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From: Peter Otten <__peter__@web.de>
Subject: Re: unittest: assertRaises() with an instance instead of a type
Date: Thu, 29 Mar 2012 08:55:53 +0200
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Ben Finney wrote:

> Steven D'Aprano <steve+comp.lang.python@pearwood.info> writes:
> 
>> (By the way, I have to question the design of an exception with error
>> codes. That seems pretty poor design to me. Normally the exception *type*
>> acts as equivalent to an error code.)
> 
> Have a look at Python's built-in OSError. The various errors from the
> operating system can only be distinguished by the numeric code the OS
> returns, so that's what to test on in one's unit tests.
 
The core devs are working to fix that:

$ python3.2 -c'open("does-not-exist")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
IOError: [Errno 2] No such file or directory: 'does-not-exist'
$ python3.3 -c'open("does-not-exist")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
FileNotFoundError: [Errno 2] No such file or directory: 'does-not-exist'

$ python3.2 -c'open("unwritable", "w")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
IOError: [Errno 13] Permission denied: 'unwritable'
$ python3.3 -c'open("unwritable", "w")'
Traceback (most recent call last):
  File "<string>", line 1, in <module>
PermissionError: [Errno 13] Permission denied: 'unwritable'

http://www.python.org/dev/peps/pep-3151/