Python Threading
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What is Threading?
Threading lets a program run multiple pieces of code seemingly at the same time within one process, which is most useful for I/O-bound tasks (waiting on network requests or disk reads) where a thread can idle without blocking the rest of the program -- CPU-bound work benefits far less due to Python's GIL.
Example: What is Threading?
import threading
print(threading.active_count())Creating a Basic Thread
Creating a thread means instantiating threading.Thread(target=some_function) and then calling .start() on it -- start() launches the thread running concurrently with the rest of your program, rather than running it immediately and blocking like a normal function call would.
Example: Creating a Basic Thread
import threading
def task():
print("Running in a thread")
t = threading.Thread(target=task)
t.start()
t.join()Joining Threads
By default, your main program keeps executing immediately after start() without waiting for the thread to finish -- calling .join() on a thread object blocks the calling code until that specific thread has completed, which matters when later code depends on the thread's work being done.
Example: Joining Threads
import threading
def task():
print("Thread work done")
t = threading.Thread(target=task)
t.start()
t.join()
print("Main continues after thread finishes")Threading with Arguments
The args parameter to Thread() accepts a tuple of arguments to pass into the target function when the thread starts -- Thread(target=download, args=(url,)) is how you launch download(url) to run concurrently rather than calling it directly.
Example: Threading with Arguments
import threading
def greet(name):
print("Hello,", name)
t = threading.Thread(target=greet, args=("Alex",))
t.start()
t.join()Thread Safety and Locks
Because threads in the same process share memory, two threads modifying the same variable at the same moment can corrupt it through a race condition -- threading.Lock() lets you mark a section of code so only one thread can execute it at a time, preventing that kind of collision.
Example: Thread Safety and Locks
import threading
counter = 0
lock = threading.Lock()
def increment():
global counter
with lock:
counter += 1
threads = [threading.Thread(target=increment) for _ in range(5)]
for t in threads:
t.start()
for t in threads:
t.join()
print(counter)Chapter Quiz — Complete all 12 topics to unlock
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