Is it safe to call print in a Python signal handler?

Is it safe to call print in a Python signal handler?

We learned earlier that because Python has two signal handlers, the onerous restrictions on what functions a signal handler may call do not apply to Python, because CPython does not call the user-supplied Python signal handler inside the low-level C signal handler, where those restrictions do apply, but arranges for it to be called later, when the interpreter is in a consistent state.

We also learned that Python signal handlers are unexpectedly reentrant – if a signal arrives while a Python signal handler is running, the signal handler can be called again in the middle of the first call.

What happens if a signal handler is reentered in the middle of a call to print ? Let's stress-test it by sending ourselves a rapid barrage of signals:

import os , signal , subprocess def sighandler ( _signo , _frame ): print ( "signal received" ) signal . signal ( signal . SIGUSR1 , sighandler ) subprocess . run ( "for x in {1..50}; do kill -USR1 %s ; done" % os . getpid (), shell = True )

Running this program on my machine produced:

File "multiple_signals.py", line 6, in sighandler print("signal received") File "multiple_signals.py", line 6, in sighandler print("signal received") File "multiple_signals.py", line 6, in sighandler print("signal received") [Previous line repeated 2 more times] RuntimeError: reentrant call inside <_io.BufferedWriter name='<stdout>'>