🚀 Python Create / Start / Join Threads – Control Concurrency Effectively

🧲 Introduction – Why Use Python Threads?

Multithreading in Python enables your programs to run multiple operations concurrently. Whether you’re managing network requests, file I/O, or background tasks like logging or monitoring, threads help keep your application responsive and efficient.

This guide focuses on the three core thread operations:

• Creating a thread
• Starting a thread
• Joining a thread (waiting for completion)

🎯 In this guide, you’ll learn:

• How to create threads using threading.Thread
• How to start and run threads concurrently
• How to join threads to the main program
• Examples using both functions and classes
• Best practices for managing thread life

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✅ Step 1: Create a Thread with threading.Thread

Import the threading module:

import threading

🧪 Example 1 – Using a Function

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

t = threading.Thread(target=task)

📌 At this stage, the thread is created but not started. It’s in the “new” state.

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▶️ Step 2: Start the Thread

Use .start() to schedule the thread for execution:

t.start()
print("Main thread continues")

✅ The thread is now “runnable”, and once the OS schedules it, it becomes “running”.

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🧵 Step 3: Join the Thread (Wait for Completion)

t.join()
print("Thread has finished")

🔒 join() makes the main thread wait until the target thread completes.

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🧱 Example – Full Workflow

import threading
import time

def background_task():
    print("Starting background task...")
    time.sleep(2)
    print("Background task finished.")

thread = threading.Thread(target=background_task)
thread.start()

print("Main thread is running...")
thread.join()
print("Main thread exits after thread completes.")

✅ Output:

Starting background task...  
Main thread is running...  
Background task finished.  
Main thread exits after thread completes.

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🧪 Example – Thread with Arguments

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

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

💡 args=() passes arguments as a tuple to the function.

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🧬 Example – Thread via Subclassing

You can also create a thread by subclassing threading.Thread.

class MyThread(threading.Thread):
    def run(self):
        print("Hello from subclassed thread!")

t = MyThread()
t.start()
t.join()

✅ Recommended when you need to maintain state or extend thread behavior.

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⏱️ Multiple Threads Example

def show_number(n):
    print(f"Thread {n} starting")
    time.sleep(1)
    print(f"Thread {n} done")

threads = []

for i in range(3):
    t = threading.Thread(target=show_number, args=(i,))
    threads.append(t)
    t.start()

for t in threads:
    t.join()

print("All threads completed")

💡 Use .join() in a loop to wait for all threads.

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📘 Best Practices for Start/Join Threads

✅ Do This	❌ Avoid This
Use join() to wait for thread finish	Relying on time.sleep() as a timer
Handle exceptions inside thread	Crashing thread without try-except
Start all threads before joining	Calling join() too early in the loop
Use args=() for parameter passing	Modifying global variables carelessly

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📌 Summary – Recap & Next Steps

Creating, starting, and joining threads is the foundation of Python multithreading. These steps enable you to run tasks concurrently while maintaining control over program execution.

🔍 Key Takeaways:

• ✅ threading.Thread(target=func) creates a thread
• ✅ start() schedules the thread for execution
• ✅ join() pauses the main thread until the thread finishes
• ✅ Threads can be created with both functions and class-based approaches

⚙️ Real-World Relevance:
Used in parallel downloads, non-blocking I/O, background tasks, game loops, and real-time applications.

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❓ FAQ – Python Create / Start / Join Threads

❓ What does join() do in Python threads?

✅ It waits for the thread to finish before moving forward.

❓ Can I pass arguments to a thread function?

✅ Yes, use args=(arg1, arg2, ...) in the Thread() constructor.

❓ Is start() the same as run()?

❌ No. start() schedules the thread.
Calling run() manually runs it in the current thread, not in a new thread.

❓ How many times can I start a thread?

❌ Only once. After calling start(), you cannot restart the same thread object.

❓ Should I use join() always?

✅ Yes, if your main thread depends on the result or you need to control flow. Otherwise, threads may outlive the main process.

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