Tuple Unpacking in raise

[Steven Bethard]

James Stroud wrote:
> Hello All,
> 
> Is this a bug? Why is this tuple getting unpacked by raise? Am I missing some 
> subtle logic? Why does print not work the same way as raise? Both are 
> statements. Why does raise need to be so special?
> 
> py> sometup = 1,2
> py> print sometup
> (1, 2)
> py> print 1,2,3, sometup
> 1 2 3 (1, 2)
> py> class MyErr(Exception):
> ...   def __init__(self, atup):
> ...     Exception.__init__(self, "Error with %s-%s" % atup)
> ...
> py> raise MyErr, sometup
> Traceback (most recent call last):
>   File "<stdin>", line 1, in ?
> TypeError: __init__() takes exactly 2 arguments (3 given)
> py> e = MyErr(sometup)
> py> print e
> Error with 1-2

Well, it's not a bug, because that's what the documentation says it'll do:

"The second object is used to determine the exception value: If it is an 
instance of the class, the instance becomes the exception value. If the 
second object is a tuple, it is used as the argument list for the class 
constructor; if it is None, an empty argument list is used, and any 
other object is treated as a single argument to the constructor."[1]

In the example above (as well as almost all code), there's no need to 
use the two argument version of raise.  You can simply write:

     raise MyErr(sometup)

If you need the three argument version of raise, I believe you can still 
write it as:

     raise MyErr(sometup), None, tb

Note that Guido has mentioned a few times that in Python 3.0, he wants 
tracebacks to be attributes of the Exception objects so that all raise 
statements are like the one argument version.

STeVe

P.S. If you insist on using the two argument version of raise, you can 
do it like this:

py> class E(Exception):
...     def __init__(self, atup):
...         Exception.__init__(self, "Error with %s-%s" % atup)
...
py> raise E, ((1, 2),)
Traceback (most recent call last):
   File "<interactive input>", line 1, in ?
E: Error with 1-2

But that seems a lot less elegant than simply using the one argument 
version.

[1] http://docs.python.org/ref/raise.html