DelayedInterrupt class - delaying the handling of process signals in Python

delayedinterrupt.py

import signal
import logging


# class based on: http://stackoverflow.com/a/21919644/487556
class DelayedInterrupt(object):
    def __init__(self, signals):
        if not isinstance(signals, list) and not isinstance(signals, tuple):
            signals = [signals]
        self.sigs = signals        

    def __enter__(self):
        self.signal_received = {}
        self.old_handlers = {}
        for sig in self.sigs:
            self.signal_received[sig] = False
            self.old_handlers[sig] = signal.getsignal(sig)
            def handler(s, frame):
                self.signal_received[sig] = (s, frame)
                # Note: in Python 3.5, you can use signal.Signals(sig).name
                logging.info('Signal %s received. Delaying KeyboardInterrupt.' % sig)
            self.old_handlers[sig] = signal.getsignal(sig)
            signal.signal(sig, handler)

    def __exit__(self, type, value, traceback):
        for sig in self.sigs:
            signal.signal(sig, self.old_handlers[sig])
            if self.signal_received[sig] and self.old_handlers[sig]:
                self.old_handlers[sig](*self.signal_received[sig])

test_delayedinterrupt.py

import os
import signal
from delayedinterrupt import DelayedInterrupt
from mock import Mock


def test_delayed_interrupt_with_one_signal():
    # check behavior without DelayedInterrupt
    a = Mock()
    b = Mock()
    c = Mock()
    try:
        a()
        os.kill(os.getpid(), signal.SIGINT)
        b()
    except KeyboardInterrupt:
        c()
    a.assert_called_with()
    b.assert_not_called()
    c.assert_called_with()

    # test behavior with DelayedInterrupt
    a = Mock()
    b = Mock()
    c = Mock()
    try:
        with DelayedInterrupt(signal.SIGINT):
            a()
            os.kill(os.getpid(), signal.SIGINT)
            b()
    except KeyboardInterrupt:
        c()
    a.assert_called_with()
    b.assert_called_with()
    c.assert_called_with()


def test_delayed_interrupt_with_multiple_signals():
    a = Mock()
    b = Mock()
    c = Mock()
    try:
        with DelayedInterrupt([signal.SIGTERM, signal.SIGINT]):
            a()
            os.kill(os.getpid(), signal.SIGINT)
            os.kill(os.getpid(), signal.SIGTERM)
            b()
    except KeyboardInterrupt:
        c()
    a.assert_called_with()
    b.assert_called_with()
    c.assert_called_with()

This has a serious bug: handler captures sig by reference, not value, so all intercepted signals will be treated as though they were the last signal in the list. In test_delayed_interrupt_with_multiple_signals, SIGTERM is never delivered; instead it's "converted" into SIGINT. To fix this add sig=sig to the parameter list.

Another way to fix that bug is using functools.partial

To fix this add sig=sig to the parameter list.

@benrg Can you clarify which method's parameter list you are referring to?

@irvinlim He's referring to line 18

def handler(s, frame):
    self.signal_received[sig] = (s, frame)
                       #  ^ this sig is captured by reference

should be

def handler(s, frame, sig=sig):
    self.signal_received[sig] = (s, frame)

See this answer on SO

Wanted to use this for my project so I refactored it in a way I consider more maintainable, and added an option for early exits if an interrupt is repeated enough times, as well as a subclass for similarly ignoring (not delaying) exceptions, unless early exit is triggered.

import logging
import signal

class DelaySignals:
    def __init__( self, signals_to_delay:list[signal.Signals]=signal.SIGINT, unless_repeated_n_times:int=False ):
        """
        A class which intercepts chosen incoming signals after __enter__ and delays them till __exit__ is reached.
        
        Args:
            signals_to_delay (list[signal.Signals], optional):  The signal or list/tuple of signals to delay. Defaults to signal.SIGINT.
            unless_repeated_n_times (int|bool, optional):       If a signal is received N amount of times, allow it through by exiting the class prematurely. Defaults to False.
            
        Example usage:
            ```py
            with DelaySignals( signal.SIGINT ):
                time.sleep(10)  # If a SIGINT signal  (KeyboardInterrupt)  is received in this scope, it is delayed until the scope is exited
            ```
    
        Based on: 
            http://stackoverflow.com/a/21919644/487556
            https://gist.github.com/tcwalther/ae058c64d5d9078a9f333913718bba95
        """
        self.signals_to_delay           = signals_to_delay if type(signals_to_delay) in [list, tuple] else [signals_to_delay]
        self.unless_repeated_n_times    = unless_repeated_n_times


    def __enter__( self ):
        self.inboxes = {}
        for sig_type in self.signals_to_delay:
            self.inboxes[sig_type] = {'received': [], 'handler': signal.getsignal(sig_type)}
            signal.signal( sig_type, self.signal_handler )


    def signal_handler( self, sig, frame ):
        self.inboxes[sig]['received'].append( (sig, frame) )
        
        if self.unless_repeated_n_times and len( self.inboxes[sig]['received'] ) >= self.unless_repeated_n_times: 
            logging.warn( f"{__class__.__name__}: Signal {sig} repeated enough times ({self.unless_repeated_n_times}) to pass through, exiting early." )
            self.__exit__()
        else:
            logging.info( f"{__class__.__name__}: Signal {sig} handled." )

    
    def __exit__( self, *_ ):
        for sig, inbox in self.inboxes.items():
            signal.signal( sig, inbox['handler'] )
            if inbox['handler']:    
                for msg in inbox['received']: inbox['handler']( *msg )
            else:                   
                logging.warn(f"{__class__.__name__}: Signal {sig} had no prior handler, skipping.")


class IgnoreSignals(DelaySignals):
    def __exit__( self, *_ ): 
        for sig, inbox in self.inboxes.items():
            signal.signal( sig, inbox['handler'] )
            if inbox['handler']:
                if len(inbox['received']) >= self.unless_repeated_n_times: inbox['handler']( *inbox['received'][0] )
            else:
                logging.warn(f"{__class__.__name__}: Signal {sig} had no prior handler, skipping.")