)
Here's the code:
from concurrent.futures import Future
import threading
import time
class Worker(object):
def __init__(self, fn, args=()):
self.future = Future()
self._fn = fn
self._args = args
def start(self, cb=None):
self._cb = cb
self.future.set_running_or_notify_cancel()
thread = threading.Thread(target=self.run, args=())
thread.daemon = True #this will continue thread execution after the main thread runs out of code - you can still ctrl + c or kill the process
thread.start()
return thread
def run(self):
try:
self.future.set_result(self._fn(*self._args))
except BaseException as e:
self.future.set_exception(e)
if(self._cb):
self._cb(self.future.result())
...and the test function:
def test(*args):
print('args are', args)
time.sleep(2)
raise Exception('foo')

I know I'm a bit late to the party here but I was having a very similar problem but it included using tkinter as a GUI, and the mainloop made it impossible to use any of the solutions that depend on .join(). Therefore I adapted the solution given in the EDIT of the original question, but made it more general to make it easier to understand for others.
Here is the new thread class in action:
import threading
import traceback
import logging
class ExceptionThread(threading.Thread):
def __init__(self, *args, **kwargs):
threading.Thread.__init__(self, *args, **kwargs)
def run(self):
try:
if self._target:
self._target(*self._args, **self._kwargs)
except Exception:
logging.error(traceback.format_exc())
def test_function_1(input):
raise IndexError(input)
if __name__ == "__main__":
input = 'useful'
t1 = ExceptionThread(target=test_function_1, args=[input])
t1.start()
Of course you can always have it handle the exception some other way from logging, such as printing it out, or having it output to the console.
This allows you to use the ExceptionThread class exactly like you would the Thread class, without any special modifications.

Similar way like RickardSjogren's without Queue, sys etc. but also without some listeners to signals: execute directly an exception handler which corresponds to an except block.
#!/usr/bin/env python3
import threading
class ExceptionThread(threading.Thread):
def __init__(self, callback=None, *args, **kwargs):
"""
Redirect exceptions of thread to an exception handler.
:param callback: function to handle occured exception
:type callback: function(thread, exception)
:param args: arguments for threading.Thread()
:type args: tuple
:param kwargs: keyword arguments for threading.Thread()
:type kwargs: dict
"""
self._callback = callback
super().__init__(*args, **kwargs)
def run(self):
try:
if self._target:
self._target(*self._args, **self._kwargs)
except BaseException as e:
if self._callback is None:
raise e
else:
self._callback(self, e)
finally:
# Avoid a refcycle if the thread is running a function with
# an argument that has a member that points to the thread.
del self._target, self._args, self._kwargs, self._callback
Only self._callback and the except block in run() is additional to normal threading.Thread.

I use this version, it's minimal and it works well.
class SafeThread(threading.Thread):
def __init__(self, *args, **kwargs):
super(SafeThread, self).__init__(*args, **kwargs)
self.exception = None
def run(self) -> None:
try:
super(SafeThread, self).run()
except Exception as ex:
self.exception = ex
traceback.print_exc()
def join(self, *args, **kwargs) -> None:
super(SafeThread, self).join(*args, **kwargs)
if self.exception:
raise self.exception
To use it, simply replace threading.Thread with SafeThread e.g
t = SafeThread(target = some_function, args = (some, args,))
t.start()
# do something else here if you want as the thread runs in the background
t.join()

As a noobie to Threading, it took me a long time to understand how to implement Mateusz Kobos's code (above). Here's a clarified version to help understand how to use it.
#!/usr/bin/env python
import sys
import threading
import Queue
class ExThread(threading.Thread):
def __init__(self):
threading.Thread.__init__(self)
self.__status_queue = Queue.Queue()
def run_with_exception(self):
"""This method should be overriden."""
raise NotImplementedError
def run(self):
"""This method should NOT be overriden."""
try:
self.run_with_exception()
except Exception:
self.__status_queue.put(sys.exc_info())
self.__status_queue.put(None)
def wait_for_exc_info(self):
return self.__status_queue.get()
def join_with_exception(self):
ex_info = self.wait_for_exc_info()
if ex_info is None:
return
else:
raise ex_info[1]
class MyException(Exception):
pass
class MyThread(ExThread):
def __init__(self):
ExThread.__init__(self)
# This overrides the "run_with_exception" from class "ExThread"
# Note, this is where the actual thread to be run lives. The thread
# to be run could also call a method or be passed in as an object
def run_with_exception(self):
# Code will function until the int
print "sleeping 5 seconds"
import time
for i in 1, 2, 3, 4, 5:
print i
time.sleep(1)
# Thread should break here
int("str")
# I'm honestly not sure why these appear here? So, I removed them.
# Perhaps Mateusz can clarify?
# thread_name = threading.current_thread().name
# raise MyException("An error in thread '{}'.".format(thread_name))
if __name__ == '__main__':
# The code lives in MyThread in this example. So creating the MyThread
# object set the code to be run (but does not start it yet)
t = MyThread()
# This actually starts the thread
t.start()
print
print ("Notice 't.start()' is considered to have completed, although"
" the countdown continues in its new thread. So you code "
"can tinue into new processing.")
# Now that the thread is running, the join allows for monitoring of it
try:
t.join_with_exception()
# should be able to be replace "Exception" with specific error (untested)
except Exception, e:
print
print "Exceptioon was caught and control passed back to the main thread"
print "Do some handling here...or raise a custom exception "
thread_name = threading.current_thread().name
e = ("Caught a MyException in thread: '" +
str(thread_name) +
"' [" + str(e) + "]")
raise Exception(e) # Or custom class of exception, such as MyException

One method I am fond of is based on the observer pattern. I define a signal class which my thread uses to emit exceptions to listeners. It can also be used to return values from threads. Example:
import threading
class Signal:
def __init__(self):
self._subscribers = list()
def emit(self, *args, **kwargs):
for func in self._subscribers:
func(*args, **kwargs)
def connect(self, func):
self._subscribers.append(func)
def disconnect(self, func):
try:
self._subscribers.remove(func)
except ValueError:
raise ValueError('Function {0} not removed from {1}'.format(func, self))
class WorkerThread(threading.Thread):
def __init__(self, *args, **kwargs):
super(WorkerThread, self).__init__(*args, **kwargs)
self.Exception = Signal()
self.Result = Signal()
def run(self):
if self._Thread__target is not None:
try:
self._return_value = self._Thread__target(*self._Thread__args, **self._Thread__kwargs)
except Exception as e:
self.Exception.emit(e)
else:
self.Result.emit(self._return_value)
if __name__ == '__main__':
import time
def handle_exception(exc):
print exc.message
def handle_result(res):
print res
def a():
time.sleep(1)
raise IOError('a failed')
def b():
time.sleep(2)
return 'b returns'
t = WorkerThread(target=a)
t2 = WorkerThread(target=b)
t.Exception.connect(handle_exception)
t2.Result.connect(handle_result)
t.start()
t2.start()
print 'Threads started'
t.join()
t2.join()
print 'Done'
I do not have enough experience of working with threads to claim that this is a completely safe method. But it has worked for me and I like the flexibility.

A simple way of catching thread's exception and communicating back to the caller method could be by passing dictionary or a list to worker method.
Example (passing dictionary to worker method):
import threading
def my_method(throw_me):
raise Exception(throw_me)
def worker(shared_obj, *args, **kwargs):
try:
shared_obj['target'](*args, **kwargs)
except Exception as err:
shared_obj['err'] = err
shared_obj = {'err':'', 'target': my_method}
throw_me = "Test"
th = threading.Thread(target=worker, args=(shared_obj, throw_me), kwargs={})
th.start()
th.join()
if shared_obj['err']:
print(">>%s" % shared_obj['err'])

Wrap Thread with exception storage.
import threading
import sys
class ExcThread(threading.Thread):
def __init__(self, target, args = None):
self.args = args if args else []
self.target = target
self.exc = None
threading.Thread.__init__(self)
def run(self):
try:
self.target(*self.args)
raise Exception('An error occured here.')
except Exception:
self.exc=sys.exc_info()
def main():
def hello(name):
print(!"Hello, {name}!")
thread_obj = ExcThread(target=hello, args=("Jack"))
thread_obj.start()
thread_obj.join()
exc = thread_obj.exc
if exc:
exc_type, exc_obj, exc_trace = exc
print(exc_type, ':',exc_obj, ":", exc_trace)
main()

I like this class:
https://gist.github.com/earonesty/b88d60cb256b71443e42c4f1d949163e
import threading
from typing import Any
class PropagatingThread(threading.Thread):
"""A Threading Class that raises errors it caught, and returns the return value of the target on join."""
def __init__(self, *args, **kwargs):
self._target = None
self._args = ()
self._kwargs = {}
super().__init__(*args, **kwargs)
self.exception = None
self.return_value = None
assert self._target
def run(self):
"""Don't override this if you want the behavior of this class, use target instead."""
try:
if self._target:
self.return_value = self._target(*self._args, **self._kwargs)
except Exception as e:
self.exception = e
finally:
# see super().run() for why this is necessary
del self._target, self._args, self._kwargs
def join(self, timeout=None) -> Any:
super().join(timeout)
if self.exception:
raise self.exception
return self.return_value

Using naked excepts is not a good practice because you usually catch more than you bargain for.
I would suggest modifying the except to catch ONLY the exception that you would like to handle. I don't think that raising it has the desired effect, because when you go to instantiate TheThread in the outer try, if it raises an exception, the assignment is never going to happen.
Instead you might want to just alert on it and move on, such as:
def run(self):
try:
shul.copytree(self.sourceFolder, self.destFolder)
except OSError, err:
print err
Then when that exception is caught, you can handle it there. Then when the outer try catches an exception from TheThread, you know it won't be the one you already handled, and will help you isolate your process flow.

I think the other solutions are somewhat complex if the only thing you want is to actually see somewhere the exception instead of being oblivious and totally blind.
The solution is to create a custom Thread that takes a logger from the main thread and logs any exceptions.
class ThreadWithLoggedException(threading.Thread):
"""
Similar to Thread but will log exceptions to passed logger.
Args:
logger: Logger instance used to log any exception in child thread
Exception is also reachable via <thread>.exception from the main thread.
"""
def __init__(self, *args, **kwargs):
try:
self.logger = kwargs.pop("logger")
except KeyError:
raise Exception("Missing 'logger' in kwargs")
super().__init__(*args, **kwargs)
self.exception = None
def run(self):
try:
if self._target is not None:
self._target(*self._args, **self._kwargs)
except Exception as exception:
thread = threading.current_thread()
self.exception = exception
self.logger.exception(f"Exception in child thread {thread}: {exception}")
finally:
del self._target, self._args, self._kwargs
Example:
logger = logging.getLogger(__name__)
logger.setLevel(logging.INFO)
logger.addHandler(logging.StreamHandler())
def serve():
raise Exception("Earth exploded.")
th = ThreadWithLoggedException(target=serve, logger=logger)
th.start()
Output in main thread:
Exception in child thread <ThreadWithLoggedException(Thread-1, started 139922384414464)>: Earth exploded.
Traceback (most recent call last):
File "/core/utils.py", line 108, in run
self._target(*self._args, **self._kwargs)
File "/myapp.py", line 105, in serve
raise Exception("Earth exploded.")
Exception: Earth exploded.

pygolang provides sync.WorkGroup which, in particular, propagates exception from spawned worker threads to the main thread. For example:
#!/usr/bin/env python
"""This program demostrates how with sync.WorkGroup an exception raised in
spawned thread is propagated into main thread which spawned the worker."""
from __future__ import print_function
from golang import sync, context
def T1(ctx, *argv):
print('T1: run ... %r' % (argv,))
raise RuntimeError('T1: problem')
def T2(ctx):
print('T2: ran ok')
def main():
wg = sync.WorkGroup(context.background())
wg.go(T1, [1,2,3])
wg.go(T2)
try:
wg.wait()
except Exception as e:
print('Tmain: caught exception: %r\n' %e)
# reraising to see full traceback
raise
if __name__ == '__main__':
main()
gives the following when run:
T1: run ... ([1, 2, 3],)
T2: ran ok
Tmain: caught exception: RuntimeError('T1: problem',)
Traceback (most recent call last):
File "./x.py", line 28, in <module>
main()
File "./x.py", line 21, in main
wg.wait()
File "golang/_sync.pyx", line 198, in golang._sync.PyWorkGroup.wait
pyerr_reraise(pyerr)
File "golang/_sync.pyx", line 178, in golang._sync.PyWorkGroup.go.pyrunf
f(pywg._pyctx, *argv, **kw)
File "./x.py", line 10, in T1
raise RuntimeError('T1: problem')
RuntimeError: T1: problem
The original code from the question would be just:
wg = sync.WorkGroup(context.background())
def _(ctx):
shul.copytree(sourceFolder, destFolder)
wg.go(_)
# waits for spawned worker to complete and, on error, reraises
# its exception on the main thread.
wg.wait()

Python 2.7: Child thread not catching KeyboardInterrupt

import sys
import time
import threading
class exThread(threading.Thread):
def __init__(self, threadId):
threading.Thread.__init__(self)
self.threadId = threadId
def run(self):
try:
while 1:
pass
except KeyboardInterrupt:
print "Ctrl-C caught by thread"
finally:
print "Thread's finally clause being executed"
sys.exit() # Same as thread.exit()
cond = True
def func():
pass
try:
th = exThread(1)
th.start()
while True:
if cond:
func()
except KeyboardInterrupt:
print "Ctrl-C caught by main thread"
sys.exit(0)
finally:
print "Executing the finally clause from main thread"
On executing above code, when I press Ctrl-C, the main thread exits after printing from its finally clause. Now, since the child thread is a non-daemon, it is still running in the try: except KeyboardInterrupt block. However, this child thread doesn't seem to respond to Ctrl-C, even though it is supposed to catch KeyboardInterrupt exception. Why?

As far as I know, KeyboardInterrup is only raised in the main thread. To be able to stop the other threads, make them check for an event from time to time, and then exit voluntarily.
Another option is to mark the child thread as daemon=True so that it will auto-terminate when the main thread terminates:
th.daemon = True
Additional reading:
How daemon threads work Daemon Threads Explanation
But also, daemon threads can be harmful: http://joeshaw.org/2009/02/24/605/

Multiple subprocesses with timeouts

I'm using a recipe that relies on SIGALRM to set alarm interrupt --
Using module 'subprocess' with timeout
The problem is that I have more than one Python script using signal.ALARM process to set time-outs, and only the latest alarm gets called. What is a good way to improve this multiple Python functions setting time-outs?

Except for simple, quick hacks, avoid SIGALRM. It's a very old, limited mechanism, not suited to anything more complex: you can only set a single alarm, and it interrupts any system call at the time rather than just the one you intend to interrupt.
It's much cleaner to use a timeout thread to kill the process, for example:
import subprocess, signal, os, threading, errno
from contextlib import contextmanager
class TimeoutThread(object):
def __init__(self, seconds):
self.seconds = seconds
self.cond = threading.Condition()
self.cancelled = False
self.thread = threading.Thread(target=self._wait)
def run(self):
"""Begin the timeout."""
self.thread.start()
def _wait(self):
with self.cond:
self.cond.wait(self.seconds)
if not self.cancelled:
self.timed_out()
def cancel(self):
"""Cancel the timeout, if it hasn't yet occured."""
with self.cond:
self.cancelled = True
self.cond.notify()
self.thread.join()
def timed_out(self):
"""The timeout has expired."""
raise NotImplementedError
class KillProcessThread(TimeoutThread):
def __init__(self, seconds, pid):
super(KillProcessThread, self).__init__(seconds)
self.pid = pid
def timed_out(self):
try:
os.kill(self.pid, signal.SIGKILL)
except OSError as e:
# If the process is already gone, ignore the error.
if e.errno not in (errno.EPERM, errno. ESRCH):
raise e
#contextmanager
def processTimeout(seconds, pid):
timeout = KillProcessThread(seconds, pid)
timeout.run()
try:
yield
finally:
timeout.cancel()
def example():
proc = subprocess.Popen(["sleep", "5"], stdin=subprocess.PIPE, stdout=subprocess.PIPE)
with processTimeout(1, proc.pid):
print proc.communicate()
resultcode = proc.wait()
if resultcode < 0:
print "error: %i" % resultcode
if __name__ == '__main__':
example()
Depending on what you're timing out, you may want to use a lighter signal than SIGKILL to allow the timing-out process to clean up after itself.

Catch a thread's exception in the caller thread?

I'm very new to Python and multithreaded programming in general. Basically, I have a script that will copy files to another location. I would like this to be placed in another thread so I can output .... to indicate that the script is still running.
The problem that I am having is that if the files cannot be copied it will throw an exception. This is OK if running in the main thread; however, having the following code does not work:
try:
threadClass = TheThread(param1, param2, etc.)
threadClass.start() ##### **Exception takes place here**
except:
print "Caught an exception"
In the thread class itself, I tried to re-throw the exception, but it does not work. I have seen people on here ask similar questions, but they all seem to be doing something more specific than what I am trying to do (and I don't quite understand the solutions offered). I have seen people mention the usage of sys.exc_info(), however I do not know where or how to use it.
Edit: The code for the thread class is below:
class TheThread(threading.Thread):
def __init__(self, sourceFolder, destFolder):
threading.Thread.__init__(self)
self.sourceFolder = sourceFolder
self.destFolder = destFolder
def run(self):
try:
shul.copytree(self.sourceFolder, self.destFolder)
except:
raise

The problem is that thread_obj.start() returns immediately. The child thread that you spawned executes in its own context, with its own stack. Any exception that occurs there is in the context of the child thread, and it is in its own stack. One way I can think of right now to communicate this information to the parent thread is by using some sort of message passing, so you might look into that.
Try this on for size:
import sys
import threading
import queue
class ExcThread(threading.Thread):
def __init__(self, bucket):
threading.Thread.__init__(self)
self.bucket = bucket
def run(self):
try:
raise Exception('An error occured here.')
except Exception:
self.bucket.put(sys.exc_info())
def main():
bucket = queue.Queue()
thread_obj = ExcThread(bucket)
thread_obj.start()
while True:
try:
exc = bucket.get(block=False)
except queue.Empty:
pass
else:
exc_type, exc_obj, exc_trace = exc
# deal with the exception
print exc_type, exc_obj
print exc_trace
thread_obj.join(0.1)
if thread_obj.isAlive():
continue
else:
break
if __name__ == '__main__':
main()

There are a lot of really weirdly complicated answers to this question. Am I oversimplifying this, because this seems sufficient for most things to me.
from threading import Thread
class PropagatingThread(Thread):
def run(self):
self.exc = None
try:
if hasattr(self, '_Thread__target'):
# Thread uses name mangling prior to Python 3.
self.ret = self._Thread__target(*self._Thread__args, **self._Thread__kwargs)
else:
self.ret = self._target(*self._args, **self._kwargs)
except BaseException as e:
self.exc = e
def join(self, timeout=None):
super(PropagatingThread, self).join(timeout)
if self.exc:
raise self.exc
return self.ret
If you're certain you'll only ever be running on one or the other version of Python, you could reduce the run() method down to just the mangled version (if you'll only be running on versions of Python before 3), or just the clean version (if you'll only be running on versions of Python starting with 3).
Example usage:
def f(*args, **kwargs):
print(args)
print(kwargs)
raise Exception('I suck at this')
t = PropagatingThread(target=f, args=(5,), kwargs={'hello':'world'})
t.start()
t.join()
And you'll see the exception raised on the other thread when you join.
If you are using six or on Python 3 only, you can improve the stack trace information you get when the exception is re-raised. Instead of only the stack at the point of the join, you can wrap the inner exception in a new outer exception, and get both stack traces with
six.raise_from(RuntimeError('Exception in thread'),self.exc)
or
raise RuntimeError('Exception in thread') from self.exc

The concurrent.futures module makes it simple to do work in separate threads (or processes) and handle any resulting exceptions:
import concurrent.futures
import shutil
def copytree_with_dots(src_path, dst_path):
with concurrent.futures.ThreadPoolExecutor(max_workers=1) as executor:
# Execute the copy on a separate thread,
# creating a future object to track progress.
future = executor.submit(shutil.copytree, src_path, dst_path)
while future.running():
# Print pretty dots here.
pass
# Return the value returned by shutil.copytree(), None.
# Raise any exceptions raised during the copy process.
return future.result()
concurrent.futures is included with Python 3.2, and is available as the backported futures module for earlier versions.

Although it is not possible to directly catch an exception thrown in a different thread, here's a code to quite transparently obtain something very close to this functionality. Your child thread must subclass the ExThread class instead of threading.Thread and the parent thread must call the child_thread.join_with_exception() method instead of child_thread.join() when waiting for the thread to finish its job.
Technical details of this implementation: when the child thread throws an exception, it is passed to the parent through a Queue and thrown again in the parent thread. Notice that there's no busy waiting in this approach .
#!/usr/bin/env python
import sys
import threading
import Queue
class ExThread(threading.Thread):
def __init__(self):
threading.Thread.__init__(self)
self.__status_queue = Queue.Queue()
def run_with_exception(self):
"""This method should be overriden."""
raise NotImplementedError
def run(self):
"""This method should NOT be overriden."""
try:
self.run_with_exception()
except BaseException:
self.__status_queue.put(sys.exc_info())
self.__status_queue.put(None)
def wait_for_exc_info(self):
return self.__status_queue.get()
def join_with_exception(self):
ex_info = self.wait_for_exc_info()
if ex_info is None:
return
else:
raise ex_info[1]
class MyException(Exception):
pass
class MyThread(ExThread):
def __init__(self):
ExThread.__init__(self)
def run_with_exception(self):
thread_name = threading.current_thread().name
raise MyException("An error in thread '{}'.".format(thread_name))
def main():
t = MyThread()
t.start()
try:
t.join_with_exception()
except MyException as ex:
thread_name = threading.current_thread().name
print "Caught a MyException in thread '{}': {}".format(thread_name, ex)
if __name__ == '__main__':
main()

If an exception occurs in a thread, the best way is to re-raise it in the caller thread during join. You can get information about the exception currently being handled using the sys.exc_info() function. This information can simply be stored as a property of the thread object until join is called, at which point it can be re-raised.
Note that a Queue.Queue (as suggested in other answers) is not necessary in this simple case where the thread throws at most 1 exception and completes right after throwing an exception. We avoid race conditions by simply waiting for the thread to complete.
For example, extend ExcThread (below), overriding excRun (instead of run).
Python 2.x:
import threading
class ExcThread(threading.Thread):
def excRun(self):
pass
def run(self):
self.exc = None
try:
# Possibly throws an exception
self.excRun()
except:
import sys
self.exc = sys.exc_info()
# Save details of the exception thrown but don't rethrow,
# just complete the function
def join(self):
threading.Thread.join(self)
if self.exc:
msg = "Thread '%s' threw an exception: %s" % (self.getName(), self.exc[1])
new_exc = Exception(msg)
raise new_exc.__class__, new_exc, self.exc[2]
Python 3.x:
The 3 argument form for raise is gone in Python 3, so change the last line to:
raise new_exc.with_traceback(self.exc[2])

concurrent.futures.as_completed
https://docs.python.org/3.7/library/concurrent.futures.html#concurrent.futures.as_completed
The following solution:
returns to the main thread immediately when an exception is called
requires no extra user defined classes because it does not need:
an explicit Queue
to add an except else around your work thread
Source:
#!/usr/bin/env python3
import concurrent.futures
import time
def func_that_raises(do_raise):
for i in range(3):
print(i)
time.sleep(0.1)
if do_raise:
raise Exception()
for i in range(3):
print(i)
time.sleep(0.1)
with concurrent.futures.ThreadPoolExecutor(max_workers=2) as executor:
futures = []
futures.append(executor.submit(func_that_raises, False))
futures.append(executor.submit(func_that_raises, True))
for future in concurrent.futures.as_completed(futures):
print(repr(future.exception()))
Possible output:
0
0
1
1
2
2
0
Exception()
1
2
None
It is unfortunately not possible to kill futures to cancel the others as one fails:
concurrent.futures; Python: concurrent.futures How to make it cancelable?
threading: Is there any way to kill a Thread?
C pthreads: Kill Thread in Pthread Library
If you do something like:
for future in concurrent.futures.as_completed(futures):
if future.exception() is not None:
raise future.exception()
then the with catches it, and waits for the second thread to finish before continuing. The following behaves similarly:
for future in concurrent.futures.as_completed(futures):
future.result()
since future.result() re-raises the exception if one occurred.
If you want to quit the entire Python process, you might get away with os._exit(0), but this likely means you need a refactor.
Custom class with perfect exception semantics
I ended up coding the perfect interface for myself at: The right way to limit maximum number of threads running at once? section "Queue example with error handling". That class aims to be both convenient, and give you total control over submission and result / error handling.
Tested on Python 3.6.7, Ubuntu 18.04.

In Python 3.8, we can use threading.excepthook to hook the uncaught exceptions in all the child threads! For example,
threading.excepthook = thread_exception_handler
Referer: https://stackoverflow.com/a/60002752/5093308

What im doing is, simple overriding join and run method of the Thread:
class RaisingThread(threading.Thread):
def run(self):
self._exc = None
try:
super().run()
except Exception as e:
self._exc = e
def join(self, timeout=None):
super().join(timeout=timeout)
if self._exc:
raise self._exc
Used as followed:
def foo():
time.sleep(2)
print('hi, from foo!')
raise Exception('exception from foo')
t = RaisingThread(target=foo)
t.start()
try:
t.join()
except Exception as e:
print(e)
Result:
hi, from foo!
exception from foo!

This was a nasty little problem, and I'd like to throw my solution in. Some other solutions I found (async.io for example) looked promising but also presented a bit of a black box. The queue / event loop approach sort of ties you to a certain implementation. The concurrent futures source code, however, is around only 1000 lines, and easy to comprehend. It allowed me to easily solve my problem: create ad-hoc worker threads without much setup, and to be able to catch exceptions in the main thread.
My solution uses the concurrent futures API and threading API. It allows you to create a worker which gives you both the thread and the future. That way, you can join the thread to wait for the result:
worker = Worker(test)
thread = worker.start()
thread.join()
print(worker.future.result())
...or you can let the worker just send a callback when done:
worker = Worker(test)
thread = worker.start(lambda x: print('callback', x))
...or you can loop until the event completes:
worker = Worker(test)
thread = worker.start()
while True:
print("waiting")
if worker.future.done():
exc = worker.future.exception()
print('exception?', exc)
result = worker.future.result()
print('result', result)
break
time.sleep(0.25)
Here's the code:
from concurrent.futures import Future
import threading
import time
class Worker(object):
def __init__(self, fn, args=()):
self.future = Future()
self._fn = fn
self._args = args
def start(self, cb=None):
self._cb = cb
self.future.set_running_or_notify_cancel()
thread = threading.Thread(target=self.run, args=())
thread.daemon = True #this will continue thread execution after the main thread runs out of code - you can still ctrl + c or kill the process
thread.start()
return thread
def run(self):
try:
self.future.set_result(self._fn(*self._args))
except BaseException as e:
self.future.set_exception(e)
if(self._cb):
self._cb(self.future.result())
...and the test function:
def test(*args):
print('args are', args)
time.sleep(2)
raise Exception('foo')

I know I'm a bit late to the party here but I was having a very similar problem but it included using tkinter as a GUI, and the mainloop made it impossible to use any of the solutions that depend on .join(). Therefore I adapted the solution given in the EDIT of the original question, but made it more general to make it easier to understand for others.
Here is the new thread class in action:
import threading
import traceback
import logging
class ExceptionThread(threading.Thread):
def __init__(self, *args, **kwargs):
threading.Thread.__init__(self, *args, **kwargs)
def run(self):
try:
if self._target:
self._target(*self._args, **self._kwargs)
except Exception:
logging.error(traceback.format_exc())
def test_function_1(input):
raise IndexError(input)
if __name__ == "__main__":
input = 'useful'
t1 = ExceptionThread(target=test_function_1, args=[input])
t1.start()
Of course you can always have it handle the exception some other way from logging, such as printing it out, or having it output to the console.
This allows you to use the ExceptionThread class exactly like you would the Thread class, without any special modifications.

Similar way like RickardSjogren's without Queue, sys etc. but also without some listeners to signals: execute directly an exception handler which corresponds to an except block.
#!/usr/bin/env python3
import threading
class ExceptionThread(threading.Thread):
def __init__(self, callback=None, *args, **kwargs):
"""
Redirect exceptions of thread to an exception handler.
:param callback: function to handle occured exception
:type callback: function(thread, exception)
:param args: arguments for threading.Thread()
:type args: tuple
:param kwargs: keyword arguments for threading.Thread()
:type kwargs: dict
"""
self._callback = callback
super().__init__(*args, **kwargs)
def run(self):
try:
if self._target:
self._target(*self._args, **self._kwargs)
except BaseException as e:
if self._callback is None:
raise e
else:
self._callback(self, e)
finally:
# Avoid a refcycle if the thread is running a function with
# an argument that has a member that points to the thread.
del self._target, self._args, self._kwargs, self._callback
Only self._callback and the except block in run() is additional to normal threading.Thread.

I use this version, it's minimal and it works well.
class SafeThread(threading.Thread):
def __init__(self, *args, **kwargs):
super(SafeThread, self).__init__(*args, **kwargs)
self.exception = None
def run(self) -> None:
try:
super(SafeThread, self).run()
except Exception as ex:
self.exception = ex
traceback.print_exc()
def join(self, *args, **kwargs) -> None:
super(SafeThread, self).join(*args, **kwargs)
if self.exception:
raise self.exception
To use it, simply replace threading.Thread with SafeThread e.g
t = SafeThread(target = some_function, args = (some, args,))
t.start()
# do something else here if you want as the thread runs in the background
t.join()

As a noobie to Threading, it took me a long time to understand how to implement Mateusz Kobos's code (above). Here's a clarified version to help understand how to use it.
#!/usr/bin/env python
import sys
import threading
import Queue
class ExThread(threading.Thread):
def __init__(self):
threading.Thread.__init__(self)
self.__status_queue = Queue.Queue()
def run_with_exception(self):
"""This method should be overriden."""
raise NotImplementedError
def run(self):
"""This method should NOT be overriden."""
try:
self.run_with_exception()
except Exception:
self.__status_queue.put(sys.exc_info())
self.__status_queue.put(None)
def wait_for_exc_info(self):
return self.__status_queue.get()
def join_with_exception(self):
ex_info = self.wait_for_exc_info()
if ex_info is None:
return
else:
raise ex_info[1]
class MyException(Exception):
pass
class MyThread(ExThread):
def __init__(self):
ExThread.__init__(self)
# This overrides the "run_with_exception" from class "ExThread"
# Note, this is where the actual thread to be run lives. The thread
# to be run could also call a method or be passed in as an object
def run_with_exception(self):
# Code will function until the int
print "sleeping 5 seconds"
import time
for i in 1, 2, 3, 4, 5:
print i
time.sleep(1)
# Thread should break here
int("str")
# I'm honestly not sure why these appear here? So, I removed them.
# Perhaps Mateusz can clarify?
# thread_name = threading.current_thread().name
# raise MyException("An error in thread '{}'.".format(thread_name))
if __name__ == '__main__':
# The code lives in MyThread in this example. So creating the MyThread
# object set the code to be run (but does not start it yet)
t = MyThread()
# This actually starts the thread
t.start()
print
print ("Notice 't.start()' is considered to have completed, although"
" the countdown continues in its new thread. So you code "
"can tinue into new processing.")
# Now that the thread is running, the join allows for monitoring of it
try:
t.join_with_exception()
# should be able to be replace "Exception" with specific error (untested)
except Exception, e:
print
print "Exceptioon was caught and control passed back to the main thread"
print "Do some handling here...or raise a custom exception "
thread_name = threading.current_thread().name
e = ("Caught a MyException in thread: '" +
str(thread_name) +
"' [" + str(e) + "]")
raise Exception(e) # Or custom class of exception, such as MyException

One method I am fond of is based on the observer pattern. I define a signal class which my thread uses to emit exceptions to listeners. It can also be used to return values from threads. Example:
import threading
class Signal:
def __init__(self):
self._subscribers = list()
def emit(self, *args, **kwargs):
for func in self._subscribers:
func(*args, **kwargs)
def connect(self, func):
self._subscribers.append(func)
def disconnect(self, func):
try:
self._subscribers.remove(func)
except ValueError:
raise ValueError('Function {0} not removed from {1}'.format(func, self))
class WorkerThread(threading.Thread):
def __init__(self, *args, **kwargs):
super(WorkerThread, self).__init__(*args, **kwargs)
self.Exception = Signal()
self.Result = Signal()
def run(self):
if self._Thread__target is not None:
try:
self._return_value = self._Thread__target(*self._Thread__args, **self._Thread__kwargs)
except Exception as e:
self.Exception.emit(e)
else:
self.Result.emit(self._return_value)
if __name__ == '__main__':
import time
def handle_exception(exc):
print exc.message
def handle_result(res):
print res
def a():
time.sleep(1)
raise IOError('a failed')
def b():
time.sleep(2)
return 'b returns'
t = WorkerThread(target=a)
t2 = WorkerThread(target=b)
t.Exception.connect(handle_exception)
t2.Result.connect(handle_result)
t.start()
t2.start()
print 'Threads started'
t.join()
t2.join()
print 'Done'
I do not have enough experience of working with threads to claim that this is a completely safe method. But it has worked for me and I like the flexibility.

A simple way of catching thread's exception and communicating back to the caller method could be by passing dictionary or a list to worker method.
Example (passing dictionary to worker method):
import threading
def my_method(throw_me):
raise Exception(throw_me)
def worker(shared_obj, *args, **kwargs):
try:
shared_obj['target'](*args, **kwargs)
except Exception as err:
shared_obj['err'] = err
shared_obj = {'err':'', 'target': my_method}
throw_me = "Test"
th = threading.Thread(target=worker, args=(shared_obj, throw_me), kwargs={})
th.start()
th.join()
if shared_obj['err']:
print(">>%s" % shared_obj['err'])

Wrap Thread with exception storage.
import threading
import sys
class ExcThread(threading.Thread):
def __init__(self, target, args = None):
self.args = args if args else []
self.target = target
self.exc = None
threading.Thread.__init__(self)
def run(self):
try:
self.target(*self.args)
raise Exception('An error occured here.')
except Exception:
self.exc=sys.exc_info()
def main():
def hello(name):
print(!"Hello, {name}!")
thread_obj = ExcThread(target=hello, args=("Jack"))
thread_obj.start()
thread_obj.join()
exc = thread_obj.exc
if exc:
exc_type, exc_obj, exc_trace = exc
print(exc_type, ':',exc_obj, ":", exc_trace)
main()

I like this class:
https://gist.github.com/earonesty/b88d60cb256b71443e42c4f1d949163e
import threading
from typing import Any
class PropagatingThread(threading.Thread):
"""A Threading Class that raises errors it caught, and returns the return value of the target on join."""
def __init__(self, *args, **kwargs):
self._target = None
self._args = ()
self._kwargs = {}
super().__init__(*args, **kwargs)
self.exception = None
self.return_value = None
assert self._target
def run(self):
"""Don't override this if you want the behavior of this class, use target instead."""
try:
if self._target:
self.return_value = self._target(*self._args, **self._kwargs)
except Exception as e:
self.exception = e
finally:
# see super().run() for why this is necessary
del self._target, self._args, self._kwargs
def join(self, timeout=None) -> Any:
super().join(timeout)
if self.exception:
raise self.exception
return self.return_value

Using naked excepts is not a good practice because you usually catch more than you bargain for.
I would suggest modifying the except to catch ONLY the exception that you would like to handle. I don't think that raising it has the desired effect, because when you go to instantiate TheThread in the outer try, if it raises an exception, the assignment is never going to happen.
Instead you might want to just alert on it and move on, such as:
def run(self):
try:
shul.copytree(self.sourceFolder, self.destFolder)
except OSError, err:
print err
Then when that exception is caught, you can handle it there. Then when the outer try catches an exception from TheThread, you know it won't be the one you already handled, and will help you isolate your process flow.

I think the other solutions are somewhat complex if the only thing you want is to actually see somewhere the exception instead of being oblivious and totally blind.
The solution is to create a custom Thread that takes a logger from the main thread and logs any exceptions.
class ThreadWithLoggedException(threading.Thread):
"""
Similar to Thread but will log exceptions to passed logger.
Args:
logger: Logger instance used to log any exception in child thread
Exception is also reachable via <thread>.exception from the main thread.
"""
def __init__(self, *args, **kwargs):
try:
self.logger = kwargs.pop("logger")
except KeyError:
raise Exception("Missing 'logger' in kwargs")
super().__init__(*args, **kwargs)
self.exception = None
def run(self):
try:
if self._target is not None:
self._target(*self._args, **self._kwargs)
except Exception as exception:
thread = threading.current_thread()
self.exception = exception
self.logger.exception(f"Exception in child thread {thread}: {exception}")
finally:
del self._target, self._args, self._kwargs
Example:
logger = logging.getLogger(__name__)
logger.setLevel(logging.INFO)
logger.addHandler(logging.StreamHandler())
def serve():
raise Exception("Earth exploded.")
th = ThreadWithLoggedException(target=serve, logger=logger)
th.start()
Output in main thread:
Exception in child thread <ThreadWithLoggedException(Thread-1, started 139922384414464)>: Earth exploded.
Traceback (most recent call last):
File "/core/utils.py", line 108, in run
self._target(*self._args, **self._kwargs)
File "/myapp.py", line 105, in serve
raise Exception("Earth exploded.")
Exception: Earth exploded.

pygolang provides sync.WorkGroup which, in particular, propagates exception from spawned worker threads to the main thread. For example:
#!/usr/bin/env python
"""This program demostrates how with sync.WorkGroup an exception raised in
spawned thread is propagated into main thread which spawned the worker."""
from __future__ import print_function
from golang import sync, context
def T1(ctx, *argv):
print('T1: run ... %r' % (argv,))
raise RuntimeError('T1: problem')
def T2(ctx):
print('T2: ran ok')
def main():
wg = sync.WorkGroup(context.background())
wg.go(T1, [1,2,3])
wg.go(T2)
try:
wg.wait()
except Exception as e:
print('Tmain: caught exception: %r\n' %e)
# reraising to see full traceback
raise
if __name__ == '__main__':
main()
gives the following when run:
T1: run ... ([1, 2, 3],)
T2: ran ok
Tmain: caught exception: RuntimeError('T1: problem',)
Traceback (most recent call last):
File "./x.py", line 28, in <module>
main()
File "./x.py", line 21, in main
wg.wait()
File "golang/_sync.pyx", line 198, in golang._sync.PyWorkGroup.wait
pyerr_reraise(pyerr)
File "golang/_sync.pyx", line 178, in golang._sync.PyWorkGroup.go.pyrunf
f(pywg._pyctx, *argv, **kw)
File "./x.py", line 10, in T1
raise RuntimeError('T1: problem')
RuntimeError: T1: problem
The original code from the question would be just:
wg = sync.WorkGroup(context.background())
def _(ctx):
shul.copytree(sourceFolder, destFolder)
wg.go(_)
# waits for spawned worker to complete and, on error, reraises
# its exception on the main thread.
wg.wait()