Handling a thread’s exception in the caller thread in Python

Multithreading in Python can be achieved by using the threading library. For invoking a thread, the caller thread creates a thread object and calls the start method on it. Once the join method is called, that initiates its execution and executes the run method of the class object. 

For Exception handling, try-except blocks are used that catch the exceptions raised across the try block and are handled accordingly in the except block

Example:

Python3

# Importing the modules
import threading
import sys

# Custom Thread Class
class MyThread(threading.Thread):
    def someFunction(self):
        print("Hello World")
    def run(self):
          self.someFunction()
    def join(self):
        threading.Thread.join(self)

# Driver function
def main():
    t = MyThread()
    t.start()
    t.join()

# Driver code
if __name__ == '__main__':
    main()

Output:

Hello World

For catching and handling a thread’s exception in the caller thread we use a variable that stores the raised exception (if any) in the called thread, and when the called thread is joined, the join function checks whether the value of exc is None, if it is then no exception is generated, otherwise, the generated exception that is stored in exc is raised again. This happens in the caller thread and hence can be handled in the caller thread itself.

Example: The example creates a thread t of type MyThread, the run() method for the thread calls the someFunction() method, that raises the MyException, thus whenever the thread is run, it will raise an exception. To catch the exception in the caller thread we maintain a separate variable exc, which is set to the exception raised when the called thread raises an exception. This exc is finally checked in the join() method and if is not None, then join simply raises the same exception. Thus, the caught exception is raised in the caller thread, as join returns in the caller thread (Here The Main Thread) and is thus handled correspondingly.

Python3

# Importing the modules
import threading
import sys

# Custom Exception Class
class MyException(Exception):
    pass

# Custom Thread Class
class MyThread(threading.Thread):

  # Function that raises the custom exception
    def someFunction(self):
        name = threading.current_thread().name
        raise MyException("An error in thread "+ name)

    def run(self):

        # Variable that stores the exception, if raised by someFunction
        self.exc = None
        try:
            self.someFunction()
        except BaseException as e:
            self.exc = e

    def join(self):
        threading.Thread.join(self)
        # Since join() returns in caller thread
        # we re-raise the caught exception
        # if any was caught
        if self.exc:
            raise self.exc

# Driver function
def main():

    # Create a new Thread t
    # Here Main is the caller thread
    t = MyThread()
    t.start()

    # Exception handled in Caller thread
    try:
        t.join()
    except Exception as e:
        print("Exception Handled in Main, Details of the Exception:", e)

# Driver code
if __name__ == '__main__':
    main()

Output:

Exception Handled in Main, Details of the Exception: An error in thread Thread-1