Tuple Unpacking in raise

[James Stroud]

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

James

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/

 

[Steven Bethard]

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

 

[Konstantin Veretennicov]

James Stroud wrote:
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)

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

Another workaround would be to use __init__(self, *atup), but raising
an explicitly constructed exception is preferable (IMO).

- kv

 

[James Stroud]

Thank you Steven and Konstantin, that clears things up.

Sometimes I forget how incomplete my Python Essential Reference is.

James

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

--
James Stroud
UCLA-DOE Institute for Genomics and Proteomics
Box 951570
Los Angeles, CA 90095

http://www.jamesstroud.com/