Python: bare except statement - why it is bad.

Posted by Moser on 20 Sep 2017

The bare except statement is something I often see in the main loop of a script that should run indefinitely. Pylint will warn you about this problem, but in my opinion we should start to treat this as an error.

Consider the following code snippet:

import sys
import time

def risky_business():
  time.sleep(1)

while(True):
  try:
    risky_business()
  except:
    print("Problems?", sys.exc_info())

This will work nicely to keep your program running whatever happends inside you risky_business function. But let’s try to stop your program by pressing CTRL-C.

$ python a.py 
^C('Problems?', (<type 'exceptions.KeyboardInterrupt'>, KeyboardInterrupt(), <traceback object at 0x7f234f6c9dd0>))

We caught the KeyboardInterrupt exception and just carried on. But we catch more which we should not. Let’s try to exit the program from the business function, maybe because the user asked for it or we got signaled.

def risky_business():
  time.sleep(1)
  sys.exit(0)
$ python a.py 
('Problems?', (<type 'exceptions.SystemExit'>, SystemExit(0,), <traceback object at 0x7f49b7ab5e18>))
('Problems?', (<type 'exceptions.SystemExit'>, SystemExit(0,), <traceback object at 0x7f49b7ab5ea8>))
...

So why is this happening? Both KeyboardInterrupt and SystemExit are exceptions (derived from BaseException). An except statement without any restriction will catch any exception derived from BaseException. Here is the complete hierachy of the builtin exceptions.

We can fix our problem by restricting the kind of exception that we want to catch:

def risky_business():
  time.sleep(1)

while(True):
  try:
    risky_business()
  except Exception:
    print("Problems?", sys.exc_info())

When we press CTRL-C now, the program exits with the right exception.

$ python a.py
^CTraceback (most recent call last):
  File "a.py", line 10, in <module>
    risky_business()
  File "a.py", line 6, in risky_business
    time.sleep(1)
KeyboardInterrupt

Pylint will still warn you about this line, because catching Exception is still very broad and you should only do this when you have a good reason, e.g. the reason mentioned above (to keep a service running even if it could not handle a certain input or request).

So next time you see a bare except statement in your code, take a moment to reconsider if you really want to catch SystemExit, KeyboardInterrupt (and GeneratorExit which I did not mention above). If that is the case, you might want to make it explicit so that the next person reading the code does not have to check again.

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