`try ... except not` construction

Question

Recently I met an example of code I've never seen before:

try:
    # a simple bunch of code
    if sample == 0:
        return True
    else:
        raise ExampleError()
except not ExampleError:
    raise AnotherExampleError()

How does it work (if it works at all)?

Answer 1

This is not going to be successful on any version of Python as far as I know. Because the not operator always results in a Boolean value (True or False) this is trying to catch one of those values here, in this case False. Since you can't throw True or False there's no use for this.

I think the intent of the author was something like this:

try:
    raise ExampleError()
except ExampleError e:
    throw e
except:
    raise AnotherExampleError()

Answer 2

EDIT: The answer below was for Python 3, I didn't realise the question related to Python 2.7. in Python 2, as it seems, the interpreter does not complain if the expression after except does not result in a subtype of BaseException. However, the behavior is still wrong, it will just ignore that except block in all cases.

---

That's a funny construction that is syntactically valid but semantically wrong. I suppose the author of the code meant to express something like "run this except block for any exception type but ExampleError". However, what is really happening is more like:

try:
    # ...
except (not bool(ExampleError)):
    # ...

When an exception is raised in the try block, Python goes through the different except blocks looking for one that matches the exception type. When it sees except not ExampleError, equivalent to except (not bool(ExampleError)), it results in except False, which is invalid because False is not a subtype of BaseException (or a tuple of subtypes of BaseException). So the code may even run if no exceptions are raised but is wrong.

Answer 3

A quick test shows that the code will throw a TypeError if it reaches that line:

try:
  raise BaseException
except not BaseException:
  print("Test1")
except BaseException:
  print("Test2")

Exception:

During handling of the above exception, another exception occurred:

Traceback (most recent call last): File "main.py", line 3, in except not BaseException: TypeError: catching classes that do not inherit from BaseException is not allowed