How to catch a thread's exception in the caller thread in Python?

Python Server Side Programming Programming

I have the following script which copies files from one location to another.

If the files cannot be copied, an exception is thrown.

The following code does not work:

class TheThread(threading.Thread):
    def __init__(self, FromFolder, ToFolder):
        threading.Thread.__init__(self)
        self.FromFolder = FromFolder
        self.ToFolder = ToFolder
    def run(self):
        try:
           shul.copytree(self.FromFolder, self.ToFolder)
        except:
           raise
try:
    threadClass = TheThread(arg1, arg2,…)
    threadClass.start()   #Exception occurs here
except:
    print "An exception is caught"

How to catch the exception?

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1 Answer

Manogna

Answered on 6th Dec, 2017

The problem is that thread_obj.start() returns immediately. The child thread that you started executes in its own context, in its own stack. Any exception that occurs there is in the context of the child thread. You have to communicate this information to the parent thread by passing some message.

The code can be re-written as follows:

import sys
import threading
import Queue
class ExcThread(threading.Thread):
def __init__(self, foo):
threading.Thread.__init__(self)
self.foo = foo
def run(self):
try:
raise Exception('An error occurred here.')
except Exception:
self.foo.put(sys.exc_info())
def main():
foo = Queue.Queue()
thread_obj = ExcThread(foo)
thread_obj.start()


while True:
try:
exc = foo.get(block=False)
except Queue.Empty:
pass
else:
exc_type, exc_obj, exc_trace = exc
print exc_type, exc_obj
print exc_trace


thread_obj.join(0.1)
if thread_obj.isAlive():
continue

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How to catch a thread's exception in the caller thread in Python?

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