except BaseException or except Exception

I've recently become the 'python expert' in a small company that had a bunch of people who were writing python code, but didn't really have anyone that had studied python. Our code had a lot of cases of this:

try:
    <probably not foolish thing>
except:
    <deal with exceptions>

I've been dutifully correcting it to, and teaching the others to correct it to, this:

try:
    <probably not foolish thing>
except Exception as err:
    <deal with err>

But today I was code reviewing a change where a colleague had applied an automated linter, and one of those instances had been replaced by

try:
    <probably not foolish thing>
except BaseException as err:
    <deal with err>

I had a feeling this was wrong. But I didn't want to just go with a feeling on it and tried to google what circumstances I should use which in. I failed, so I asked twitter.

Essentially, Exception is a subclass of BaseException. So are several things that you probably really do want to kill your program (the one most noted in the twitter replies was ctrl + c / keyboard interrupt). So, except BaseException does what a bare except did, which is why the linter suggested it. But except Exception is almost certainly what you meant.


From the python docs

BaseException
 +-- SystemExit
 +-- KeyboardInterrupt
 +-- GeneratorExit
 +-- Exception
      +-- StopIteration
      +-- StopAsyncIteration
      +-- ArithmeticError
      |    +-- FloatingPointError
      |    +-- OverflowError
      |    +-- ZeroDivisionError
      +-- AssertionError
      +-- AttributeError
      +-- BufferError
      +-- EOFError
      +-- ImportError
      |    +-- ModuleNotFoundError
      +-- LookupError
      |    +-- IndexError
      |    +-- KeyError
      +-- MemoryError
      +-- NameError
      |    +-- UnboundLocalError
      +-- OSError
      |    +-- BlockingIOError
      |    +-- ChildProcessError
      |    +-- ConnectionError
      |    |    +-- BrokenPipeError
      |    |    +-- ConnectionAbortedError
      |    |    +-- ConnectionRefusedError
      |    |    +-- ConnectionResetError
      |    +-- FileExistsError
      |    +-- FileNotFoundError
      |    +-- InterruptedError
      |    +-- IsADirectoryError
      |    +-- NotADirectoryError
      |    +-- PermissionError
      |    +-- ProcessLookupError
      |    +-- TimeoutError
      +-- ReferenceError
      +-- RuntimeError
      |    +-- NotImplementedError
      |    +-- RecursionError
      +-- SyntaxError
      |    +-- IndentationError
      |         +-- TabError
      +-- SystemError
      +-- TypeError
      +-- ValueError
      |    +-- UnicodeError
      |         +-- UnicodeDecodeError
      |         +-- UnicodeEncodeError
      |         +-- UnicodeTranslateError
      +-- Warning
           +-- DeprecationWarning
           +-- PendingDeprecationWarning
           +-- RuntimeWarning
           +-- SyntaxWarning
           +-- UserWarning
           +-- FutureWarning
           +-- ImportWarning
           +-- UnicodeWarning
           +-- BytesWarning
           +-- ResourceWarning