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Exception Messages

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  Exception Messages Wanderer <wanderer@dialup4less.com> - 2012-10-15 09:00 -0700
    Re: Exception Messages MRAB <python@mrabarnett.plus.com> - 2012-10-15 17:17 +0100
      Re: Exception Messages John Gordon <gordon@panix.com> - 2012-10-15 16:22 +0000
        Re: Exception Messages MRAB <python@mrabarnett.plus.com> - 2012-10-15 17:34 +0100
          Re: Exception Messages Wanderer <wanderer@dialup4less.com> - 2012-10-15 10:18 -0700
            Re: Exception Messages Wanderer <wanderer@dialup4less.com> - 2012-10-15 10:20 -0700
            Re: Exception Messages Wanderer <wanderer@dialup4less.com> - 2012-10-15 10:20 -0700
            Re: Exception Messages MRAB <python@mrabarnett.plus.com> - 2012-10-15 18:34 +0100
              Re: Exception Messages Wanderer <wanderer@dialup4less.com> - 2012-10-15 10:44 -0700
              Re: Exception Messages Wanderer <wanderer@dialup4less.com> - 2012-10-15 10:44 -0700
          Re: Exception Messages Wanderer <wanderer@dialup4less.com> - 2012-10-15 10:18 -0700
        Re: Exception Messages Dave Angel <d@davea.name> - 2012-10-15 12:42 -0400
        Re: Exception Messages Terry Reedy <tjreedy@udel.edu> - 2012-10-15 15:15 -0400
    Re: Exception Messages Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-10-16 00:47 +0000

#31315 — Exception Messages

How do you get Exceptions to print messages? I have an exception defined like this

class PvCamError(Exception):
    def __init__(self, msg):
        self.msg = msg

But when the error is raised I get this:

Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\ipython-0.12.1-py2.7.egg\IPython\core\interactiveshell.py", line 2538, in run_code
    exec code_obj in self.user_global_ns, self.user_ns
  File "<ipython-input-1-fb48da797583>", line 1, in <module>
    import S477Test
  File "U:\workspace\camera\src\S477Test.py", line 13, in <module>
    camera.getSerialNum()
  File "U:\workspace\camera\src\S477.py", line 131, in getSerialNum
    num = self.pl.getParamValue(pvcamConstants.PARAM_HEAD_SER_NUM_ALPHA)
  File "U:\workspace\camera\src\pvcam.py", line 261, in getParamValue
    raise PvCamError("Unhandled Type: {0}".format(attype))
PvCamError

Why wasn't the message printed out?

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#31317

On 2012-10-15 17:00, Wanderer wrote:
> How do you get Exceptions to print messages? I have an exception defined like this
>
> class PvCamError(Exception):
>      def __init__(self, msg):
>          self.msg = msg
>
> But when the error is raised I get this:
>
> Traceback (most recent call last):
>    File "C:\Python27\lib\site-packages\ipython-0.12.1-py2.7.egg\IPython\core\interactiveshell.py", line 2538, in run_code
>      exec code_obj in self.user_global_ns, self.user_ns
>    File "<ipython-input-1-fb48da797583>", line 1, in <module>
>      import S477Test
>    File "U:\workspace\camera\src\S477Test.py", line 13, in <module>
>      camera.getSerialNum()
>    File "U:\workspace\camera\src\S477.py", line 131, in getSerialNum
>      num = self.pl.getParamValue(pvcamConstants.PARAM_HEAD_SER_NUM_ALPHA)
>    File "U:\workspace\camera\src\pvcam.py", line 261, in getParamValue
>      raise PvCamError("Unhandled Type: {0}".format(attype))
> PvCamError
>
> Why wasn't the message printed out?
>
You didn't add a __str__ method:

class PvCamError(Exception):
     def __init__(self, msg):
         self.msg = msg
     def __str__(self):
         return self.msg

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#31318

In <mailman.2217.1350317845.27098.python-list@python.org> MRAB <python@mrabarnett.plus.com> writes:

> > Why wasn't the message printed out?
>
> You didn't add a __str__ method:

> class PvCamError(Exception):
>      def __init__(self, msg):
>          self.msg = msg
>      def __str__(self):
>          return self.msg

Wouldn't PvCamError inherit __str__() from Exception?

-- 
John Gordon                   A is for Amy, who fell down the stairs
gordon@panix.com              B is for Basil, assaulted by bears
                                -- Edward Gorey, "The Gashlycrumb Tinies"

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#31319

On 2012-10-15 17:22, John Gordon wrote:
> In <mailman.2217.1350317845.27098.python-list@python.org> MRAB <python@mrabarnett.plus.com> writes:
>
>> > Why wasn't the message printed out?
>>
>> You didn't add a __str__ method:
>
>> class PvCamError(Exception):
>>      def __init__(self, msg):
>>          self.msg = msg
>>      def __str__(self):
>>          return self.msg
>
> Wouldn't PvCamError inherit __str__() from Exception?
>
Yes, but you've put the message in msg, and Exception doesn't have that
attribute.

An alternative approach would be:

class PvCamError(Exception):
     pass

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#31324

On Monday, October 15, 2012 12:34:53 PM UTC-4, MRAB wrote:

> 
> Yes, but you've put the message in msg, and Exception doesn't have that
> 
> attribute.
> 

That's weird. I got this Exception class definition idea from this post by Guido van Rostrum, Where he gives this main function to look like

import sys
import getopt

class Usage(Exception):
    def __init__(self, msg):
        self.msg = msg

def main(argv=None):
    if argv is None:
        argv = sys.argv
    try:
        try:
            opts, args = getopt.getopt(argv[1:], "h", ["help"])
        except getopt.error, msg:
             raise Usage(msg)
        # more code, unchanged
    except Usage, err:
        print >>sys.stderr, err.msg
        print >>sys.stderr, "for help use --help"
        return 2

if __name__ == "__main__":
    sys.exit(main())




http://www.artima.com/weblogs/viewpost.jsp?thread=4829

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#31326

On Monday, October 15, 2012 1:18:52 PM UTC-4, Wanderer wrote:
> On Monday, October 15, 2012 12:34:53 PM UTC-4, MRAB wrote:
> 
> 
> 
> > 
> 
> > Yes, but you've put the message in msg, and Exception doesn't have that
> 
> > 
> 
> > attribute.
> 
> > 
> 
> 
> 
> That's weird. I got this Exception class definition idea from this post by Guido van Rostrum, Where he gives this main function to look like
> 
> 
> 
> import sys
> 
> import getopt
> 
> 
> 
> class Usage(Exception):
> 
>     def __init__(self, msg):
> 
>         self.msg = msg
> 
> 
> 
> def main(argv=None):
> 
>     if argv is None:
> 
>         argv = sys.argv
> 
>     try:
> 
>         try:
> 
>             opts, args = getopt.getopt(argv[1:], "h", ["help"])
> 
>         except getopt.error, msg:
> 
>              raise Usage(msg)
> 
>         # more code, unchanged
> 
>     except Usage, err:
> 
>         print >>sys.stderr, err.msg
> 
>         print >>sys.stderr, "for help use --help"
> 
>         return 2
> 
> 
> 
> if __name__ == "__main__":
> 
>     sys.exit(main())
> 
> 
> 
> 
> 
> 
> 
> 
> 
> http://www.artima.com/weblogs/viewpost.jsp?thread=4829

Oops. I meant Guido van Rossum

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#31327

On Monday, October 15, 2012 1:18:52 PM UTC-4, Wanderer wrote:
> On Monday, October 15, 2012 12:34:53 PM UTC-4, MRAB wrote:
> 
> 
> 
> > 
> 
> > Yes, but you've put the message in msg, and Exception doesn't have that
> 
> > 
> 
> > attribute.
> 
> > 
> 
> 
> 
> That's weird. I got this Exception class definition idea from this post by Guido van Rostrum, Where he gives this main function to look like
> 
> 
> 
> import sys
> 
> import getopt
> 
> 
> 
> class Usage(Exception):
> 
>     def __init__(self, msg):
> 
>         self.msg = msg
> 
> 
> 
> def main(argv=None):
> 
>     if argv is None:
> 
>         argv = sys.argv
> 
>     try:
> 
>         try:
> 
>             opts, args = getopt.getopt(argv[1:], "h", ["help"])
> 
>         except getopt.error, msg:
> 
>              raise Usage(msg)
> 
>         # more code, unchanged
> 
>     except Usage, err:
> 
>         print >>sys.stderr, err.msg
> 
>         print >>sys.stderr, "for help use --help"
> 
>         return 2
> 
> 
> 
> if __name__ == "__main__":
> 
>     sys.exit(main())
> 
> 
> 
> 
> 
> 
> 
> 
> 
> http://www.artima.com/weblogs/viewpost.jsp?thread=4829

Oops. I meant Guido van Rossum

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#31329

On 2012-10-15 18:18, Wanderer wrote:
> On Monday, October 15, 2012 12:34:53 PM UTC-4, MRAB wrote:
>
>>
>> Yes, but you've put the message in msg, and Exception doesn't have that
>>
>> attribute.
>>
>
> That's weird. I got this Exception class definition idea from this post by Guido van Rostrum, Where he gives this main function to look like
>
> import sys
> import getopt
>
> class Usage(Exception):
>      def __init__(self, msg):
>          self.msg = msg
>
> def main(argv=None):
>      if argv is None:
>          argv = sys.argv
>      try:
>          try:
>              opts, args = getopt.getopt(argv[1:], "h", ["help"])
>          except getopt.error, msg:
>               raise Usage(msg)
>          # more code, unchanged
>      except Usage, err:
>          print >>sys.stderr, err.msg
>          print >>sys.stderr, "for help use --help"
>          return 2
>
> if __name__ == "__main__":
>      sys.exit(main())
>
>
>
>
> http://www.artima.com/weblogs/viewpost.jsp?thread=4829
>
>
Note how it explicitly prints err.msg.

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#31330

On Monday, October 15, 2012 1:34:24 PM UTC-4, MRAB wrote:
> On 2012-10-15 18:18, Wanderer wrote:
> 
> > On Monday, October 15, 2012 12:34:53 PM UTC-4, MRAB wrote:
> 
> >
> 
> >>
> 
> >> Yes, but you've put the message in msg, and Exception doesn't have that
> 
> >>
> 
> >> attribute.
> 
> >>
> 
> >
> 
> > That's weird. I got this Exception class definition idea from this post by Guido van Rostrum, Where he gives this main function to look like
> 
> >
> 
> > import sys
> 
> > import getopt
> 
> >
> 
> > class Usage(Exception):
> 
> >      def __init__(self, msg):
> 
> >          self.msg = msg
> 
> >
> 
> > def main(argv=None):
> 
> >      if argv is None:
> 
> >          argv = sys.argv
> 
> >      try:
> 
> >          try:
> 
> >              opts, args = getopt.getopt(argv[1:], "h", ["help"])
> 
> >          except getopt.error, msg:
> 
> >               raise Usage(msg)
> 
> >          # more code, unchanged
> 
> >      except Usage, err:
> 
> >          print >>sys.stderr, err.msg
> 
> >          print >>sys.stderr, "for help use --help"
> 
> >          return 2
> 
> >
> 
> > if __name__ == "__main__":
> 
> >      sys.exit(main())
> 
> >
> 
> >
> 
> >
> 
> >
> 
> > http://www.artima.com/weblogs/viewpost.jsp?thread=4829
> 
> >
> 
> >
> 
> Note how it explicitly prints err.msg.

Not in the raise statement. 

Adding the def __str__ made it work for me.
Thanks

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#31331

On Monday, October 15, 2012 1:34:24 PM UTC-4, MRAB wrote:
> On 2012-10-15 18:18, Wanderer wrote:
> 
> > On Monday, October 15, 2012 12:34:53 PM UTC-4, MRAB wrote:
> 
> >
> 
> >>
> 
> >> Yes, but you've put the message in msg, and Exception doesn't have that
> 
> >>
> 
> >> attribute.
> 
> >>
> 
> >
> 
> > That's weird. I got this Exception class definition idea from this post by Guido van Rostrum, Where he gives this main function to look like
> 
> >
> 
> > import sys
> 
> > import getopt
> 
> >
> 
> > class Usage(Exception):
> 
> >      def __init__(self, msg):
> 
> >          self.msg = msg
> 
> >
> 
> > def main(argv=None):
> 
> >      if argv is None:
> 
> >          argv = sys.argv
> 
> >      try:
> 
> >          try:
> 
> >              opts, args = getopt.getopt(argv[1:], "h", ["help"])
> 
> >          except getopt.error, msg:
> 
> >               raise Usage(msg)
> 
> >          # more code, unchanged
> 
> >      except Usage, err:
> 
> >          print >>sys.stderr, err.msg
> 
> >          print >>sys.stderr, "for help use --help"
> 
> >          return 2
> 
> >
> 
> > if __name__ == "__main__":
> 
> >      sys.exit(main())
> 
> >
> 
> >
> 
> >
> 
> >
> 
> > http://www.artima.com/weblogs/viewpost.jsp?thread=4829
> 
> >
> 
> >
> 
> Note how it explicitly prints err.msg.

Not in the raise statement. 

Adding the def __str__ made it work for me.
Thanks

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#31325

On Monday, October 15, 2012 12:34:53 PM UTC-4, MRAB wrote:

> 
> Yes, but you've put the message in msg, and Exception doesn't have that
> 
> attribute.
> 

That's weird. I got this Exception class definition idea from this post by Guido van Rostrum, Where he gives this main function to look like

import sys
import getopt

class Usage(Exception):
    def __init__(self, msg):
        self.msg = msg

def main(argv=None):
    if argv is None:
        argv = sys.argv
    try:
        try:
            opts, args = getopt.getopt(argv[1:], "h", ["help"])
        except getopt.error, msg:
             raise Usage(msg)
        # more code, unchanged
    except Usage, err:
        print >>sys.stderr, err.msg
        print >>sys.stderr, "for help use --help"
        return 2

if __name__ == "__main__":
    sys.exit(main())




http://www.artima.com/weblogs/viewpost.jsp?thread=4829

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#31320

On 10/15/2012 12:34 PM, MRAB wrote:
> On 2012-10-15 17:22, John Gordon wrote:
>> In <mailman.2217.1350317845.27098.python-list@python.org> MRAB
>> <python@mrabarnett.plus.com> writes:
>>
>>> > Why wasn't the message printed out?
>>>
>>> You didn't add a __str__ method:
>>
>>> class PvCamError(Exception):
>>>      def __init__(self, msg):
>>>          self.msg = msg
>>>      def __str__(self):
>>>          return self.msg
>>
>> Wouldn't PvCamError inherit __str__() from Exception?
>>
> Yes, but you've put the message in msg, and Exception doesn't have that
> attribute.
>
> An alternative approach would be:
>
> class PvCamError(Exception):
>     pass
>

Wouldn't another approach be for this __init__() method to call the
base-class __init__() method with whatever data it needs?



-- 

DaveA

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#31335

On 10/15/2012 12:22 PM, John Gordon wrote:
> In <mailman.2217.1350317845.27098.python-list@python.org> MRAB <python@mrabarnett.plus.com> writes:
>
>>> Why wasn't the message printed out?
>>
>> You didn't add a __str__ method:
>
>> class PvCamError(Exception):
>>       def __init__(self, msg):
>>           self.msg = msg
>>       def __str__(self):
>>           return self.msg
>
> Wouldn't PvCamError inherit __str__() from Exception?

Exception instances get a .args attibute set to the arguments of the 
class call and printed in the .__str__ message.

 >>> dir(Exception())
['__cause__', '__class__', '__context__', '__delattr__', '__dict__', 
'__dir__', '__doc__', '__eq__', '__format__', '__ge__', 
'__getattribute__', '__gt__', '__hash__', '__init__', '__le__', 
'__lt__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', 
'__repr__', '__setattr__', '__setstate__', '__sizeof__', '__str__', 
'__subclasshook__', '__suppress_context__', '__traceback__', 'args', 
'with_traceback']
 >>> Exception('abc', 1)
Exception('abc', 1)

So class MyError(Exception) will get that basic default behavior. For 
fancier printing, one needs a fancier .__str__ method ;-).

-- 
Terry Jan Reedy

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#31350

On Mon, 15 Oct 2012 09:00:15 -0700, Wanderer wrote:

> How do you get Exceptions to print messages? I have an exception defined
> like this
> 
> class PvCamError(Exception):
>     def __init__(self, msg):
>         self.msg = msg

Please don't invent yet another interface for exception messages. 
Exceptions already have two defined interfaces for accessing the error 
message:

exception.message  # older, now obsolete, and gone in Python 3.
exception.args  # recommended way

Please use the same interface that nearly all exceptions already have and 
use an "args" argument. The easiest way to do this is also the simplest:

class PvCamError(Exception): pass


If you *must* support "msg", do something like this:

class PvCamError(Exception):
    def __init__(self, msg, *args):
        super(PvCamError, self).__init__(msg, *args)  # see footnote [1]
        self.msg = msg





[1] If you're using Python 3, you can abbreviate the super call to:

    super().__init__(self, msg, *args)



-- 
Steven

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