Avoid using bare except in Python

In Python, keyboard interrupts and system exits are propagated using exceptions (i.e. KeyboardInterrupt and SystemExit). As a result, a bare except clause is going to catch something like the user hitting Ctrl + C.

Consider the following code. If the user were to try exiting the program, the keyboard interrupt would be caught by the except clause. This would be undesirable, as it prevents the user from actually exiting the program until they provide valid input.

while True:
  try:
    s = input('Input a number:')
    x = int(s)
  except:
    print('Not a number, try again!')

A way to prevent this would be to use Exception which will ensure that the user will not be trapped. The only problem with this approach is that Exception is generic and will handle pretty much anything thrown at it.

while True:
  try:
    s = input('Input a number:')
    x = int(s)
  except Exception:
    print('Not a number, try again!')

The correct way to handle errors is to specify the type of error you expect. For example, in this code sample, ValueError would be appropriate.

while True:
  try:
    s = input('Input a number:')
    x = int(s)
  except ValueError:
    print('Not a number, try again!')

As a rule of thumb, you should only handle expected failure states using except with an appropriate error type. In the case of unexpected errors, it might be better to simply let the program fail naturally and exit.