Daniel Arbuckle's Mastering Python

The semaphore object

The semaphore objects look a lot like lock objects. The difference is that, where the lock objects always ensured that exactly one process has acquired the lock at a given time, semaphore objects ensure that no more than a fixed number of processes can acquire it at the same time.

This can be seen using the following code example:

This is useful for doing things such as limiting the number of worker processes that can access the hard disk at the same time.