I was somewhat surprised to discover that Python 3 no longer allows an
exception to be raised in an except clause (or rather that it reports it
as a separate exception that occurred during the handling of the first).
So the following code:
>>> d = {}
>>> try:
... val = d['nosuch']
... except:
... raise AttributeError("No attribute 'nosuch'")
...
Give the traceback I expected and wanted in Python 2:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
AttributeError: No attribute 'nosuch'
but in Python 3.1 the traceback looks like this:
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
KeyError: 'nosuch'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
AttributeError: No attribute 'nosuch'
Modifying the code a little allows me to change the error message, but
not much else:
>>> d = {}
>>> try:
... val = d['nosuch']
... except KeyError as e:
... raise AttributeError("No attribute 'nosuch'") from e
...
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
KeyError: 'nosuch'
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "<stdin>", line 4, in <module>
AttributeError: No attribute 'nosuch'
>>>
In a class's __getattr__() method this means that instead of being able
to say
try:
value = _attrs[name]
except KeyError:
raise AttributeError ...
I am forced to write
if name not in _attrs:
raise AttributeError ...
value = _attrs[name]
which requires an unnecessary second lookup on the attribute name. What
is the correct paradigm for this situation?
regards
Steve
--
Steve Holden +1 571 484 6266 +1 800 494 3119
PyCon 2011 Atlanta March 9-17 http://us.pycon.org/
See Python Video! http://python.mirocommunity.org/
Holden Web LLC http://www.holdenweb.com/
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