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The signal.signal() function allows defining custom handlers to beexecuted when a signal is received. A small number of default handlers areinstalled: SIGPIPE is ignored (so write errors on pipes and socketscan be reported as ordinary Python exceptions) and SIGINT istranslated into a KeyboardInterrupt exception if the parent processhas not changed it.

A handler for a particular signal, once set, remains installed until it isexplicitly reset (Python emulates the BSD style interface regardless of theunderlying implementation), with the exception of the handler forSIGCHLD, which follows the underlying implementation.

A Python signal handler does not get executed inside the low-level (C) signalhandler. Instead, the low-level signal handler sets a flag which tells thevirtual machine to execute the corresponding Python signal handlerat a later point(for example at the next bytecode instruction).This has consequences:

It makes little sense to catch synchronous errors like SIGFPE orSIGSEGV that are caused by an invalid operation in C code. Pythonwill return from the signal handler to the C code, which is likely to raisethe same signal again, causing Python to apparently hang. From Python 3.3onwards, you can use the faulthandler module to report on synchronouserrors.

A long-running calculation implemented purely in C (such as regularexpression matching on a large body of text) may run uninterrupted for anarbitrary amount of time, regardless of any signals received. The Pythonsignal handlers will be called when the calculation finishes.

This is one of two standard signal handling options; it will simply performthe default function for the signal. For example, on most systems thedefault action for SIGQUIT is to dump core and exit, while thedefault action for SIGCHLD is to simply ignore it.

Raised to signal an error from the underlying setitimer() orgetitimer() implementation. Expect this error if an invalidinterval timer or a negative time is passed to setitimer().This error is a subtype of OSError.

If time is non-zero, this function requests that a SIGALRM signal besent to the process in time seconds. Any previously scheduled alarm iscanceled (only one alarm can be scheduled at any time). The returned value isthen the number of seconds before any previously set alarm was to have beendelivered. If time is zero, no alarm is scheduled, and any scheduled alarm iscanceled. If the return value is zero, no alarm is currently scheduled.

Return the current signal handler for the signal signalnum. The returned valuemay be a callable Python object, or one of the special valuessignal.SIG_IGN, signal.SIG_DFL or None. Here,signal.SIG_IGN means that the signal was previously ignored,signal.SIG_DFL means that the default way of handling the signal waspreviously in use, and None means that the previous signal handler was notinstalled from Python.

Send signal sig to the process referred to by file descriptor pidfd.Python does not currently support the siginfo parameter; it must beNone. The flags argument is provided for future extensions; no flagvalues are currently defined.

Send the signal signalnum to the thread thread_id, another thread in thesame process as the caller. The target thread can be executing any code(Python or not). However, if the target thread is executing the Pythoninterpreter, the Python signal handlers will be executed by the mainthread of the main interpreter. Therefore, the only point of sending asignal to a particular Python thread would be to force a running system callto fail with InterruptedError.

Sets given interval timer (one of signal.ITIMER_REAL,signal.ITIMER_VIRTUAL or signal.ITIMER_PROF) specifiedby which to fire after seconds (float is accepted, different fromalarm()) and after that every interval seconds (if intervalis non-zero). The interval timer specified by which can be cleared bysetting seconds to zero.

When an interval timer fires, a signal is sent to the process.The signal sent is dependent on the timer being used;signal.ITIMER_REAL will deliver SIGALRM,signal.ITIMER_VIRTUAL sends SIGVTALRM,and signal.ITIMER_PROF will deliver SIGPROF.

Set the wakeup file descriptor to fd. When a signal is received, thesignal number is written as a single byte into the fd. This can be used bya library to wakeup a poll or select call, allowing the signal to be fullyprocessed.

Set the handler for signal signalnum to the function handler. handler canbe a callable Python object taking two arguments (see below), or one of thespecial values signal.SIG_IGN or signal.SIG_DFL. The previoussignal handler will be returned (see the description of getsignal()above). (See the Unix man page signal(2) for further information.)

The handler is called with two arguments: the signal number and the currentstack frame (None or a frame object; for a description of frame objects,see the description in the type hierarchy or see theattribute descriptions in the inspect module).

On Windows, signal() can only be called with SIGABRT,SIGFPE, SIGILL, SIGINT, SIGSEGV,SIGTERM, or SIGBREAK.A ValueError will be raised in any other case.Note that not all systems define the same set of signal names; anAttributeError will be raised if a signal name is not defined asSIG* module level constant.

Suspend execution of the calling thread until the delivery of one of thesignals specified in the signal set sigset. The function accepts the signal(removes it from the pending list of signals), and returns the signal number.

Suspend execution of the calling thread until the delivery of one of thesignals specified in the signal set sigset. The function accepts thesignal and removes it from the pending list of signals. If one of thesignals in sigset is already pending for the calling thread, the functionwill return immediately with information about that signal. The signalhandler is not called for the delivered signal. The function raises anInterruptedError if it is interrupted by a signal that is not insigset.

Changed in version 3.5: The function is now retried with the recomputed timeout if interruptedby a signal not in sigset and the signal handler does not raise anexception (see _______ for the rationale).

Here is a minimal example program. It uses the alarm() function to limitthe time spent waiting to open a file; this is useful if the file is for aserial device that may not be turned on, which would normally cause theos.open() to hang indefinitely. The solution is to set a 5-second alarmbefore opening the file; if the operation takes too long, the alarm signal willbe sent, and the handler raises an exception.

Piping output of your program to tools like head(1) willcause a SIGPIPE signal to be sent to your process when the receiverof its standard output closes early. This results in an exceptionlike BrokenPipeError: [Errno 32] Broken pipe. To handle thiscase, wrap your entry point to catch this exception as follows:

If a signal handler raises an exception, the exception will be propagated tothe main thread and may be raised after any bytecode instruction. Mostnotably, a KeyboardInterrupt may appear at any point during execution.Most Python code, including the standard library, cannot be made robust againstthis, and so a KeyboardInterrupt (or any other exception resulting froma signal handler) may on rare occasions put the program in an unexpected state.

For many programs, especially those that merely want to exit onKeyboardInterrupt, this is not a problem, but applications that arecomplex or require high reliability should avoid raising exceptions from signalhandlers. They should also avoid catching KeyboardInterrupt as a meansof gracefully shutting down. Instead, they should install their ownSIGINT handler. Below is an example of an HTTP server that avoidsKeyboardInterrupt:

When the fetch request is initiated, we pass in the AbortSignal as an option inside the request's options object (the {signal} below). This associates the signal and controller with the fetch request and allows us to abort it by calling AbortController.abort(), as seen below in the second event listener.

In these cases, you can register to receive signals sent only by particularsenders. In the case of django.db.models.signals.pre_save, the senderwill be the model class being saved, so you can indicate that you only wantsignals sent by some model:

In some circumstances, the code connecting receivers to signals may runmultiple times. This can cause your receiver function to be registered morethan once, and thus called as many times for a signal event. For example, theready() method may be executed more than onceduring testing. More generally, this occurs everywhere your project imports themodule where you define the signals, because signal registration runs as manytimes as it is imported.

If this behavior is problematic (such as when using signals tosend an email whenever a model is saved), pass a unique identifier asthe dispatch_uid argument to identify your receiver function. Thisidentifier will usually be a string, although any hashable object willsuffice. The end result is that your receiver function will only bebound to the signal once for each unique dispatch_uid value:

To send a signal, call either Signal.send(), Signal.send_robust(),await Signal.asend(), orawait Signal.asend_robust(). You must provide thesender argument (which is a class most of the time) and may provide as manyother keyword arguments as you like.

send() differs from send_robust() in how exceptions raised by receiverfunctions are handled. send() does not catch any exceptions raised byreceivers; it simply allows errors to propagate. Thus not all receivers maybe notified of a signal in the face of an error.

To disconnect a receiver from a signal, call Signal.disconnect(). Thearguments are as described in Signal.connect(). The method returnsTrue if a receiver was disconnected and False if not. When senderis passed as a lazy reference to ., this method alwaysreturns None. 5376163bf9

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