Question

I’m talking to a measurement device. I basically send commands and receive answers. But I’m providing a method ask that sends a command and reads back the answer. If I lock this method I get a deadlock due to the called methods read and write locking aswell. If I don’t lock another thread could steal the answer or write before I’m reading. How would you implement this?

import threading

class Device(object):
    lock = threading.Lock()
    def ask(self, value):
        # can't use lock here would block
        self.write(value) # another thread could start reading the answer
        return self.read()

    def read(self):
        with self.lock:
            # read values from device

    def write(self, value):
        with self.lock:
            # send command to device

Answer 1

Use threading.RLock() to avoid contention within a single thread:

A reentrant lock is a synchronization primitive that may be acquired
multiple times by the same thread. Internally, it uses the concepts of
“owning thread” and “recursion level” in addition to the
locked/unlocked state used by primitive locks. In the locked state,
some thread owns the lock; in the unlocked state, no thread owns it.