12.4 给关键部分加锁

## 问题 Your program uses threads and you want to lock critical sections of code to avoid raceconditions. ## 解决方案 To make mutable objects safe to use by multiple threads, use Lock objects in the threading library, as shown here: import threading class SharedCounter: ‘''A counter object that can be shared by multiple threads.‘''def __init__(self, initial_value = 0): > self._value = initial_valueself._value_lock = threading.Lock() def incr(self,delta=1): ‘''Increment the counter with locking‘''with self._value_lock: > self._value += delta def decr(self,delta=1): ‘''Decrement the counter with locking‘''with self._value_lock: > self._value -= delta A Lock guarantees mutual exclusion when used with the with statement—that is, onlyone thread is allowed to execute the block of statements under the with statement at atime. The with statement acquires the lock for the duration of the indented statementsand releases the lock when control flow exits the indented block. ## 讨论 Thread scheduling is inherently nondeterministic. Because of this, failure to use locksin threaded programs can result in randomly corrupted data and bizarre behaviorknown as a “race condition.” To avoid this, locks should always be used whenever sharedmutable state is accessed by multiple threads. In older Python code, it is common to see locks explicitly acquired and released. Forexample, in this variant of the last example: import threading class SharedCounter: ‘''A counter object that can be shared by multiple threads.‘''def __init__(self, initial_value = 0): > self._value = initial_valueself._value_lock = threading.Lock() def incr(self,delta=1):‘''Increment the counter with locking‘''self._value_lock.acquire()self._value += deltaself._value_lock.release()def decr(self,delta=1):‘''Decrement the counter with locking‘''self._value_lock.acquire()self._value -= deltaself._value_lock.release() The with statement is more elegant and less prone to error—especially in situationswhere a programmer might forget to call the release() method or if a program happensto raise an exception while holding a lock (the with statement guarantees that locks arealways released in both cases).To avoid the potential for deadlock, programs that use locks should be written in a waysuch that each thread is only allowed to acquire one lock at a time. If this is not possible,you may need to introduce more advanced deadlock avoidance into your program, asdescribed in Recipe 12.5.In the threading library, you’ll find other synchronization primitives, such as RLockand Semaphore objects. As a general rule of thumb, these are more special purpose andshould not be used for simple locking of mutable state. An RLock or re-entrant lockobject is a lock that can be acquired multiple times by the same thread. It is primarilyused to implement code based locking or synchronization based on a construct knownas a “monitor.” With this kind of locking, only one thread is allowed to use an entirefunction or the methods of a class while the lock is held. For example, you could im‐plement the SharedCounter class like this: import threading class SharedCounter: ‘''A counter object that can be shared by multiple threads.‘''_lock = threading.RLock()def __init__(self, initial_value = 0): > self._value = initial_value def incr(self,delta=1): ‘''Increment the counter with locking‘''with SharedCounter._lock: > self._value += delta def decr(self,delta=1): ‘''Decrement the counter with locking‘''with SharedCounter._lock: > self.incr(-delta) In this variant of the code, there is just a single class-level lock shared by all instancesof the class. Instead of the lock being tied to the per-instance mutable state, the lock ismeant to synchronize the methods of the class. Specifically, this lock ensures that onlyone thread is allowed to be using the methods of the class at once. However, unlike astandard lock, it is OK for methods that already have the lock to call other methods thatalso use the lock (e.g., see the decr() method).One feature of this implementation is that only one lock is created, regardless of howmany counter instances are created. Thus, it is much more memory-efficient in situa‐tions where there are a large number of counters. However, a possible downside is thatit may cause more lock contention in programs that use a large number of threads andmake frequent counter updates.A Semaphore object is a synchronization primitive based on a shared counter. If thecounter is nonzero, the with statement decrements the count and a thread is allowed toproceed. The counter is incremented upon the conclusion of the with block. If thecounter is zero, progress is blocked until the counter is incremented by another thread.Although a semaphore can be used in the same manner as a standard Lock, the addedcomplexity in implementation negatively impacts performance. Instead of simple lock‐ing, Semaphore objects are more useful for applications involving signaling betweenthreads or throttling. For example, if you want to limit the amount of concurrency in apart of code, you might use a semaphore, as follows: from threading import Semaphoreimport urllib.request # At most, five threads allowed to run at once_fetch_url_sema = Semaphore(5) def fetch_url(url):with _fetch_url_sema:return urllib.request.urlopen(url) If you’re interested in the underlying theory and implementation of thread synchroni‐zation primitives, consult almost any textbook on operating systems.