Python:为什么即使在退出后也会调用except

Question

请检查下面的代码，

import sys
try:
    seq=eval(raw_input("Enter seq number: "))
    if seq <= 0 or seq >= 9999:
       print "Sequence number not in range [0001-9999]"
       sys.exit(1)
except:
      print "!!! Sequence number not in range [0001-9999]"
      sys.exit(1)

我给 eval raw_input 函数一个字符串。

$> python test.py
Enter seq number: "12"


Sequence number not in range [0001-9999]
!!! Sequence number not in range [0001-9999]

为什么在收到 exit 调用后还不退出？

Answer 1

sys.exit 只是引发一个异常 (SystemExit)，然后被捕获。作为演示:

import sys
import traceback

try:
    sys.exit(1)
except:
    print "This threw the following exception:"
    traceback.print_exc()
# This threw the following exception:
# Traceback (most recent call last):
#   File "test.py", line 5, in <module>
#     sys.exit(1)
# SystemExit: 1