# 15.3 `threading` -- Higher-level threading interface

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

The `dummy_threading` module is provided for situations where `threading` cannot be used because `thread` is missing.

This module defines the following functions and objects:

`activeCount`( | )

  Return the number of `Thread` objects currently alive. The returned count is equal to the length of the list returned by `enumerate()`.

`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 `Thread` objects currently alive. 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.

class `local`
  A class that represents thread-local data. Thread-local data are data whose values are thread specific. To manage thread-local data, just create an instance of `local` (or a subclass) and store attributes on it:

      mydata = threading.local()
      mydata.x = 1

  The instance's values will be different for separate threads.

  For more details and extensive examples, see the documentation string of the `_threading_local` module.

  New in version 2.4.

`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.

`settrace`( | func)

  Set a trace function for all threads started from the `threading` module. The func will be passed to `sys.settrace()` for each thread, before its `run()` method is called. New in version 2.3.

`setprofile`( | func)

  Set a profile function for all threads started from the `threading` module. The func will be passed to `sys.setprofile()` for each thread, before its `run()` method is called. New in version 2.3.

`stack_size`( | [size])

  Return the thread stack size used when creating new threads. The optional size argument specifies the stack size to be used for subsequently created threads, and must be 0 (use platform or configured default) or a positive integer value of at least 32,768 (32kB). If changing the thread stack size is unsupported, a `ThreadError` is raised. If the specified stack size is invalid, a `ValueError` is raised and the stack size is unmodified. 32kB is currently the minimum supported stack size value to guarantee sufficient stack space for the interpreter itself. Note that some platforms may have particular restrictions on values for the stack size, such as requiring a minimum stack size > 32kB or requiring allocation in multiples of the system memory page size - platform documentation should be referred to for more information (4kB pages are common; using multiples of 4096 for the stack size is the suggested approach in the absence of more specific information). Availability: Windows, systems with POSIX threads. New in version 2.5.

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.