Advanced Python 6 — Thread Lifecycle & Difference between start() and run()

What is a Thread Lifecycle?

The thread lifecycle describes the different stages a thread goes through from creation to completion.

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In Python, a thread goes through these main stages:

1. New
2. Runnable
3. Running
4. Waiting / Blocked
5. Terminated

Let’s understand each one.

Example:

import threading
import time

def task():
    print("[Running] Thread is now running")
    
    print("[Waiting] Thread going to sleep (waiting state)")
    time.sleep(2)
    
    print("[Running] Thread resumed after sleep")
    print("[Terminated] Thread finishing execution")

print("[New] Thread object created")
t = threading.Thread(target=task, name="DemoThread")

print("[Runnable] Thread is about to start")
t.start()

print("[Main] Waiting for thread to finish")
t.join()

print("[Dead] Thread has terminated")

Difference Between start() and run()

start() Method

What it does

• Creates a new thread
• Calls run() internally
• Runs code concurrently

import threading

def task():
    print("Running in a separate thread")

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

✔ New thread is created
✔ Runs in parallel (logically)
✔ Correct way to start a thread

run() Method

What it does

• Runs the function like a normal function
• Does NOT create a new thread

t = threading.Thread(target=task)
t.run()

❌ No new thread
❌ Runs in the main thread
❌ No concurrency

EXAMPLE

import threading
import time

def task():
    print(f"Task running in thread: {threading.current_thread().name}")
    time.sleep(1)
    print(f"Task finished in thread: {threading.current_thread().name}")

print("Main thread:", threading.current_thread().name)

print("\n--- Using start() ---")
t1 = threading.Thread(target=task, name="Worker-Thread")
t1.start()
t1.join()

print("\n--- Using run() ---")
t2 = threading.Thread(target=task, name="Fake-Thread")
t2.run()

print("\nBack to main thread:", threading.current_thread().name)


### OUTPUT
Main thread: MainThread

--- Using start() ---
Task running in thread: Worker-Thread
Task finished in thread: Worker-Thread

--- Using run() ---
Task running in thread: MainThread
Task finished in thread: MainThread

Back to main thread: MainThread

Final Takeaway

• A thread goes through creation → execution → waiting → termination
• start() creates a new thread
• run() is just a normal function call
• Always use start() for real multithreading

start() gives you concurrency; run() does not.