How to prevent a block of code from being interrupted by KeyboardInterrupt in Python?

Question

I'm writing a program that caches some results via the pickle module. What happens at the moment is that if I hit ctrl-c at while the dump operation is occurring, dump gets interrupted and the resulting file is corrupted (i.e. only partially written, so it cannot be loaded again.

Is there a way to make dump, or in general a block of code, uninterruptable? My current workaround looks something like this:

try:
  file = open(path, 'w')
  dump(obj, file)
  file.close()
except KeyboardInterrupt:
  file.close()
  file.open(path,'w')
  dump(obj, file)
  file.close()
  raise

It seems silly to restart the operation if it is interrupted, so I am searching for a way to defer the interrupt. How do I do this?

Answer 1

Put the function in a thread, and wait for the thread to finish.

Python threads cannot be interrupted except with a special C api.

import time
from threading import Thread

def noInterrupt():
    for i in xrange(4):
        print i
        time.sleep(1)

a = Thread(target=noInterrupt)
a.start()
a.join()
print "done"


0
1
2
3
Traceback (most recent call last):
  File "C:\Users\Admin\Desktop\test.py", line 11, in <module>
    a.join()
  File "C:\Python26\lib\threading.py", line 634, in join
    self.__block.wait()
  File "C:\Python26\lib\threading.py", line 237, in wait
    waiter.acquire()
KeyboardInterrupt

See how the interrupt was deferred until the thread finished?

Here it is adapted to your use:

import time
from threading import Thread

def noInterrupt(path, obj):
    try:
        file = open(path, 'w')
        dump(obj, file)
    finally:
        file.close()

a = Thread(target=noInterrupt, args=(path,obj))
a.start()
a.join()

Answer 2

Use the signal module to disable SIGINT for the duration of the process:

s = signal.signal(signal.SIGINT, signal.SIG_IGN)
do_important_stuff()
signal.signal(signal.SIGINT, s)

Answer 3

In my opinion using threads for this is an overkill. You can make sure the file is being saved correctly by simply doing it in a loop until a successful write was done:

def saveToFile(obj, filename):
    file = open(filename, 'w')
    cPickle.dump(obj, file)
    file.close()
    return True

done = False
while not done:
    try:
        done = saveToFile(obj, 'file')
    except KeyboardInterrupt:
        print 'retry'
        continue