incorrect DeprecationWarning?

exarkun at exarkun at
Sat Sep 5 15:07:52 CEST 2009

On 12:20 pm, alan.isaac at wrote:
>>Alan G Isaac wrote:
>>>Python 2.6.2 (r262:71605, Apr 14 2009, 22:40:02) [MSC v.1500 32 bit
>>>(Intel)] on win32
>>>Type "help", "copyright", "credits" or "license" for more 
>>>>>>class MyError(Exception):
>>>... def __init__(self, message):
>>>... Exception.__init__(self)
>>>... self.message = message
>>>>>>e = MyError('msg')
>>>__main__:4: DeprecationWarning: BaseException.message has been
>>>deprecated as of Python 2.6
>>>So? Why would that mean I cannot add such an attribute
>>>to derived classes?
>On 9/4/2009 6:42 PM, Terry Reedy wrote:
>>It does not mean that. Try printing e.message and you should see 
>>I believe what it does mean is the the special meaning of
>>exception.message (I have forgotten what it is) is gone in Python 3.
>>In Py3
>>class MyError(Exception):
>>def __init__(self, message):
>>self.message = message
>>e = MyError('msg')
>># 'msg'
>>No warning any more.
>I think you are missing my point.
>I understand it is just a DeprecationWarning.
>But **why** should I receive a deprecation warning
>when I am **not** using the deprecated practice?
>Since I am **not** using the deprecated practice, the
>warning is incorrect. (See the class definition above.)
>And this incorrect warning affects a lot of people!

You are using the deprecated practice.  Attributes are not scoped to a 
particular class.  There is only one "message" attribute on your 
"MyError" instance.  It does not belong just to "MyError".  It does not 
belong just to "Exception".  It does not belong just to "BaseException". 
It is shared by all of them.  Because "BaseException" deprecates 
instances of it having a "message" attribute, any instance of any 
subclass of "BaseException" which uses this attribute will get the 
deprecation warning.  Perhaps you weren't intending to use the "message" 
attribute as "BaseException" was using it, but this doesn't matter. 
There is only one "message" attribute, and "BaseException" already 
claimed it, and then deprecated it.
>What anyone who is **not** using the deprecated practice
>should expect in Python 2.6 is the Py3 behavior.  That is
>not what we get: we get instead an incorrect deprecation

Possibly so, but there is no way for the runtime to know that you're not 
trying to use the deprecated behavior.  All it can tell is that you're 
using the deprecated attribute name.  Perhaps you can come up with a way 
for it to differentiate between these two cases and contribute a patch, 


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