RuntimeError: calling start() on an already running thread

This error occurs when you attempt to call .start() on a thread that is already running. In Python, a thread can be started only once, and trying to restart an already running thread results in a RuntimeError.

1. Understanding the Cause

Python’s threading.Thread objects cannot be restarted once they have been started. If a thread is already running, calling .start() again on the same thread object will trigger this error.

Incorrect Code (Starting a Running Thread Again)

import threading
import time

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

t = threading.Thread(target=task)
t.start()  # First start - Works fine
t.start()  # Second start - Causes RuntimeError

Output:

RuntimeError: calling start() on an already running thread

2. How to Fix the Issue?

Solution 1: Create a New Thread Object

Once a thread is started, you cannot restart it. If you need to run the function again, create a new thread instance instead.

import threading
import time

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

t = threading.Thread(target=task)
t.start()
t.join()  # Wait for the thread to complete

# Create a new thread instance before starting again
t = threading.Thread(target=task)
t.start()

Solution 2: Use a Loop Instead of Restarting

If you want the thread to keep running, place the logic inside a loop instead of restarting it.

import threading
import time

def task():
    while True:
        print("Thread running...")
        time.sleep(2)  # Wait before repeating

t = threading.Thread(target=task, daemon=True)
t.start()

This avoids the need to restart the thread and allows it to run continuously.

Solution 3: Use a Thread Pool

If you need to run multiple tasks with threading, use ThreadPoolExecutor, which automatically manages threads.

from concurrent.futures import ThreadPoolExecutor
import time

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

with ThreadPoolExecutor(max_workers=2) as executor:
    executor.submit(task)
    executor.submit(task)  # Runs in a separate thread

This method prevents manual thread management errors.

3. Summary of Fixes

Cause	Fix
Trying to restart a running thread	Create a new thread instance before restarting
Want continuous execution	Use a `while` loop inside the thread
Need multiple executions	Use `ThreadPoolExecutor` to manage threads