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pythonpython-2.7cpython

Is a python dict's len() atomic with respect to the GIL?

I'm asking about CPython, python2.7. Say I have a dict, and a few threads that will insert values from time to time by calling add():

d = {}
dlock = threading.Lock()
def add(key, value):
    with dlock:
        d[key] = value

Is it safe to get the size of the dict from a separate thread without grabbing the lock, relying just on the GIL?

def count():
    return len(d)

Assuming that I don't care about getting precisely the correct value, just any value that was correct at some point during count().

Solution

  • I wouldn't recommend it, but yeah, the GIL will protect that. There's no opportunity for the GIL to be released during the len call.