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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Exception Messages
Date: Mon, 15 Oct 2012 15:15:30 -0400
References: <8b5cb95f-424b-45a8-b19b-42f92f531a75@googlegroups.com> <mailman.2217.1350317845.27098.python-list@python.org> <k5hd7c$emf$1@reader1.panix.com>
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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