在python中按下CTRL + C时如何优雅地终止循环

Question

我对 python 比较陌生，并且遇到了以下问题。 我有一个脚本，可以逐个处理文件，并根据输入文件名将输出写入单独的文件。 有时我需要中断脚本，但我想让它完成处理当前文件然后终止（以避免结果文件信息不完整）。 如何在 python 中编码这种行为？

这是我尝试过的。

a) Try-except 块

x = 1
print "Script started."
while True:
 try:
  print "Processing file #",x,"started...",
  # do something time-cosnuming
  time.sleep(1)
  x += 1
  print " finished."
 except KeyboardInterrupt:
  print "Bye"
  print "x=",x
  sys.exit()

sys.exit()

输出：

Script started.
Processing file # 1 started...  finished.
Processing file # 2 started...  finished.
Processing file # 3 started... Bye
x= 3

迭代 #3 没有优雅地完成。

b) sys.excepthook

OriginalExceptHook = sys.excepthook
def NewExceptHook(type, value, traceback):
global Terminator
    Terminator = True
    if type == KeyboardInterrupt:
        #exit("\nExiting by CTRL+C.")   # this line was here originally
        print("\n\nExiting by CTRL+C.\n\n")
    else:
        OriginalExceptHook(type, value, traceback)
sys.excepthook = NewExceptHook

global Terminator
Terminator = False

x = 1
while True:
  print "Processing file #",x,"started...",
  # do something time-cosnuming
  time.sleep(1)
  x += 1
  print " finished."
  if Terminator:
   print "I'll be back!"
   break

print "Bye"
print "x=",x
sys.exit()

输出：

Script started.
Processing file # 1 started...  finished.
Processing file # 2 started...  finished.
Processing file # 3 started...

Exiting by CTRL+C.

迭代 #3 没有优雅地完成。

更新#1

@mguijarr ，我稍微修改了这样的代码：

import time, sys

x = 1
print "Script started."
stored_exception=None

while True:
    try:
        print "Processing file #",x,"started...",
        # do something time-cosnuming
        time.sleep(1)
        print "Processing file #",x,"part two...",
        time.sleep(1)
        print " finished."
        if stored_exception:
            break
        x += 1
    except KeyboardInterrupt:
        print "[CTRL+C detected]",
        stored_exception=sys.exc_info()

print "Bye"
print "x=",x

if stored_exception:
    raise stored_exception[0], stored_exception[1], stored_exception[2]

sys.exit()

输出是（在 Win7-64 位上使用“Python 2.7.6 :: Anaconda 2.0.0 (64-bit)”测试）：

Script started.
Processing file # 1 started... Processing file # 1 part two...  finished.
Processing file # 2 started... Processing file # 2 part two...  finished.
Processing file # 3 started... [CTRL+C detected] Processing file # 3 started... Processing file # 3 part two...  finished.
Bye
x= 3
Traceback (most recent call last):
  File "test2.py", line 12, in <module>
    time.sleep(1)
KeyboardInterrupt

在这种情况下，迭代 #3 被有效地重新启动，这看起来很奇怪并且不是所需的行为。 有没有可能避免这种情况？

我删除了“打印”语句中的逗号并添加了更多内容以查看迭代实际上已重新启动：

import time, sys

x = 1
y = 0
print "Script started."
stored_exception=None

while True:
    try:
        y=x*1000
        y+=1
        print "Processing file #",x,y,"started..."
        y+=1
        # do something time-cosnuming
        y+=1
        time.sleep(1)
        y+=1
        print "Processing file #",x,y,"part two..."
        y+=1
        time.sleep(1)
        y+=1
        print " finished.",x,y
        y+=1
        if stored_exception:
            break
        y+=1
        x += 1
        y+=1
    except KeyboardInterrupt:
        print "[CTRL+C detected]",
        stored_exception=sys.exc_info()

print "Bye"
print "x=",x
print "y=",y

if stored_exception:
    raise stored_exception[0], stored_exception[1], stored_exception[2]

sys.exit()

输出是：

Script started.
Processing file # 1 1001 started...
Processing file # 1 1004 part two...
 finished. 1 1006
Processing file # 2 2001 started...
Processing file # 2 2004 part two...
[CTRL+C detected] Processing file # 2 2001 started...
Processing file # 2 2004 part two...
 finished. 2 2006
Bye
x= 2
y= 2007
Traceback (most recent call last):
  File "test2.py", line 20, in <module>
    time.sleep(1)
KeyboardInterrupt

Answer 1

我会简单地使用一个异常处理程序，它会捕获KeyboardInterrupt并存储异常。 然后，在迭代完成的那一刻，如果有异常挂起，我会中断循环并重新引发异常（让正常的异常处理有机会发生）。

这有效（使用 Python 2.7 测试）：

x = 1
print "Script started."
stored_exception=None

while True:
    try:
        print "Processing file #",x,"started...",
        # do something time-cosnuming
        time.sleep(1)
        print " finished."
        if stored_exception:
            break
        x += 1
    except KeyboardInterrupt:
        stored_exception=sys.exc_info()

print "Bye"
print "x=",x

if stored_exception:
    raise stored_exception[0], stored_exception[1], stored_exception[2]

sys.exit()

编辑：正如在评论中发现的那样，这个答案对原始海报并不满意，这是一个基于线程的解决方案：

import time
import sys
import threading

print "Script started."

class MyProcessingThread(threading.Thread):
    def __init__(self):
        threading.Thread.__init__(self)

    def run(self):
        print "Processing file #",x,"started...",
        # do something time-cosnuming
        time.sleep(1)
        print " finished."

for x in range(1,4):
    task = MyProcessingThread()
    task.start()
    try:
        task.join()
    except KeyboardInterrupt:
        break

print "Bye"
print "x=",x

sys.exit()

Answer 2

你可以写一个信号处理函数

import signal,sys,time                          
terminate = False                            

def signal_handling(signum,frame):           
    global terminate                         
    terminate = True                         

signal.signal(signal.SIGINT,signal_handling) 
x=1                                          
while True:                                  
    print "Processing file #",x,"started..." 
    time.sleep(1)                            
    x+=1                                     
    if terminate:                            
        print "I'll be back"                 
        break                                
print "bye"                                  
print x

按 Ctrl+c 会发送一个 SIGINT 中断，它将输出：

Processing file # 1 started...
Processing file # 2 started...
^CI'll be back
bye
3

Answer 3

我觉得创建一个具有处理用户异常状态的类更优雅一些，因为我不必弄乱不能跨不同模块工作的全局变量

import signal
import time

class GracefulExiter():

    def __init__(self):
        self.state = False
        signal.signal(signal.SIGINT, self.change_state)

    def change_state(self, signum, frame):
        print("exit flag set to True (repeat to exit now)")
        signal.signal(signal.SIGINT, signal.SIG_DFL)
        self.state = True

    def exit(self):
        return self.state


x = 1
flag = GracefulExiter()
while True:
    print("Processing file #",x,"started...")
    time.sleep(1)
    x+=1
    print(" finished.")
    if flag.exit():
        break