Interview question

How do you handle exceptions in threads and async tasks? Threads और async tasks में exceptions कैसे handle करें?

Answer

Exceptions raised inside a thread's target function don't automatically propagate to the main thread - they're silently printed but don't crash the calling code. Async tasks behave differently and require explicit handling too.

import threading

def risky_task():
    raise ValueError('Something went wrong')

t = threading.Thread(target=risky_task)
t.start()
t.join()
print('Main thread continues normally')
# The exception is printed to stderr by the threading module,
# but the main thread is NOT interrupted or notified automatically

# FIX: capture the exception yourself and store/communicate it
class ThreadWithException(threading.Thread):
    def __init__(self, target):
        super().__init__()
        self.target = target
        self.exception = None

    def run(self):
        try:
            self.target()
        except Exception as e:
            self.exception = e  # store it for the main thread to check

t = ThreadWithException(risky_task)
t.start()
t.join()
if t.exception:
    print(f'Thread raised: {t.exception}')

# Using concurrent.futures - exceptions ARE captured and re-raised via .result()
from concurrent.futures import ThreadPoolExecutor
with ThreadPoolExecutor() as executor:
    future = executor.submit(risky_task)
    try:
        future.result()  # re-raises the original exception HERE
    except ValueError as e:
        print(f'Caught: {e}')

# Asyncio - exceptions in a task propagate when you await it
import asyncio
async def risky_async():
    raise ValueError('Async error')

async def main():
    try:
        await risky_async()
    except ValueError as e:
        print(f'Caught async: {e}')
asyncio.run(main())

Thread की target function में raise हुई exceptions main thread तक automatically propagate नहीं होतीं - चुपचाप print हो जाती हैं पर calling code को crash नहीं करतीं।

import threading

def risky_task():
    raise ValueError('कुछ गलत हुआ')

t = threading.Thread(target=risky_task)
t.start()
t.join()
print('Main thread सामान्य रूप से जारी')
# Exception stderr पर print होता है पर main thread notify नहीं होता

# FIX: exception खुद capture करना
class ThreadWithException(threading.Thread):
    def __init__(self, target):
        super().__init__()
        self.target = target
        self.exception = None

    def run(self):
        try:
            self.target()
        except Exception as e:
            self.exception = e

t = ThreadWithException(risky_task)
t.start()
t.join()
if t.exception:
    print(f'Thread ने raise किया: {t.exception}')

# concurrent.futures से exceptions .result() पर re-raise होती हैं
from concurrent.futures import ThreadPoolExecutor
with ThreadPoolExecutor() as executor:
    future = executor.submit(risky_task)
    try:
        future.result()
    except ValueError as e:
        print(f'Caught: {e}')

# Asyncio - task की exception await पर propagate होती है
import asyncio
async def risky_async():
    raise ValueError('Async error')

async def main():
    try:
        await risky_async()
    except ValueError as e:
        print(f'Caught async: {e}')
asyncio.run(main())

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