Multithreading and Multiprocessing in Python - Interview Questions and Answers

Multithreading uses multiple threads within the same process to execute tasks concurrently, while multiprocessing creates separate processes, each with its own memory space. Multiprocessing avoids the Global Interpreter Lock (GIL), making it better for CPU-bound tasks.

The GIL is a mechanism in CPython that ensures only one thread executes Python bytecode at a time. This means Python threads cannot fully utilize multi-core processors for CPU-bound tasks.

Since the GIL restricts threads from running in parallel for CPU-bound tasks, multiprocessing is preferred when you need true parallel execution.

The threading module in Python provides support for creating and managing threads.

The multiprocessing module provides support for process-based parallelism.

import threading

def print_message():
    print("Hello from thread")

t = threading.Thread(target=print_message)
t.start()
t.join()

import multiprocessing

def print_message():
    print("Hello from process")

p = multiprocessing.Process(target=print_message)
p.start()
p.join()

import threading

def greet(name):
    print(f"Hello, {name}")

t = threading.Thread(target=greet, args=("Alice",))
t.start()
t.join()

import multiprocessing

def greet(name):
    print(f"Hello, {name}")

p = multiprocessing.Process(target=greet, args=("Alice",))
p.start()
p.join()

import threading
import time

def task():
    print("Starting task")
    time.sleep(2)
    print("Task completed")

t = threading.Thread(target=task)
t.start()
t.join()

t.is_alive()  # Returns True if the thread is still running

Use .join() on the thread to make the main program wait for it to finish.

import threading

def task():
    print(f"Thread name: {threading.current_thread().name}")

t = threading.Thread(target=task, name="WorkerThread")
t.start()
t.join()

Thread runs in the same memory space, while Process runs in a separate memory space, making multiprocessing better for CPU-intensive tasks.

Use t.daemon = True before starting the thread.

Daemon threads run in the background and exit automatically when the main program exits.

Use shared objects like lists, dictionaries, or synchronization mechanisms like threading.Lock.

Use multiprocessing.Queue, multiprocessing.Value, or multiprocessing.Array.

A Lock is used to prevent multiple threads from accessing shared resources at the same time.

import threading

lock = threading.Lock()

def task():
    with lock:
        print("Thread is running with lock")

t1 = threading.Thread(target=task)
t2 = threading.Thread(target=task)
t1.start()
t2.start()
t1.join()
t2.join()

A Semaphore is a synchronization primitive that limits the number of threads that can access a resource at the same time.

import threading

semaphore = threading.Semaphore(2)

def task():
    with semaphore:
        print(f"Thread {threading.current_thread().name} is running")
        import time
        time.sleep(2)

threads = [threading.Thread(target=task) for _ in range(5)]

for t in threads:
    t.start()
for t in threads:
    t.join()

An Event is a flag that can be set or cleared to control thread execution.

import threading

event = threading.Event()

def wait_for_event():
    print("Thread waiting for event...")
    event.wait()
    print("Event received, thread running!")

t = threading.Thread(target=wait_for_event)
t.start()
import time
time.sleep(2)
event.set()
t.join()

A Barrier allows multiple threads to synchronize at a common point before proceeding.

import threading

barrier = threading.Barrier(3)

def task():
    print(f"{threading.current_thread().name} waiting at barrier")
    barrier.wait()
    print(f"{threading.current_thread().name} passed barrier")

threads = [threading.Thread(target=task) for _ in range(3)]

for t in threads:
    t.start()
for t in threads:
    t.join()

• threading.Queue is used for inter-thread communication.
• multiprocessing.Queue is used for inter-process communication.

Use queue.Queue for thread-safe operations.

import queue
import threading

q = queue.Queue()

def worker():
    while not q.empty():
        item = q.get()
        print(f"Processing {item}")
        q.task_done()

for i in range(5):
    q.put(i)

threads = [threading.Thread(target=worker) for _ in range(2)]

for t in threads:
    t.start()
for t in threads:
    t.join()

Python does not provide a direct way to terminate threads. Use flags or threading.Event.

It creates thread-local storage, allowing data to be unique per thread.

It allows parallel execution of a function using multiple processes.

import multiprocessing

def square(n):
    return n * n

with multiprocessing.Pool(4) as pool:
    result = pool.map(square, [1, 2, 3, 4, 5])
print(result)

from multiprocessing import Manager, Process

def worker(shared_list):
    shared_list.append("Hello from process")

with Manager() as manager:
    shared_list = manager.list()
    p = Process(target=worker, args=(shared_list,))
    p.start()
    p.join()
    print(shared_list)

• apply() runs a function in one process and returns the result.
• apply_async() runs a function asynchronously.
• map() applies a function to all elements of an iterable in parallel.
• map_async() does the same but asynchronously.

Value allows sharing a single value between processes, while Array allows sharing a list.

Use multiprocessing.Queue, which is process-safe by default.

It prevents multiple processes from accessing a shared resource simultaneously.

import multiprocessing

lock = multiprocessing.Lock()

def task():
    with lock:
        print("Process is running with lock")

p1 = multiprocessing.Process(target=task)
p2 = multiprocessing.Process(target=task)
p1.start()
p2.start()
p1.join()
p2.join()

It allows a process to wait until another process meets a certain condition.

ThreadPoolExecutor is used for I/O-bound tasks, while ProcessPoolExecutor is used for CPU-bound tasks.

from concurrent.futures import ProcessPoolExecutor

def square(n):
    return n * n

with ProcessPoolExecutor() as executor:
    results = executor.map(square, [1, 2, 3, 4, 5])

print(list(results))

Use synchronization primitives like Lock, Semaphore, or Queue.

• os.fork() is Unix-specific and creates a child process by duplicating the parent process.
• multiprocessing.Process is cross-platform and starts a new process.

No, due to the GIL, threads execute one at a time. Only multiprocessing achieves true parallel execution.

• Deadlocks due to improper locking.
• Pickling errors when passing non-picklable objects.
• Resource leaks if processes are not properly closed.

A daemon thread runs in the background and terminates when all non-daemon threads finish.

import threading
import time

def background_task():
    while True:
        print("Daemon thread running...")
        time.sleep(2)

thread = threading.Thread(target=background_task, daemon=True)
thread.start()
time.sleep(5)
print("Main thread exiting")

Here, the daemon thread stops when the main thread exits.

Use the join(timeout) method.

import threading

def task():
    import time
    time.sleep(10)

thread = threading.Thread(target=task)
thread.start()
thread.join(timeout=5)  # Waits for 5 seconds
if thread.is_alive():
    print("Thread is still running, timeout occurred")

A worker thread executes tasks asynchronously in a pool, improving performance in I/O-bound applications.

Use ThreadPoolExecutor from concurrent.futures.

from concurrent.futures import ThreadPoolExecutor

def worker(n):
    return n * n

with ThreadPoolExecutor(max_workers=4) as executor:
    results = executor.map(worker, [1, 2, 3, 4, 5])

print(list(results))

Pipe allows bidirectional communication between two processes, while Queue supports multiple producers and consumers.

from multiprocessing import Pipe, Process

def sender(conn):
    conn.send("Hello from child process")
    conn.close()

parent_conn, child_conn = Pipe()
p = Process(target=sender, args=(child_conn,))
p.start()
print(parent_conn.recv())  # Receiving data
p.join()

Catch exceptions inside the thread and use a shared variable to store errors.

• Zombie process: A child process that finishes execution but still has an entry in the process table.
• Orphan process: A child process whose parent has terminated.

• Use timeouts when acquiring locks.
• Use thread-safe data structures like queue.Queue().
• Avoid circular wait conditions.

• CPU-bound: Use multiprocessing to utilize multiple cores.
• I/O-bound: Use threading or asyncio since the GIL allows I/O tasks to run concurrently.

Use cProfile for performance profiling.

import cProfile
import threading

def task():
    sum(range(100000))

thread = threading.Thread(target=task)
cProfile.run("thread.start(); thread.join()")

Process affinity binds a process to specific CPU cores. Use psutil to control it.

Use the terminate() method in multiprocessing.Process.

from multiprocessing import Process
import time

def task():
    while True:
        print("Running...")
        time.sleep(1)

p = Process(target=task)
p.start()
time.sleep(5)
p.terminate()
p.join()
print("Process terminated")

Use multiprocessing.shared_memory to avoid unnecessary copying.

It occurs when low-priority threads are indefinitely delayed by high-priority threads. Use fair scheduling or adjust thread priorities.

Use Semaphore in threading or BoundedSemaphore in multiprocessing.

Yes, but avoid if __name__ == "__main__" issues by placing multiprocessing code inside functions.

• Windows: Uses spawn (creates a fresh process).
• Linux: Uses fork (copies the parent process).

It yields the CPU to another thread or process, useful for cooperative multitasking.

• asyncio is best for single-threaded concurrency using coroutines.
• threading is best for I/O-bound tasks.
• multiprocessing is best for CPU-bound tasks.

• Use logging instead of print().
• Use threading.enumerate() to track active threads.
• Use GDB with Python extensions for deep debugging.

Barrier synchronizes multiple threads, making them wait at a checkpoint before proceeding.

import threading

barrier = threading.Barrier(3)

def task(n):
    print(f"Thread-{n} waiting at barrier")
    barrier.wait()
    print(f"Thread-{n} passed the barrier")

for i in range(3):
    threading.Thread(target=task, args=(i,)).start()

Use threading.enumerate() to list all active threads.

import threading

def worker():
    import time
    time.sleep(2)

t1 = threading.Thread(target=worker)
t2 = threading.Thread(target=worker)
t1.start()
t2.start()

print(threading.enumerate())  # Lists all running threads

Use the logging module with ThreadingHandler or QueueHandler.

It provides shared objects like lists and dictionaries across processes.

from multiprocessing import Manager, Process

def worker(shared_list):
    shared_list.append(1)

with Manager() as manager:
    shared_list = manager.list()
    processes = [Process(target=worker, args=(shared_list,)) for _ in range(3)]
    for p in processes:
        p.start()
    for p in processes:
        p.join()
    print(shared_list)  # Output: [1, 1, 1]

Python does not automatically resolve deadlocks. Use timeouts, lock hierarchy, or try-except blocks.

Array allows sharing data between processes without using a Manager.

from multiprocessing import Array, Process

def worker(arr):
    arr[0] = 99

shared_array = Array('i', [1, 2, 3])
p = Process(target=worker, args=(shared_array,))
p.start()
p.join()
print(shared_array[:])  # Output: [99, 2, 3]

JoinableQueue extends Queue with a task_done() method for better synchronization.

It uses a queue where tasks with higher priority are processed first.

No, Python only allows one running event loop per thread.

GIL prevents true parallel execution of Python threads, limiting performance in CPU-bound tasks.

Use multiprocessing, cython, numba, or JIT-optimized code.

• os.fork() creates a process by duplicating the current one (Linux only).
• multiprocessing.Process() creates a new process that starts from the function entry.

Yes, using libraries like Ray or Dask.

Use concurrent.futures.ProcessPoolExecutor.

• subprocess is for running external programs.
• multiprocessing is for parallel execution of Python code.