CODEWITHSUNDEEP
pythonpython-3.xmultithreadingpython-multithreading
TurtleTread
TurtleTread

Reputation: 1314

python thread.start() doesn't run to completion before switching context?

Can someone explain how the Python threading works? Does thread.start() not run the target function to completion before switching back to another context like main below?

import time
import threading

def threadfunc():
    # time.sleep(1)
    print('thread print 1', flush=True)
    print('thread print 2', flush=True)
    #time.sleep(1)

print('before thread', flush=True)

thread1 = threading.Thread(target=threadfunc)
thread1.start()

print('after thread', flush=True)

Output:

before thread
thread print 1
after thread
thread print 2 #shouldn't this be after "print 1"?

Upvotes: 0

Views: 856

Answers (1)

Sam Mason
Sam Mason

Reputation: 16214

as the comments explained, that's not how threads or the GIL work in Python

lots of the C code underlying CPython will release the GIL, IO operations (as Solomon noted) will do this, while some libraries (e.g. NumPy) explicitly release it just to help multithreaded code go faster

the interpreter will also try and switch threads every few milliseconds

see https://stackoverflow.com/a/49573860/1358308 for some more details

Upvotes: 2

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