This module provides mechanisms to use signal handlers in Python. Show
General rules¶The A handler for a particular signal, once set, remains installed until it is explicitly reset (Python emulates the BSD style interface regardless of the underlying implementation), with the exception of the handler for
On WebAssembly platforms Execution of Python signal handlers¶A Python signal handler does not get executed inside the low-level (C) signal handler. Instead, the low-level signal handler sets a flag which tells the virtual machine to execute the corresponding Python signal handler at a later point(for example at the next bytecode instruction). This has consequences:
Signals and threads¶Python signal handlers are always executed in the main Python thread of the main interpreter, even if the signal was received in another thread. This means that signals can’t be used as a means of inter-thread communication. You can use the synchronization primitives from the Besides, only the main thread of the main interpreter is allowed to set a new signal handler. Module contents¶The signal module defines three enums: classsignal.Signals¶
New in version 3.5. classsignal.Handlers¶
New in version 3.5. class signal.Sigmasks¶
Availability: Unix. See the man page sigprocmask(2) and pthread_sigmask(3) for further information. New in version 3.5. The variables defined in the This is one of two standard signal handling options; it will simply perform the default function for the signal. For example, on most systems the default action for This is another standard signal handler, which will simply ignore the given signal. signal.SIGABRT¶Abort signal from abort(3). signal.SIGALRM¶Timer signal from alarm(2). Availability: Unix. signal.SIGBREAK¶Interrupt from keyboard (CTRL + BREAK). Availability: Windows. signal.SIGBUS¶Bus error (bad memory access). Availability: Unix. signal.SIGCHLD¶Child process stopped or terminated. Availability: Unix. signal.SIGCLD¶Alias to Continue the process if it is currently stopped Availability: Unix. signal.SIGFPE¶Floating-point exception. For example, division by zero. See also
Hangup detected on controlling terminal or death of controlling process. Availability: Unix. signal.SIGILL¶Illegal instruction. signal.SIGINT¶Interrupt from keyboard (CTRL + C). Default action is to raise Kill signal. It cannot be caught, blocked, or ignored. Availability: Unix. signal.SIGPIPE¶Broken pipe: write to pipe with no readers. Default action is to ignore the signal. Availability: Unix. signal.SIGSEGV¶Segmentation fault: invalid memory reference. signal.SIGSTKFLT¶
Availability: Linux. On architectures where the signal is available. See the man page signal(7) for further information. New in version 3.11. signal.SIGTERM¶Termination signal. signal.SIGUSR1¶User-defined signal 1. Availability: Unix. signal.SIGUSR2¶User-defined signal 2. Availability: Unix. signal.SIGWINCH¶Window resize signal. Availability: Unix. SIG*All the signal numbers are defined symbolically. For example, the hangup
signal is defined as The signal
corresponding to the Ctrl+C keystroke event. This signal can only be used with Availability: Windows. New in version 3.2. signal.CTRL_BREAK_EVENT¶The signal corresponding to the Ctrl+Break keystroke event. This signal can only be used with Availability: Windows. New in version 3.2. signal.NSIG¶One more than the number of the highest signal number. Use
Decrements interval timer in real time, and delivers
Decrements interval timer only when the process is executing, and delivers SIGVTALRM upon expiration. signal.ITIMER_PROF¶Decrements interval timer both when the process executes and when the system is executing on behalf of the process. Coupled with ITIMER_VIRTUAL, this timer is usually used to profile the time spent by the application in user and kernel space. SIGPROF is delivered upon expiration. signal.SIG_BLOCK¶A possible value for the how parameter to New in version 3.3. signal.SIG_UNBLOCK¶A possible value for the how parameter to
New in version 3.3. signal.SIG_SETMASK¶A possible value
for the how parameter to New in version 3.3. The Raised to signal an error from the underlying New
in version 3.3: This error used to be a subtype of The If time is non-zero, this function requests that a
Availability: Unix. See the man page alarm(2) for further information. signal.getsignal(signalnum)¶Return the current signal handler for the signal signalnum. The returned value may be a callable Python object, or one of the special values
Returns the description of signal signalnum, such as “Interrupt” for New in version 3.8. signal.valid_signals()¶Return the set of valid signal numbers on this platform. This can be less than New in version 3.8. signal.pause()¶Cause the process to sleep until a signal is received; the appropriate handler will then be called. Returns nothing. Availability: Unix. See the man page signal(2) for further information. See also Sends a signal to the calling process. Returns nothing. New in version 3.8. signal.pidfd_send_signal(pidfd, sig, siginfo=None, flags=0)¶Send signal sig to the process referred to by file descriptor pidfd. Python does not currently support the siginfo parameter; it must be See the pidfd_send_signal(2) man page for more information. Availability: Linux >= 5.1 New in version 3.9. signal.pthread_kill(thread_id, signalnum)¶Send the signal signalnum to the thread thread_id, another thread in the same process as the caller. The target thread can be executing any code
(Python or not). However, if the target thread is executing the Python interpreter, the Python signal handlers will be executed by the main thread of the main interpreter. Therefore, the only point of sending a signal to a particular Python thread would be to force a running system call to fail with
Use If signalnum is 0, then no signal is sent, but error checking is still performed; this can be used to check if the target thread is still running. Raises an auditing event Availability: Unix. See the man page pthread_kill(3) for further information. See also New in version 3.3. signal.pthread_sigmask(how, mask)¶Fetch and/or change the signal mask of the calling thread. The signal mask is the set of signals whose delivery is currently blocked for the caller. Return the old signal mask as a set of signals. The behavior of the call is dependent on the value of how, as follows.
mask is a set of signal numbers (e.g. { For
example,
Availability: Unix. See the man page sigprocmask(2) and pthread_sigmask(3) for further information. See also New in version 3.3. signal.setitimer(which, seconds, interval=0.0)¶Sets given interval timer (one of When an interval timer fires, a signal is sent to the process. The signal sent is dependent on the timer being used; The old values are returned as a tuple: (delay, interval). Attempting to pass an invalid interval timer will cause an Availability: Unix. signal.getitimer(which)¶Returns current value of a given interval timer specified by which. Availability: Unix. signal.set_wakeup_fd(fd, *, warn_on_full_buffer=True)¶Set the wakeup file descriptor to fd. When a signal is received, the signal number is written as a single byte into the fd. This can be used by a library to wakeup a poll or select call, allowing the signal to be fully processed. The old wakeup fd is returned (or -1 if file descriptor wakeup was not enabled). If fd is -1, file descriptor wakeup is disabled. If not -1, fd must be non-blocking. It is up to the library to remove any bytes from fd before calling poll or select again. When
threads are enabled, this function can only be called from the main thread of the main interpreter; attempting to call it from other threads will cause a There are two common ways to use this function. In both approaches, you use the fd to wake up when a signal arrives, but then they differ in how they determine which signal or signals have arrived. In the first approach, we read the data out of the fd’s buffer, and the byte values give you the signal numbers. This is simple, but in rare cases it can run into a problem: generally the fd will have a limited amount of buffer space, and if too many signals arrive too quickly, then the buffer may become full, and some signals may be lost. If you use this approach, then
you should set In the second approach, we use the wakeup fd only for wakeups, and ignore the actual byte values. In this case, all we care about is whether the fd’s buffer is empty or non-empty; a full buffer doesn’t indicate a problem at all. If you use this approach, then you should set Changed in version 3.5: On Windows, the function now also supports socket handles. Changed in version 3.7: Added Change system call restart behaviour: if flag is Availability: Unix. See the man page siginterrupt(3) for further information. Note that installing a signal handler with Set the handler
for signal signalnum to the function handler. handler can be a callable Python object taking two arguments (see below), or one of the special values When threads are enabled, this function can only be called from the main thread of the main interpreter;
attempting to call it from other threads will cause a The handler is called with two arguments: the signal number and the current stack frame ( On Windows, Examine the set of signals that are pending for delivery to the calling thread (i.e., the signals which have been raised while blocked). Return the set of the pending signals. Availability: Unix. See the man page sigpending(2) for further information. See also New in version 3.3. signal.sigwait(sigset)¶Suspend execution of the calling thread until the delivery of one of the signals specified in the signal set sigset. The function accepts the signal (removes it from the pending list of signals), and returns the signal number. Availability: Unix. See the man page sigwait(3) for further information. See also New in version 3.3. signal.sigwaitinfo(sigset)¶Suspend execution of the calling thread until the delivery of one of the signals specified in the signal set sigset. The function accepts the signal and removes it from the pending list of signals. If one of the signals in sigset is already pending for the calling thread, the function will return immediately with information about that signal. The signal handler is not called for the delivered signal. The function raises an
The return value is an object representing the data contained in the Availability: Unix. See the man page sigwaitinfo(2) for further information. See also New in version 3.3. Changed in version 3.5: The function is now retried if interrupted by a signal not in sigset and the signal handler does not raise an exception (see PEP 475 for the rationale). signal.sigtimedwait(sigset, timeout)¶Like Availability: Unix. See the man page sigtimedwait(2) for further information. See also New in version 3.3. Changed in version 3.5: The function is now retried with the recomputed timeout if interrupted by a signal not in sigset and the signal handler does not raise an exception (see PEP 475 for the rationale). Examples¶Here is a minimal example program. It uses the import signal, os def handler(signum, frame): signame = signal.Signals(signum).name print(f'Signal handler called with signal {signame} ({signum})') raise OSError("Couldn't open device!") # Set the signal handler and a 5-second alarm signal.signal(signal.SIGALRM, handler) signal.alarm(5) # This open() may hang indefinitely fd = os.open('/dev/ttyS0', os.O_RDWR) signal.alarm(0) # Disable the alarm Note on SIGPIPE¶Piping output of your program to tools like head(1) will cause a import os import sys def main(): try: # simulate large output (your code replaces this loop) for x in range(10000): print("y") # flush output here to force SIGPIPE to be triggered # while inside this try block. sys.stdout.flush() except BrokenPipeError: # Python flushes standard streams on exit; redirect remaining output # to devnull to avoid another BrokenPipeError at shutdown devnull = os.open(os.devnull, os.O_WRONLY) os.dup2(devnull, sys.stdout.fileno()) sys.exit(1) # Python exits with error code 1 on EPIPE if __name__ == '__main__': main() Do not set Note on Signal Handlers and Exceptions¶If a signal handler raises an exception, the exception will be propagated to the main thread and may be raised after any bytecode instruction. Most notably, a
To illustrate this issue, consider the following code: class SpamContext: def __init__(self): self.lock = threading.Lock() def __enter__(self): # If KeyboardInterrupt occurs here, everything is fine self.lock.acquire() # If KeyboardInterrupt occurs here, __exit__ will not be called ... # KeyboardInterrupt could occur just before the function returns def __exit__(self, exc_type, exc_val, exc_tb): ... self.lock.release() For many programs, especially those that merely want to exit on import signal import socket from selectors import DefaultSelector, EVENT_READ from http.server import HTTPServer, SimpleHTTPRequestHandler interrupt_read, interrupt_write = socket.socketpair() def handler(signum, frame): print('Signal handler called with signal', signum) interrupt_write.send(b'\0') signal.signal(signal.SIGINT, handler) def serve_forever(httpd): sel = DefaultSelector() sel.register(interrupt_read, EVENT_READ) sel.register(httpd, EVENT_READ) while True: for key, _ in sel.select(): if key.fileobj == interrupt_read: interrupt_read.recv(1) return if key.fileobj == httpd: httpd.handle_request() print("Serving on port 8000") httpd = HTTPServer(('', 8000), SimpleHTTPRequestHandler) serve_forever(httpd) print("Shutdown...") What error will you get if you type a function name incorrectly?The #NAME error occurs in Excel when the program doesn't recognize something in your formula. The most common cause is a simple misspelling of the function being used. For example, in the image below, the formula has VLOOKUP spelled incorrectly in the first instance (F5), so it produces the #NAME? error.
Which function would you use if you wanted to count the number of values but ignore cells that have text or are empty group of answer choices?COUNTA doesn't count empty or blank cells. You need the COUNTBLANK function for that.
Which function would you use if you wanted to count the number of values but ignore cells?Using COUNT function in Excel - things to remember
Below are the two simple rules by which the Excel COUNT function works. If an argument(s) of an Excel Count formula is a cell reference or range, only numbers, dates and times are counted. Blanks cells and cells containing anything but a numeric value are ignored.
What is not true about using the AND function in a custom conditional formatting rule?What is not true about using the AND function in a custom conditional formatting rule? All the test must be of the same data type, for example all numbers are all text.
|