_thread — Low-level threading API¶

This module provides low-level primitives for working with multiple threads(also calledlight-weight processes ortasks) — multiple threads ofcontrol sharing their global data space. For synchronization, simple locks(also calledmutexes orbinary semaphores) are provided.Thethreading module provides an easier to use and higher-levelthreading API built on top of this module.

This module defines the following constants and functions:

exception_thread.error¶

Raised on thread-specific errors.

Changed in version 3.3:This is now a synonym of the built-inRuntimeError.

_thread.LockType¶

This is the type of lock objects.

_thread.start_new_thread(function,args[,kwargs])¶

Start a new thread and return its identifier. The thread executes thefunctionfunction with the argument listargs (which must be a tuple).The optionalkwargs argument specifies a dictionary of keyword arguments.

When the function returns, the thread silently exits.

When the function terminates with an unhandled exception,sys.unraisablehook() is called to handle the exception. Theobjectattribute of the hook argument isfunction. By default, a stack trace isprinted and then the thread exits (but other threads continue to run).

When the function raises aSystemExit exception, it is silentlyignored.

Changed in version 3.8:sys.unraisablehook() is now used to handle unhandled exceptions.

_thread.interrupt_main()¶

Simulate the effect of asignal.SIGINT signal arriving in the mainthread. A thread can use this function to interrupt the main thread.

Ifsignal.SIGINT isn’t handled by Python (it was set tosignal.SIG_DFL orsignal.SIG_IGN), this function doesnothing.

_thread.exit()¶

Raise theSystemExit exception. When not caught, this will cause thethread to exit silently.

_thread.allocate_lock()¶

Return a new lock object. Methods of locks are described below. The lock isinitially unlocked.

_thread.get_ident()¶

Return the ‘thread identifier’ of the current thread. This is a nonzerointeger. Its value has no direct meaning; it is intended as a magic cookie tobe used e.g. to index a dictionary of thread-specific data. Thread identifiersmay be recycled when a thread exits and another thread is created.

_thread.get_native_id()¶

Return the native integral Thread ID of the current thread assigned by the kernel.This is a non-negative integer.Its value may be used to uniquely identify this particular thread system-wide(until the thread terminates, after which the value may be recycled by the OS).

Availability: Windows, FreeBSD, Linux, macOS, OpenBSD, NetBSD, AIX.

New in version 3.8.

_thread.stack_size([size])¶

Return the thread stack size used when creating new threads. The optionalsize argument specifies the stack size to be used for subsequently createdthreads, and must be 0 (use platform or configured default) or a positiveinteger value of at least 32,768 (32 KiB). Ifsize is not specified,0 is used. If changing the thread stack size isunsupported, aRuntimeError is raised. If the specified stack size isinvalid, aValueError is raised and the stack size is unmodified. 32 KiBis currently the minimum supported stack size value to guarantee sufficientstack space for the interpreter itself. Note that some platforms may haveparticular restrictions on values for the stack size, such as requiring aminimum stack size > 32 KiB or requiring allocation in multiples of the systemmemory page size - platform documentation should be referred to for moreinformation (4 KiB pages are common; using multiples of 4096 for the stack size isthe suggested approach in the absence of more specific information).

Availability: Windows, systems with POSIX threads.

_thread.TIMEOUT_MAX¶

The maximum value allowed for thetimeout parameter ofLock.acquire(). Specifying a timeout greater than this value willraise anOverflowError.

New in version 3.2.

Lock objects have the following methods:

lock.acquire(waitflag=1,timeout=-1)¶

Without any optional argument, this method acquires the lock unconditionally, ifnecessary waiting until it is released by another thread (only one thread at atime can acquire a lock — that’s their reason for existence).

If the integerwaitflag argument is present, the action depends on itsvalue: if it is zero, the lock is only acquired if it can be acquiredimmediately without waiting, while if it is nonzero, the lock is acquiredunconditionally as above.

If the floating-pointtimeout argument is present and positive, itspecifies the maximum wait time in seconds before returning. A negativetimeout argument specifies an unbounded wait. You cannot specifyatimeout ifwaitflag is zero.

The return value isTrue if the lock is acquired successfully,False if not.

Changed in version 3.2:Thetimeout parameter is new.

Changed in version 3.2:Lock acquires can now be interrupted by signals on POSIX.

lock.release()¶

Releases the lock. The lock must have been acquired earlier, but notnecessarily by the same thread.

lock.locked()¶

Return the status of the lock:True if it has been acquired by some thread,False if not.

In addition to these methods, lock objects can also be used via thewith statement, e.g.:

import_threada_lock=_thread.allocate_lock()witha_lock:print("a_lock is locked while this executes")

Caveats:

• Threads interact strangely with interrupts: theKeyboardInterruptexception will be received by an arbitrary thread. (When thesignalmodule is available, interrupts always go to the main thread.)

• Callingsys.exit() or raising theSystemExit exception isequivalent to calling_thread.exit().

• It is not possible to interrupt theacquire() method on a lock — theKeyboardInterrupt exception will happen after the lock has been acquired.

• When the main thread exits, it is system defined whether the other threadssurvive. On most systems, they are killed without executingtry …finally clauses or executing objectdestructors.

• When the main thread exits, it does not do any of its usual cleanup (exceptthattry …finally clauses are honored), and thestandard I/O files are not flushed.