python 在无限循环期间如何处理键盘中断？

Question

When I run this code:

while 1:
    try:
        pass
    except KeyboardInterrupt:
        pass

and I press ctrl+c to try to terminate the program through a KeyboardInterrupt, I am usually, but not always successful. Most of the time, my first attempt at ctrl+c terminates the program, but sometimes, I need to press ctrl+c twice.

Compare that with this code:

from time import sleep

while 1:
    try:
        sleep(0.000000000000000000000000000000000000000000000000000000000000000000001)
    except KeyboardInterrupt:
        pass

With this code, no matter how many times I press ctrl+c , the program never terminates.

My assumption is that in the first scenario, my KeyboardInterrupt usually works because the pass statements executes so quickly that I am more likely to press ctrl+c during the while loop condition check (which is not in the try block) than during the pass statement execution (which is in the try block).

And, in the second example, I assume the sleep function must take much longer to execute than the while loop condition check such that I am almost guaranteed to press ctrl+c during its execution (thus catching the KeyboardInterrupt in the try block and continuing the loop).

Can anyone confirm my assumption or give an alternate reasoning?

Answer 1

Since you are pass ing the keyboard interrupt in both cases, the program just continues with the while loop.

With the second case, due to the sleep in the body of the try block, there is a mu8ch higher chance of your KeyboardInterrupt hitting the process during that sleep - which means your except block is almost guaranteed to be executed. Which means that your program just carries on. You can test this by adding a print in the except block.

from time import sleep

while 1:
    try:
        sleep(0.000000000000000000000000000000000000000000000000000000000000000000001)
    except KeyboardInterrupt:
        print('exc')
        pass

If I run this and press Ctrl+C I get:

>python so_test.py
exc
exc
exc
exc
exc
exc
exc
exc

With each Ctrl+C the except block is executed, but your program continues on.

In the first first case however, due to the lack of any delay (ie sleep ) in the try block, the programs is just as likely to be executing outside the try block (before entering it, or after) when the KeyboardInterrupt hits your process. Which means that your exception is not handled, and that ends the program. If however, the interrupt hits the process inside the try -block, the program will continue. Check this:

while 1:
    try:
        pass
    except KeyboardInterrupt:
        print('exc')
        pass

And you can get this:

>python so_test.py
exc
exc
exc
exc
Traceback (most recent call last):
  File "so_test.py", line 5, in <module>
    pass
KeyboardInterrupt

Hope that clears up the confusion.