python interrupt, sigterm, sigkill, exception handling experiments

When thinking both Exceptions and interrupts at the same time, things can get confusing so here I write down some simple experiments that I did to clear some confusing concepts.

sigkill can’t be catched

signal handling can be done with signal package that is included in python by default. Last time I read the docs, there was SIGKILL so I blatantly thought KILL signal can also be caught. But even in the docs, it says that it cannot catch it.

I ran this under Ubuntu environment, python3.8.10

import signal

def kill\_handler(signum, frame):
    print("kill hanlder")

signal.signal(signal.SIGKILL, kill\_handler)

print("hello")

and the script even fails to run.

Traceback (most recent call last):
  File "t2.py", line 7, in <module>
    signal.signal(signal.SIGKILL, kill_handler)
  File "/usr/lib/python3.8/signal.py", line 47, in signal
    handler = _signal.signal(_enum_to_int(signalnum), _enum_to_int(handler))
OSError: [Errno 22] Invalid argument

KeyBoardInterrupt cannot be catched by try..except Exception

I just thought Ctrl+C keyboard interrupt was just another Exception subclass, so I expected the following catch statement to be able to catch it but I was surprised to see that it didn’t.

import signal

try:
    print("entering loop")
    while True:
        pass
except Exception as e:
    print("exception catched")

output:

entering loop
^CTraceback (most recent call last):
  File "t2.py", line 6, in <module>
    pass
KeyboardInterrupt

As you can see the KeyboardInterrupt was not catched by except Exception ... statement.

From the python docs:

exception KeyboardInterrupt
Raised when the user hits the interrupt key (normally Control-C or Delete). During execution, a check for interrupts is made regularly. The exception inherits from BaseException so as to not be accidentally caught by code that catches Exception and thus prevent the interpreter from exiting.

So I have mistaken KeyboardInterrupt as a subclass of Exception when it wasn’t. The proper way to handle this exception would be:

try:
    print("entering loop")
    while True:
        pass
except KeyboardInterrupt:
    print("keyboardinterrupt detected")
except Exception as e:
    print("exception catched")

output:

entering loop
^Ckeyboardinterrupt detected

SIGINT signal handler comes first than except KeyboardInterrupt

However, signal package also allows to handle keyboard interrupts through SIGINT. In this case which one would be prioritized? The following code puts this to the test:

import signal, sys

def int\_handler(signum, frame):
    print("sigint handler")
    sys.exit(0)

signal.signal(signal.SIGINT, int\_handler)

try:
    print("entering loop")
    while True:
        pass
except KeyboardInterrupt:
    print("keyboardinterrupt detected")
except Exception as e:
    print("exception catched")

output:

entering loop
^Csigint handler

The SIGINT handler comes first.

SIGTERM handler works when terminated from outside

say we run the following script:

import signal, sys, os

def term\_handler(signum, frame):
    print("sig term handler")
    sys.exit(0)

signal.signal(signal.SIGTERM, term\_handler)

print(f"pid: {os.getpid()}")
print("entering loop")
try:
    while True:
        pass
except Exception as e:
    print("exception detected")
except BaseException:
    print("base exception detected")

# output:
# pid: 18927
# entering loop

and from another shell, we terminate (not kill) it with cmd:

$ sudo kill 18927

then we get the following output from the python script running terminal:

pid: 18927
entering loop
sig term handler
base exception detected

we can confirm that SIGTERM is handled well as expected and is not somehow filtered by except Exception. But surprisingly even after sigint handler, it is caught by except BaseException statement. As it turns out, the BaseException was raised by sys.exit(0) in the sigint handler. I guess the calling of sigint handler was still inside the context of try...except and thus the call of sys.exit(0) triggered except BaseException.

SIGTERM handler is not triggered by killing the process from outside

With the same code above, I tested $ sudo kill -9 <pid> which sends a KILL signal instead of TERMINATE signal. The result terminal output was something like this:

pid: 19738
entering loop
Killed

As expected, it failed to even call sigint handler.