Name	Size	Mode	Actions
2to3.rst.txt	14647	0644	editdlrm
abc.rst.txt	7219	0644	editdlrm
aepack.rst.txt	4257	0644	editdlrm
aetools.rst.txt	3532	0644	editdlrm
aetypes.rst.txt	4257	0644	editdlrm
aifc.rst.txt	7086	0644	editdlrm
al.rst.txt	5309	0644	editdlrm
allos.rst.txt	695	0644	editdlrm
anydbm.rst.txt	4111	0644	editdlrm
archiving.rst.txt	424	0644	editdlrm
argparse.rst.txt	74685	0644	editdlrm
array.rst.txt	10524	0644	editdlrm
ast.rst.txt	10111	0644	editdlrm
asynchat.rst.txt	9206	0644	editdlrm
asyncore.rst.txt	12937	0644	editdlrm
atexit.rst.txt	3910	0644	editdlrm
audioop.rst.txt	10395	0644	editdlrm
autogil.rst.txt	1015	0644	editdlrm
base64.rst.txt	6252	0644	editdlrm
basehttpserver.rst.txt	10404	0644	editdlrm
bastion.rst.txt	2611	0644	editdlrm
bdb.rst.txt	12456	0644	editdlrm
binascii.rst.txt	6508	0644	editdlrm
binhex.rst.txt	1910	0644	editdlrm
bisect.rst.txt	5415	0644	editdlrm
bsddb.rst.txt	7575	0644	editdlrm
bz2.rst.txt	8064	0644	editdlrm
calendar.rst.txt	11282	0644	editdlrm
carbon.rst.txt	15956	0644	editdlrm
cd.rst.txt	11974	0644	editdlrm
cgi.rst.txt	22853	0644	editdlrm
cgihttpserver.rst.txt	2788	0644	editdlrm
cgitb.rst.txt	2874	0644	editdlrm
chunk.rst.txt	4955	0644	editdlrm
cmath.rst.txt	7647	0644	editdlrm
cmd.rst.txt	8565	0644	editdlrm
code.rst.txt	7115	0644	editdlrm
codecs.rst.txt	66966	0644	editdlrm
codeop.rst.txt	3774	0644	editdlrm
collections.rst.txt	41448	0644	editdlrm
colorpicker.rst.txt	913	0644	editdlrm
colorsys.rst.txt	1819	0644	editdlrm
commands.rst.txt	2595	0644	editdlrm
compileall.rst.txt	4678	0644	editdlrm
compiler.rst.txt	37464	0644	editdlrm
configparser.rst.txt	19607	0644	editdlrm
constants.rst.txt	2328	0644	editdlrm
contextlib.rst.txt	6012	0644	editdlrm
cookie.rst.txt	9543	0644	editdlrm
cookielib.rst.txt	27875	0644	editdlrm
copy.rst.txt	3351	0644	editdlrm
copy_reg.rst.txt	2329	0644	editdlrm
crypt.rst.txt	2292	0644	editdlrm
crypto.rst.txt	355	0644	editdlrm
csv.rst.txt	22757	0644	editdlrm
ctypes.rst.txt	90513	0644	editdlrm
curses.ascii.rst.txt	9046	0644	editdlrm
curses.panel.rst.txt	2740	0644	editdlrm
curses.rst.txt	74877	0644	editdlrm
custominterp.rst.txt	570	0644	editdlrm
datatypes.rst.txt	864	0644	editdlrm
datetime.rst.txt	74679	0644	editdlrm
dbhash.rst.txt	3865	0644	editdlrm
dbm.rst.txt	3117	0644	editdlrm
debug.rst.txt	446	0644	editdlrm
decimal.rst.txt	70926	0644	editdlrm
development.rst.txt	640	0644	editdlrm
difflib.rst.txt	30716	0644	editdlrm
dircache.rst.txt	1813	0644	editdlrm
dis.rst.txt	23248	0644	editdlrm
distribution.rst.txt	426	0644	editdlrm
distutils.rst.txt	1958	0644	editdlrm
dl.rst.txt	3392	0644	editdlrm
doctest.rst.txt	73985	0644	editdlrm
docxmlrpcserver.rst.txt	3802	0644	editdlrm
dumbdbm.rst.txt	2841	0644	editdlrm
dummy_thread.rst.txt	1058	0644	editdlrm
dummy_threading.rst.txt	799	0644	editdlrm
easydialogs.rst.txt	10346	0644	editdlrm
email-examples.rst.txt	1271	0644	editdlrm
email.charset.rst.txt	9654	0644	editdlrm
email.encoders.rst.txt	2376	0644	editdlrm
email.errors.rst.txt	4006	0644	editdlrm
email.generator.rst.txt	6132	0644	editdlrm
email.header.rst.txt	7531	0644	editdlrm
email.iterators.rst.txt	2421	0644	editdlrm
email.message.rst.txt	25217	0644	editdlrm
email.mime.rst.txt	9917	0644	editdlrm
email.parser.rst.txt	10327	0644	editdlrm
email.rst.txt	16111	0644	editdlrm
email.utils.rst.txt	6475	0644	editdlrm
ensurepip.rst.txt	5022	0644	editdlrm
errno.rst.txt	6708	0644	editdlrm
exceptions.rst.txt	18343	0644	editdlrm
fcntl.rst.txt	7361	0644	editdlrm
filecmp.rst.txt	5348	0644	editdlrm
fileformats.rst.txt	302	0644	editdlrm
fileinput.rst.txt	7417	0644	editdlrm
filesys.rst.txt	806	0644	editdlrm
fl.rst.txt	17646	0644	editdlrm
fm.rst.txt	2699	0644	editdlrm
fnmatch.rst.txt	3104	0644	editdlrm
formatter.rst.txt	13245	0644	editdlrm
fpectl.rst.txt	4171	0644	editdlrm
fpformat.rst.txt	1747	0644	editdlrm
fractions.rst.txt	5297	0644	editdlrm
framework.rst.txt	11444	0644	editdlrm
frameworks.rst.txt	378	0644	editdlrm
ftplib.rst.txt	15720	0644	editdlrm
functions.rst.txt	75554	0644	editdlrm
functools.rst.txt	7448	0644	editdlrm
future_builtins.rst.txt	2010	0644	editdlrm
gc.rst.txt	9014	0644	editdlrm
gdbm.rst.txt	4882	0644	editdlrm
gensuitemodule.rst.txt	3113	0644	editdlrm
getopt.rst.txt	6670	0644	editdlrm
getpass.rst.txt	1876	0644	editdlrm
gettext.rst.txt	29040	0644	editdlrm
gl.rst.txt	6009	0644	editdlrm
glob.rst.txt	2414	0644	editdlrm
grp.rst.txt	2256	0644	editdlrm
gzip.rst.txt	4828	0644	editdlrm
hashlib.rst.txt	7379	0644	editdlrm
heapq.rst.txt	13185	0644	editdlrm
hmac.rst.txt	3073	0644	editdlrm
hotshot.rst.txt	4289	0644	editdlrm
htmllib.rst.txt	7379	0644	editdlrm
htmlparser.rst.txt	11640	0644	editdlrm
httplib.rst.txt	37454	0644	editdlrm
i18n.rst.txt	409	0644	editdlrm
ic.rst.txt	5005	0644	editdlrm
idle.rst.txt	22182	0644	editdlrm
imageop.rst.txt	4001	0644	editdlrm
imaplib.rst.txt	17221	0644	editdlrm
imgfile.rst.txt	2765	0644	editdlrm
imghdr.rst.txt	2635	0644	editdlrm
imp.rst.txt	12593	0644	editdlrm
importlib.rst.txt	1126	0644	editdlrm
imputil.rst.txt	7023	0644	editdlrm
index.rst.txt	2287	0644	editdlrm
inspect.rst.txt	28151	0644	editdlrm
internet.rst.txt	950	0644	editdlrm
intro.rst.txt	2803	0644	editdlrm
io.rst.txt	39015	0644	editdlrm
ipc.rst.txt	630	0644	editdlrm
itertools.rst.txt	36324	0644	editdlrm
jpeg.rst.txt	3858	0644	editdlrm
json.rst.txt	25545	0644	editdlrm
keyword.rst.txt	617	0644	editdlrm
language.rst.txt	523	0644	editdlrm
linecache.rst.txt	1887	0644	editdlrm
locale.rst.txt	24975	0644	editdlrm
logging.config.rst.txt	31560	0644	editdlrm
logging.handlers.rst.txt	28153	0644	editdlrm
logging.rst.txt	46882	0644	editdlrm
mac.rst.txt	791	0644	editdlrm
macos.rst.txt	3824	0644	editdlrm
macosa.rst.txt	3964	0644	editdlrm
macostools.rst.txt	4017	0644	editdlrm
macpath.rst.txt	650	0644	editdlrm
mailbox.rst.txt	68129	0644	editdlrm
mailcap.rst.txt	3675	0644	editdlrm
markup.rst.txt	1249	0644	editdlrm
marshal.rst.txt	5689	0644	editdlrm
math.rst.txt	10920	0644	editdlrm
md5.rst.txt	2815	0644	editdlrm
mhlib.rst.txt	3966	0644	editdlrm
mimetools.rst.txt	4504	0644	editdlrm
mimetypes.rst.txt	9839	0644	editdlrm
mimewriter.rst.txt	3360	0644	editdlrm
mimify.rst.txt	3519	0644	editdlrm
miniaeframe.rst.txt	2564	0644	editdlrm
misc.rst.txt	248	0644	editdlrm
mm.rst.txt	447	0644	editdlrm
mmap.rst.txt	10481	0644	editdlrm
modulefinder.rst.txt	3380	0644	editdlrm
modules.rst.txt	382	0644	editdlrm
msilib.rst.txt	19062	0644	editdlrm
msvcrt.rst.txt	4346	0644	editdlrm
multifile.rst.txt	6613	0644	editdlrm
multiprocessing.rst.txt	92621	0644	editdlrm
mutex.rst.txt	1939	0644	editdlrm
netdata.rst.txt	432	0644	editdlrm
netrc.rst.txt	3119	0644	editdlrm
new.rst.txt	2653	0644	editdlrm
nis.rst.txt	2108	0644	editdlrm
nntplib.rst.txt	14531	0644	editdlrm
numbers.rst.txt	8007	0644	editdlrm
numeric.rst.txt	751	0644	editdlrm
operator.rst.txt	22092	0644	editdlrm
optparse.rst.txt	77101	0644	editdlrm
os.path.rst.txt	13094	0644	editdlrm
os.rst.txt	82580	0644	editdlrm
ossaudiodev.rst.txt	17309	0644	editdlrm
othergui.rst.txt	2721	0644	editdlrm
parser.rst.txt	15385	0644	editdlrm
pdb.rst.txt	16043	0644	editdlrm
persistence.rst.txt	826	0644	editdlrm
pickle.rst.txt	37262	0644	editdlrm
pickletools.rst.txt	1997	0644	editdlrm
pipes.rst.txt	3786	0644	editdlrm
pkgutil.rst.txt	7714	0644	editdlrm
platform.rst.txt	9554	0644	editdlrm
plistlib.rst.txt	4134	0644	editdlrm
popen2.rst.txt	7022	0644	editdlrm
poplib.rst.txt	6220	0644	editdlrm
posix.rst.txt	3630	0644	editdlrm
posixfile.rst.txt	7200	0644	editdlrm
pprint.rst.txt	9071	0644	editdlrm
profile.rst.txt	28666	0644	editdlrm
pty.rst.txt	1762	0644	editdlrm
pwd.rst.txt	2725	0644	editdlrm
pyclbr.rst.txt	3296	0644	editdlrm
pydoc.rst.txt	4083	0644	editdlrm
pyexpat.rst.txt	28891	0644	editdlrm
python.rst.txt	514	0644	editdlrm
py_compile.rst.txt	2479	0644	editdlrm
queue.rst.txt	7034	0644	editdlrm
quopri.rst.txt	2670	0644	editdlrm
random.rst.txt	13319	0644	editdlrm
re.rst.txt	55102	0644	editdlrm
readline.rst.txt	10513	0644	editdlrm
repr.rst.txt	4718	0644	editdlrm
resource.rst.txt	9595	0644	editdlrm
restricted.rst.txt	3327	0644	editdlrm
rexec.rst.txt	11742	0644	editdlrm
rfc822.rst.txt	14037	0644	editdlrm
rlcompleter.rst.txt	2494	0644	editdlrm
robotparser.rst.txt	2190	0644	editdlrm
runpy.rst.txt	6938	0644	editdlrm
sched.rst.txt	4644	0644	editdlrm
scrolledtext.rst.txt	1379	0644	editdlrm
select.rst.txt	20705	0644	editdlrm
sets.rst.txt	14973	0644	editdlrm
sgi.rst.txt	322	0644	editdlrm
sgmllib.rst.txt	10664	0644	editdlrm
sha.rst.txt	2807	0644	editdlrm
shelve.rst.txt	8319	0644	editdlrm
shlex.rst.txt	11313	0644	editdlrm
shutil.rst.txt	13491	0644	editdlrm
signal.rst.txt	10719	0644	editdlrm
simplehttpserver.rst.txt	4559	0644	editdlrm
simplexmlrpcserver.rst.txt	10881	0644	editdlrm
site.rst.txt	7880	0644	editdlrm
smtpd.rst.txt	2464	0644	editdlrm
smtplib.rst.txt	14906	0644	editdlrm
sndhdr.rst.txt	1759	0644	editdlrm
socket.rst.txt	40765	0644	editdlrm
socketserver.rst.txt	22512	0644	editdlrm
someos.rst.txt	599	0644	editdlrm
spwd.rst.txt	2825	0644	editdlrm
sqlite3.rst.txt	35448	0644	editdlrm
ssl.rst.txt	76554	0644	editdlrm
stat.rst.txt	7778	0644	editdlrm
statvfs.rst.txt	1300	0644	editdlrm
stdtypes.rst.txt	122668	0644	editdlrm
string.rst.txt	44085	0644	editdlrm
stringio.rst.txt	4166	0644	editdlrm
stringprep.rst.txt	4238	0644	editdlrm
strings.rst.txt	746	0644	editdlrm
struct.rst.txt	17100	0644	editdlrm
subprocess.rst.txt	33648	0644	editdlrm
sun.rst.txt	249	0644	editdlrm
sunau.rst.txt	7124	0644	editdlrm
sunaudio.rst.txt	5849	0644	editdlrm
symbol.rst.txt	975	0644	editdlrm
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sys.rst.txt	47520	0644	editdlrm
sysconfig.rst.txt	7639	0644	editdlrm
syslog.rst.txt	3931	0644	editdlrm
tabnanny.rst.txt	1999	0644	editdlrm
tarfile.rst.txt	28214	0644	editdlrm
telnetlib.rst.txt	7482	0644	editdlrm
tempfile.rst.txt	10504	0644	editdlrm
termios.rst.txt	3733	0644	editdlrm
test.rst.txt	17659	0644	editdlrm
textwrap.rst.txt	8552	0644	editdlrm
thread.rst.txt	6588	0644	editdlrm
threading.rst.txt	32468	0644	editdlrm
time.rst.txt	26020	0644	editdlrm
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tix.rst.txt	22693	0644	editdlrm
tk.rst.txt	1612	0644	editdlrm
tkinter.rst.txt	33456	0644	editdlrm
token.rst.txt	2451	0644	editdlrm
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ttk.rst.txt	57411	0644	editdlrm
tty.rst.txt	1011	0644	editdlrm
turtle.rst.txt	64097	0644	editdlrm
types.rst.txt	6200	0644	editdlrm
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unicodedata.rst.txt	5736	0644	editdlrm
unittest.rst.txt	82952	0644	editdlrm
unix.rst.txt	490	0644	editdlrm
urllib.rst.txt	24960	0644	editdlrm
urllib2.rst.txt	35211	0644	editdlrm
urlparse.rst.txt	16071	0644	editdlrm
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wave.rst.txt	5047	0644	editdlrm
weakref.rst.txt	12926	0644	editdlrm
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winsound.rst.txt	5066	0644	editdlrm
wsgiref.rst.txt	30567	0644	editdlrm
xdrlib.rst.txt	8077	0644	editdlrm
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xml.etree.elementtree.rst.txt	35707	0644	editdlrm
xml.rst.txt	6090	0644	editdlrm
xml.sax.handler.rst.txt	15371	0644	editdlrm
xml.sax.reader.rst.txt	12257	0644	editdlrm
xml.sax.rst.txt	6460	0644	editdlrm
xml.sax.utils.rst.txt	3560	0644	editdlrm
xmlrpclib.rst.txt	22559	0644	editdlrm
zipfile.rst.txt	18871	0644	editdlrm
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zlib.rst.txt	12766	0644	editdlrm
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__future__.rst.txt	4952	0644	editdlrm
__main__.rst.txt	535	0644	editdlrm

:mod:`signal` --- Set handlers for asynchronous events ====================================================== .. module:: signal :synopsis: Set handlers for asynchronous events. This module provides mechanisms to use signal handlers in Python. Some general rules for working with signals and their handlers: * 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 :const:`SIGCHLD`, which follows the underlying implementation. * There is no way to "block" signals temporarily from critical sections (since this is not supported by all Unix flavors). * Although Python signal handlers are called asynchronously as far as the Python user is concerned, they can only occur between the "atomic" instructions of the Python interpreter. This means that signals arriving during long calculations implemented purely in C (such as regular expression matches on large bodies of text) may be delayed for an arbitrary amount of time. * When a signal arrives during an I/O operation, it is possible that the I/O operation raises an exception after the signal handler returns. This is dependent on the underlying Unix system's semantics regarding interrupted system calls. * Because the C signal handler always returns, it makes little sense to catch synchronous errors like :const:`SIGFPE` or :const:`SIGSEGV`. * Python installs a small number of signal handlers by default: :const:`SIGPIPE` is ignored (so write errors on pipes and sockets can be reported as ordinary Python exceptions) and :const:`SIGINT` is translated into a :exc:`KeyboardInterrupt` exception. All of these can be overridden. * Some care must be taken if both signals and threads are used in the same program. The fundamental thing to remember in using signals and threads simultaneously is: always perform :func:`signal` operations in the main thread of execution. Any thread can perform an :func:`alarm`, :func:`getsignal`, :func:`pause`, :func:`setitimer` or :func:`getitimer`; only the main thread can set a new signal handler, and the main thread will be the only one to receive signals (this is enforced by the Python :mod:`signal` module, even if the underlying thread implementation supports sending signals to individual threads). This means that signals can't be used as a means of inter-thread communication. Use locks instead. The variables defined in the :mod:`signal` module are: .. data:: SIG_DFL 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 :const:`SIGQUIT` is to dump core and exit, while the default action for :const:`SIGCHLD` is to simply ignore it. .. data:: SIG_IGN This is another standard signal handler, which will simply ignore the given signal. .. data:: SIG* All the signal numbers are defined symbolically. For example, the hangup signal is defined as :const:`signal.SIGHUP`; the variable names are identical to the names used in C programs, as found in ````. The Unix man page for ':c:func:`signal`' lists the existing signals (on some systems this is :manpage:`signal(2)`, on others the list is in :manpage:`signal(7)`). Note that not all systems define the same set of signal names; only those names defined by the system are defined by this module. .. data:: CTRL_C_EVENT The signal corresponding to the :kbd:`Ctrl+C` keystroke event. This signal can only be used with :func:`os.kill`. Availability: Windows. .. versionadded:: 2.7 .. data:: CTRL_BREAK_EVENT The signal corresponding to the :kbd:`Ctrl+Break` keystroke event. This signal can only be used with :func:`os.kill`. Availability: Windows. .. versionadded:: 2.7 .. data:: NSIG One more than the number of the highest signal number. .. data:: ITIMER_REAL Decrements interval timer in real time, and delivers :const:`SIGALRM` upon expiration. .. data:: ITIMER_VIRTUAL Decrements interval timer only when the process is executing, and delivers SIGVTALRM upon expiration. .. data:: 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. The :mod:`signal` module defines one exception: .. exception:: ItimerError Raised to signal an error from the underlying :func:`setitimer` or :func:`getitimer` implementation. Expect this error if an invalid interval timer or a negative time is passed to :func:`setitimer`. This error is a subtype of :exc:`IOError`. The :mod:`signal` module defines the following functions: .. function:: alarm(time) If *time* is non-zero, this function requests that a :const:`SIGALRM` signal be sent to the process in *time* seconds. Any previously scheduled alarm is canceled (only one alarm can be scheduled at any time). The returned value is then the number of seconds before any previously set alarm was to have been delivered. If *time* is zero, no alarm is scheduled, and any scheduled alarm is canceled. If the return value is zero, no alarm is currently scheduled. (See the Unix man page :manpage:`alarm(2)`.) Availability: Unix. .. function:: 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 :const:`signal.SIG_IGN`, :const:`signal.SIG_DFL` or :const:`None`. Here, :const:`signal.SIG_IGN` means that the signal was previously ignored, :const:`signal.SIG_DFL` means that the default way of handling the signal was previously in use, and ``None`` means that the previous signal handler was not installed from Python. .. function:: pause() Cause the process to sleep until a signal is received; the appropriate handler will then be called. Returns nothing. Not on Windows. (See the Unix man page :manpage:`signal(2)`.) .. function:: setitimer(which, seconds[, interval]) Sets given interval timer (one of :const:`signal.ITIMER_REAL`, :const:`signal.ITIMER_VIRTUAL` or :const:`signal.ITIMER_PROF`) specified by *which* to fire after *seconds* (float is accepted, different from :func:`alarm`) and after that every *interval* seconds. The interval timer specified by *which* can be cleared by setting 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; :const:`signal.ITIMER_REAL` will deliver :const:`SIGALRM`, :const:`signal.ITIMER_VIRTUAL` sends :const:`SIGVTALRM`, and :const:`signal.ITIMER_PROF` will deliver :const:`SIGPROF`. The old values are returned as a tuple: (delay, interval). Attempting to pass an invalid interval timer will cause an :exc:`ItimerError`. Availability: Unix. .. versionadded:: 2.6 .. function:: getitimer(which) Returns current value of a given interval timer specified by *which*. Availability: Unix. .. versionadded:: 2.6 .. function:: set_wakeup_fd(fd) Set the wakeup fd to *fd*. When a signal is received, a ``'\0'`` byte is written to 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; attempting to call it from other threads will cause a :exc:`ValueError` exception to be raised. .. versionadded:: 2.6 .. function:: siginterrupt(signalnum, flag) Change system call restart behaviour: if *flag* is :const:`False`, system calls will be restarted when interrupted by signal *signalnum*, otherwise system calls will be interrupted. Returns nothing. Availability: Unix (see the man page :manpage:`siginterrupt(3)` for further information). Note that installing a signal handler with :func:`signal` will reset the restart behaviour to interruptible by implicitly calling :c:func:`siginterrupt` with a true *flag* value for the given signal. .. versionadded:: 2.6 .. function:: signal(signalnum, handler) 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 :const:`signal.SIG_IGN` or :const:`signal.SIG_DFL`. The previous signal handler will be returned (see the description of :func:`getsignal` above). (See the Unix man page :manpage:`signal(2)`.) When threads are enabled, this function can only be called from the main thread; attempting to call it from other threads will cause a :exc:`ValueError` exception to be raised. The *handler* is called with two arguments: the signal number and the current stack frame (``None`` or a frame object; for a description of frame objects, see the :ref:`description in the type hierarchy ` or see the attribute descriptions in the :mod:`inspect` module). On Windows, :func:`signal` can only be called with :const:`SIGABRT`, :const:`SIGFPE`, :const:`SIGILL`, :const:`SIGINT`, :const:`SIGSEGV`, or :const:`SIGTERM`. A :exc:`ValueError` will be raised in any other case. .. _signal-example: Example ------- Here is a minimal example program. It uses the :func:`alarm` function to limit the time spent waiting to open a file; this is useful if the file is for a serial device that may not be turned on, which would normally cause the :func:`os.open` to hang indefinitely. The solution is to set a 5-second alarm before opening the file; if the operation takes too long, the alarm signal will be sent, and the handler raises an exception. :: import signal, os def handler(signum, frame): print 'Signal handler called with signal', signum raise IOError("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