# Python Cookbook by David Ascher, Alex Martelli. Credit: Sami Hangaslammi
import threading
class ReadWriteLock:
""" A lock object that allows many simultaneous "read locks", but
only one "write lock." """
def __init__(self):
self._read_ready = threading.Condition(threading.Lock( ))
self._readers = 0
def acquire_read(self):
""" Acquire a read lock. Blocks only if a thread has
acquired the write lock. """
self._read_ready.acquire( )
try:
self._readers += 1
finally:
self._read_ready.release( )
def release_read(self):
""" Release a read lock. """
self._read_ready.acquire( )
try:
self._readers -= 1
if not self._readers:
self._read_ready.notifyAll( )
finally:
self._read_ready.release( )
def acquire_write(self):
""" Acquire a write lock. Blocks until there are no
acquired read or write locks. """
self._read_ready.acquire( )
while self._readers > 0:
self._read_ready.wait( )
def release_write(self):
""" Release a write lock. """
self._read_ready.release( )