7.5 threading -- Higher-level threading interface
Keywords: lib

# 7.5 `threading` -- Higher-level threading interface

This module constructs higher-level threading interfaces on top of the lower level `thread` module.

This module defines the following functions and objects:

`activeCount`()
  Return the number of currently active `Thread` objects. The returned count is equal to the length of the list returned by `enumerate()`. A function that returns the number of currently active threads.

`Condition`()
  A factory function that returns a new condition variable object. A condition variable allows one or more threads to wait until they are notified by another thread.

`currentThread`()
  Return the current `Thread` object, corresponding to the caller's thread of control. If the caller's thread of control was not created through the `threading` module, a dummy thread object with limited functionality is returned.

`enumerate`()
  Return a list of all currently active `Thread` objects. The list includes daemonic threads, dummy thread objects created by `currentThread()`, and the main thread. It excludes terminated threads and threads that have not yet been started.

`Event`()
  A factory function that returns a new event object. An event manages a flag that can be set to true with the `set()` method and reset to false with the `clear()` method. The `wait()` method blocks until the flag is true.

`Lock`()
  A factory function that returns a new primitive lock object. Once a thread has acquired it, subsequent attempts to acquire it block, until it is released; any thread may release it.

`RLock`()
  A factory function that returns a new reentrant lock object. A reentrant lock must be released by the thread that acquired it. Once a thread has acquired a reentrant lock, the same thread may acquire it again without blocking; the thread must release it once for each time it has acquired it.

`Semaphore`([value])
  A factory function that returns a new semaphore object. A semaphore manages a counter representing the number of `release()` calls minus the number of `acquire()` calls, plus an initial value. The `acquire()` method blocks if necessary until it can return without making the counter negative. If not given, value defaults to 1.

`BoundedSemaphore`([value])
  A factory function that returns a new bounded semaphore object. A bounded semaphore checks to make sure its current value doesn't exceed its initial value. If it does, `ValueError` is raised. In most situations semaphores are used to guard resources with limited capacity. If the semaphore is released too many times it's a sign of a bug. If not given, value defaults to 1.

class `Thread`
  A class that represents a thread of control. This class can be safely subclassed in a limited fashion.

class `Timer`
  A thread that executes a function after a specified interval has passed.

Detailed interfaces for the objects are documented below.

The design of this module is loosely based on Java's threading model. However, where Java makes locks and condition variables basic behavior of every object, they are separate objects in Python. Python's `Thread` class supports a subset of the behavior of Java's Thread class; currently, there are no priorities, no thread groups, and threads cannot be destroyed, stopped, suspended, resumed, or interrupted. The static methods of Java's Thread class, when implemented, are mapped to module-level functions.

All of the methods described below are executed atomically.

Subsections

- 7.5.1 Lock Objects
- 7.5.2 RLock Objects
- 7.5.3 Condition Objects
- 7.5.4 Semaphore Objects
  - 7.5.4.1 `Semaphore` Example
- 7.5.5 Event Objects
- 7.5.6 Thread Objects
- 7.5.7 Timer Objects