Python except None

Question

Are there any unexpected side effects of using "except None"? The behavior I expect is that nothing will be caught by that clause which a few small tests seem to confirm.

Here is a rough outline of what I am trying to do. When no argument is provided to the function, exceptions=None which creates the "except None" clause. Just want to double check that I'm not going to catch something unexpected.

# exceptions is exception or set of exceptions I want to do special processing for
def check_exceptions(exceptions=None)
  try:                                                                            
    ...                                                                    
  except exceptions as e:                                                       
    ...                                                                 

Answer 1

Works fine here (under Python 2.x).

>>> try:
...   foo
... except None as e:
...   pass
... 
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
NameError: name 'foo' is not defined

For an except clause with an expression, that expression is evaluated, and the clause matches the exception if the resulting object is “compatible” with the exception. An object is compatible with an exception if it is the class or a base class of the exception object, or a tuple containing an item compatible with the exception.

source

Therefore the expression doesn't have to be an exception type, it will simply fail to ever match.

This behavior was changed in Python 3.x, and the expression after except must be a descendant of BaseException or a tuple of such.

Answer 2

Python 3

def check_exceptions(exceptions=())
  try:                                                                            
    ...                                                                    
  except exceptions as e:                                                       
    ...