Lib/threading.py (part 5)

Source:

cpython 3.14 @ ab2d84fe1023/Lib/threading.py ↗

This annotation covers the higher-level threading synchronization primitives. See module_threading4_detail for Thread, Lock, RLock, and Condition.

Map

Lines	Symbol	Role
1-80	threading.Barrier	Synchronize N threads at a rendezvous point
81-160	threading.BoundedSemaphore	Semaphore that raises on over-release
161-240	threading.Event	Boolean flag with wait
241-360	threading.local	Thread-local storage
361-500	threading.Timer	Run a function after a delay

Reading

Barrier

# CPython: Lib/threading.py:580 Barrier
class Barrier:
    """Implement a Barrier: block until N threads have called wait()."""
    def __init__(self, parties, action=None, timeout=None):
        self._cond = Condition(Lock())
        self._action = action
        self._parties = parties
        self._count = 0
        self._state = 0  # 0=filling, 1=draining, -1=broken

    def wait(self, timeout=None):
        with self._cond:
            self._enter()  # raises BrokenBarrierError if broken
            index = self._count
            self._count += 1
            if self._count == self._parties:
                self._release()
            else:
                self._wait(timeout)
            return index

All parties threads call wait() and all block until the last one arrives. The last thread calls the optional action callback (once, while others wait) then releases all. Returns the thread's arrival index (0 to parties-1).

threading.Event

# CPython: Lib/threading.py:520 Event
class Event:
    """A flag object. One thread signals; others wait."""
    def __init__(self):
        self._cond = Condition(Lock())
        self._flag = False

    def wait(self, timeout=None):
        with self._cond:
            if not self._flag:
                self._cond.wait(timeout)
            return self._flag

    def set(self):
        with self._cond:
            self._flag = True
            self._cond.notify_all()

event.wait() blocks until event.set() is called. If already set, wait returns immediately. event.clear() resets the flag. Used for start signals, shutdown notifications, and one-time initialization gates.

threading.local

# CPython: Lib/threading.py:60 local
class local:
    """Thread-local storage.
    
    Attributes set on a local() instance are private to each thread.
    Other threads see a separate copy initialized from __init_subclass__.
    """
    __slots__ = '_local__impl', '__dict__'

threading.local() uses __dict__ to store per-thread values: t._local__impl maps each thread's ident to its own __dict__. Getting an attribute looks up the current thread's dict.

gopy notes

threading.Barrier is module/threading.Barrier in module/threading/module.go, implemented using a sync.WaitGroup plus a sync.Mutex for the draining phase. threading.Event uses a sync.Cond. threading.local stores values in a sync.Map[goroutineID]map[string]interface{}.