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Condition
A condition variable allows one or more threads to wait until they are
notified by another thread.
If the lock argument is given and not None, it must be a Lock or RLock
object, and it is used as the underlying lock. Otherwise, a new RLock object
is created and used as the underlying lock.
- 实质利用一个主锁，加上 N 告诉锁来实现
wait 步骤原理
- 创立告诉锁 锁定状态的 lock1
- 开释主锁
- 梗塞 lock1.acquire(), 相当如期待解锁
- lock1 解锁后，锁定主锁
- wait 完结，继续执行后续操作，并开释主锁；
- 衍生出 Semaphore, Event
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