CODEWITHSUNDEEP
pythonsignals
noname7619
noname7619

Reputation: 3758

SIGINT and exception handling in Python

import signal
import sys
import time

def sigint_handler(signal, frame):
    print "signal"
    sys.exit(0)

signal.signal(signal.SIGINT, sigint_handler)

while 1:
    try:
        print "text"
        time.sleep(2)
    except KeyboardInterrupt:
        print "keybi"
        exit(0)
    except:
        print "except"
        continue

When I press Ctrl-C I see "signal" and "except" and the program doesn't exit.

  1. Why doesn't the program exit, while it apparently reaches sys.exit(0)?

  2. Why doesn't the program flow reach the KeyboardInterrupt section?

  3. What is the concise way to make Ctrl-C work and handle each except: case separately in different places without exiting?

Upvotes: 8

Views: 14339

Answers (2)

cdarke
cdarke

Reputation: 44354

@user2357112 answered your first two questions, as to your third, you can create your own exception in the signal handler. In this example we raise MyError in the event of a SIGINT.

import signal
import time

class MyError(Exception): 
    pass

def handler(sig, frame):
    raise MyError('Received signal ' + str(sig) +
                  ' on line ' + str(frame.f_lineno) +
                  ' in ' + frame.f_code.co_filename)    

signal.signal(signal.SIGINT, handler)

try:
    while 1:
        time.sleep(1)        # Hit <CTRL>+C here
except KeyboardInterrupt:
    print('Keyboard interrupt caught')
except MyError as err:
    print("Hiccup:",err)
    # do stuff

print('Clean exit')

If you comment out the call to signal.signal() then you will get a KeyboardInterrupt, as described.

Upvotes: 3

user2357112
user2357112

Reputation: 280465

The program doesn't exit because sys.exit works by throwing a SystemExit exception, and your blanket except caught it.

The except KeyboardInterrupt doesn't fire because the SIGINT handler you installed overrides the default SIGINT handler, and the default SIGINT handler is responsible for raising a KeyboardInterrupt when you hit Ctrl-C.

As for your third question, it's unclear what you're asking.

Upvotes: 11

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