Why raising a tuple works if first element is an Exception?

Question

I have a hard time figuring this one out, it\'s about mistakes that can be done when raising an exception in Python 2.7:

try:
  raise [1, 2, 3, 4]
except Excepti         


        

Answer 1

As documented in the Python 2 reference, the raise statement takes up to 3 expressions to create the exception being raised:

raise_stmt ::= "raise" [expression ["," expression ["," expression]]]

If the first expression is a tuple, python will 'unwrap' the tuple recursively, taking the first element until it finds something other than a tuple. This behavior is being removed from Python 3 (see PEP 3109). The following is legal:

>>> raise ((Exception, 'ignored'), 'ignored'), 'something', None
Traceback (most recent call last):
  File "", line 1, in 
Exception: something

The documentation explains the rest in more detail, but the raise statement expects the first value to be a Exception class, the second value is seen as the value of the exception (the message) and the third value is a traceback. Python fills in None for the latter two values if missing.

If the first value is a instance instead, the second value must be None:

>>> raise Exception('something'), 'something', None
Traceback (most recent call last):
  File "", line 1, in 
TypeError: instance exception may not have a separate value

If you use a tuple of more than 3 items, it'll raise a syntax error:

>>> raise Exception, 'something', None, None
  File "", line 1
    raise Exception, 'something', None, None
                                      ^
SyntaxError: invalid syntax

In your case however, you raised neither a class nor an instance, so that's what Python found to be incorrect first; if I use a string it'll complain too:

>>> raise 'not an exception', 'something', None
Traceback (most recent call last):
  File "", line 1, in 
TypeError: exceptions must be old-style classes or derived from BaseException, not str

The correct syntax is of course:

>>> raise Exception, 'something', None
Traceback (most recent call last):
  File "", line 1, in 
Exception: something