Re-raising exceptions with modified message

[Christoph Zwerschke]

What is the best way to re-raise any exception with a message
supplemented with additional information (e.g. line number in a
template)? Let's say for simplicity I just want to add "sorry" to every
exception message. My naive solution was this:

try:
...
except Exception, e:
raise e.__class__, str(e) + ", sorry!"

This works pretty well for most exceptions, e.g.

>>try:

.... 1/0
.... except Exception, e:
.... raise e.__class__, str(e) + ", sorry!"
....
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
ZeroDivisionErr or: integer division or modulo by zero, sorry!

But it fails for some exceptions that cannot be instantiated with a
single string argument, like UnicodeDecodeEr ror which gets "converted"
to a TypeError:

>>try:

.... unicode('\xe4')
.... except Exception, e:
.... raise e.__class__, str(e) + ", sorry!"
....
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
TypeError: function takes exactly 5 arguments (1 given)

Another approach is using a wrapper Extension class:

class SorryEx(Excepti on):
def __init__(self, e):
self._e = e
def __getattr__(sel f, name):
return getattr(self._e , name)
def __str__(self):
return str(self._e) + ", sorry!"

try:
unicode('\xe4')
except Exception, e:
raise SorryEx(e)

But then I get the name of the wrapper class in the message:

__main__.SorryE x: 'ascii' codec can't decode byte 0xe4 in position 0:
ordinal not in range(128), sorry!

Yet another approach would be to replace the __str__ method of e, but
this does not work for new style Exceptions (Python 2.5).

Any suggestions?

-- Chris

[samwyse]

On Jul 12, 11:48 am, samwyse <samw...@gmail. comwrote:

On Jul 12, 6:31 am,samwyse<samw [e-mail] ote:

On Jul 8, 8:50 am, Christoph Zwerschke <[e-mail] wrote:

With Py 2.5 I get:

new.__class__ = old.__class__
TypeError: __class__ must be set to a class

Hmmm, under Python 2.4.X, printing repr(old.__clas s__) gives me this:
<class exceptions.Unic odeDecodeError at 0x00A24F00>
while under 2.5.X, I get this:
<type 'exceptions.Uni codeDecodeError '>

So, let's try sub-classing the type:

def modify_message( old, f):
class Empty: pass
new = Empty()
print "old.__clas s__ =", repr(old.__clas s__)
print "Empty =", repr(Empty)
new.__class__ = Empty

class Excpt(old.__cla ss__): pass
print "Excpt =", repr(Excpt)
print "Excpt.__class_ _ =", repr(Excpt.__cl ass__)
new.__class__ = Excpt

new.__dict__ = old.__dict__.co py()
new.__str__ = f
return new

Nope, that gives us the same message:

old.__class__ = <type 'exceptions.Uni codeDecodeError '>
Empty = <class __main__.Empty at 0x00AB0AB0>
Excpt = <class '__main__.Excpt '>
Excpt.__class__ = <type 'type'>
Traceback (most recent call last):
[...]
TypeError: __class__ must be set to a class

Excpt certainly appears to be a class. Does anyone smarter than me
know what's going on here?

OK, in classobject.h, we find this:

#define PyClass_Check(o p) ((op)->ob_type == &PyClass_Typ e)

That seems straightforward enough. And the relevant message appears
in classobject.c here:

static int
instance_setatt r(PyInstanceObj ect *inst, PyObject *name, PyObject *v)
[...]
if (strcmp(sname, "__class__" ) == 0) {
if (v == NULL || !PyClass_Check( v)) {
PyErr_SetString (PyExc_TypeErro r,
"__class__ must be set to a class");
return -1;
}

Back in our test code, we got these:

Empty = <class __main__.Empty at 0x00AB0AB0>
Excpt = <class '__main__.Excpt '>

The first class (Empty) passes the PyClass_Check macro, the second one
(Excpt) evidently fails. I'll need to dig deeper. Meanwhile, I still
have to wonder why the code doesn't allow __class_ to be assigned a
type instead of a class. Why can't we do this in the C code (assuming
the appropriate PyType_Check macro):

if (v == NULL || !(PyClass_Check (v) || PyType_Check(v) )) {

[samwyse]

(Yes, I probably should have said CPython in my subject, not Python.
Sorry.)

On Jul 13, 12:56 am, samwyse <samw...@gmail. comwrote:

OK, in classobject.h, we find this:

#define PyClass_Check(o p) ((op)->ob_type == &PyClass_Typ e)

That seems straightforward enough. And the relevant message appears
in classobject.c here:

static int
instance_setatt r(PyInstanceObj ect *inst, PyObject *name, PyObject *v)
[...]
if (strcmp(sname, "__class__" ) == 0) {
if (v == NULL || !PyClass_Check( v)) {
PyErr_SetString (PyExc_TypeErro r,
"__class__ must be set to a class");
return -1;
}

Back in our test code, we got these:

Empty = <class __main__.Empty at 0x00AB0AB0>
Excpt = <class '__main__.Excpt '>

The first class (Empty) passes the PyClass_Check macro, the second one
(Excpt) evidently fails. I'll need to dig deeper. Meanwhile, I still
have to wonder why the code doesn't allow __class_ to be assigned a
type instead of a class. Why can't we do this in the C code (assuming
the appropriate PyType_Check macro):

if (v == NULL || !(PyClass_Check (v) || PyType_Check(v) )) {

After a good night's sleep, I can see that Empty is a "real" class;
i.e. its repr() is handled by class_repr() in classobject.c. Excpt,
on the other hand, is a type; i.e. its repr is handled by type_repr()
in typeobject.c. (You can tell because class_repr() returns a value
formatted as "<class %s.%s at %p>" whereas type_repr returns a value
formatted as "<%s '%s.%s'>", where the first %s gets filled with
either "type" or "class".)

This is looking more and more like a failure to abide by PEP 252/253.
I think that the patch is simple, but I'm unusre of the
ramifications. I also haven't looked at the 2.4 source to see how
things used to work. Still, I think that I've got a work-around for
OP's problem, I just need to test it under both 2.4 and 2.5.

[samwyse]

On Jul 13, 12:45 am, Christoph Zwerschke <[e-mail] wrote:

samwyse wrote:

TypeError: __class__ must be set to a class

Excpt ceratinly appears to be a class. Does anyone smarter than me
know what's going on here?

Not that I want to appear smarter, but I think the problem here is that
exceptions are new-style classes now, whereas Empty is an old-style
class. But even if you define Empty as a new-style class, it will not
work, you get:

TypeError: __class__ assignment: only for heap types

This tells us that we cannot change the attributes of a built-in
exception. If it would be possible, I simply would have overridden the
__str__ method of the original exception in the first place.

-- Chris

Chris, you owe me a beer if you're ever in St. Louis, or I'm ever in
Germany.

# ----- CUT HERE -----

# Written by Sam Denton <sa*****@gmail. com>
# You may use, copy, or distribute this work,
# as long as you give credit to the original author.

# tested successfully under Python 2.4.1, 2.4.3, 2.5.1

"""
On Jul 5, 2007, at 8:53 am, Christoph Zwerschke <[e-mail] >
wrote:

What is the best way to re-raise any exception with a message
supplemented with additional information (e.g. line number in a
template)? Let's say for simplicity I just want to add "sorry" to
every exception message.

Here is an example of typical usage:

>>def typical_usage(c ode):

.... try:
.... code()
.... except Exception, e:
.... simplicity = lambda self: str(e) + ", sorry!"
.... raise modify_message( e, simplicity)

Note that if we want to re-cycle the original exception's message,
then we need our re-formatter (here called 'simplicity') to be
defined inside the exception handler. I tried verious approaches
to defining the re-formater, but all of them eventually needed a
closure; I decided that I liked this approach best.

This harness wraps the example so that doctest doesn't get upset.

>>def test_harness(co de):

.... try:
.... typical_usage(c ode)
.... except Exception, e:
.... print "%s: %s" % (e.__class__.__ name__, str(e))

Now for some test cases:

>>test_harness( lambda: 1/0)

ZeroDivisionErr or: integer division or modulo by zero, sorry!

>>test_harness( lambda: unicode('\xe4') )

UnicodeDecodeEr ror: 'ascii' codec can't decode byte 0xe4 in position
0: ordinal not in range(128), sorry!

"""

def modify_message( old, f):
"""modify_messa ge(exception, mutator) --exception

Modifies the string representation of an exception.
"""
class NewStyle(old.__ class__):
def __init__(self): pass
NewStyle.__name __ = old.__class__._ _name__
NewStyle.__str_ _ = f
new = NewStyle()
new.__dict__ = old.__dict__.co py()
return new

def _test():
import doctest
return doctest.testmod (verbose=True)

if __name__ == "__main__":
_test()

[Christoph Zwerschke]

samwyse wrote:

NewStyle.__name __ = old.__class__._ _name__

Simple, but that does the trick!

new.__dict__ = old.__dict__.co py()

Unfortunately, that does not work, since the attributes are not
writeable and thus do not appear in __dict__.

But my __getattr__ solution does not work either, since the attributes
are set to None when initialized, so __getattr__ is never called.

Need to think about this point some more...

Anyway, the beer is on me ;-)

-- Chris

[Christoph Zwerschke]

Christoph Zwerschke wrote:

But my __getattr__ solution does not work either, since the attributes
are set to None when initialized, so __getattr__ is never called.

Here is a simple solution, but it depends on the existence of the args
attribute that "will eventually be deprecated" according to the docs:

def PoliteException (e):
E = e.__class__
class PoliteException (E):
def __str__(self):
return str(e) + ", sorry!"
PoliteException .__name__ = E.__name__
return PoliteException (*e.args)

try:
unicode('\xe4')
except Exception, e:
p = PoliteException (e)
assert p.reason == e.reason
raise p

[Christoph Zwerschke]

Christoph Zwerschke wrote:

Here is a simple solution, but it depends on the existence of the args
attribute that "will eventually be deprecated" according to the docs:

Ok, here is another solution that does not depend on args:

def PoliteException (e):
E = e.__class__
class PoliteException (E):
def __init__(self):
for arg in dir(e):
if not arg.startswith( '_'):
setattr(self, arg, getattr(e, arg))
def __str__(self):
return str(e) + ", sorry!"
PoliteException .__name__ = E.__name__
return PoliteException ()

try:
unicode('\xe4')
except Exception, e:
p = PoliteException (e)
assert p.reason == e.reason
raise p

[Christoph Zwerschke]

Christoph Zwerschke wrote:

Here is a simple solution, but it depends on the existence of the args
attribute that "will eventually be deprecated" according to the docs:

Just found another amazingly simple solution that does neither use teh
..args (docs: "will eventually be deprecated") attribute nor the dir()
function (docs: "its detailed behavior may change across releases").
Instead it relies on the fact that the exception itselfs behaves like
its args tuple (checked with Py 2.3, 2.4 and 2.5).

As another twist, I set the wrapper exception module to the module of
the original exception so that the output looks more like the output of
the original exception (i.e. simply "UnicodeDecodeE rror" instead of
"__main__.Unico deDecodeError") .

The code now looks like this:

def PoliteException (e):
E = e.__class__
class PoliteException (E):
def __str__(self):
return str(e) + ", sorry!"
PoliteException .__name__ = E.__name__
PoliteException .__module__ = E.__module__
return PoliteException (*e)

try:
unicode('\xe4')
except Exception, e:
p = PoliteException (e)
assert p.reason == e.reason
raise p

[fumanchu]

On Jul 15, 2:55 am, Christoph Zwerschke <[e-mail] wrote:

Here is a simple solution, but it depends
on the existence of the args attribute that
"will eventually be deprecated" according
to the docs

If you don't mind using .args, then the solution is usually as simple
as:
try:
Thing.do(arg1, arg2)
except Exception, e:
e.args += (Thing.state, arg1, arg2)
raise
No over-engineering needed. ;)
Robert Brewer
System Architect
Amor Ministries
fu******@amor.o rg

[Gabriel Genellina]

En Mon, 16 Jul 2007 13:50:50 -0300, fumanchu <fu******@amor. orgescribió:

On Jul 15, 2:55 am, Christoph Zwerschke <[e-mail] wrote:

>Here is a simple solution, but it depends
on the existence of the args attribute that
"will eventually be deprecated" according
to the docs

If you don't mind using .args, then the solution is usually as simple
as:
try:
Thing.do(arg1, arg2)
except Exception, e:
e.args += (Thing.state, arg1, arg2)
raise
No over-engineering needed. ;)

If you read enough of this long thread, you'll see that the original
requirement was to enhance the *message* displayed by a normal traceback -
the OP has no control over the callers, but wants to add useful
information to any exception.
Your code does not qualify:

pytry:
.... open("a file that does not exist")
.... except Exception,e:
.... e.args += ("Sorry",)
.... raise
....
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
IOError: [Errno 2] No such file or directory: 'a file that does not exist'
pytry:
.... x = u"á".encode("as cii")
.... except Exception,e:
.... e.args += ("Sorry",)
.... raise
....
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
UnicodeEncodeEr ror: 'ascii' codec can't encode character u'\xe1' in
position 0:
ordinal not in range(128)

--
Gabriel Genellina