unittest: assertRaises() with an instance instead of a type

Ulrich Eckhardt ulrich.eckhardt at dominolaser.com
Thu Mar 29 09:28:15 CEST 2012

Am 28.03.2012 20:26, schrieb Terry Reedy:
> On 3/28/2012 8:28 AM, Ulrich Eckhardt wrote:
>> with self.assertRaises(MyException(SOME_FOO_ERROR)):
>>     foo()
> I presume that if this worked the way you want, all attributes would
> have to match. The message part of builtin exceptions is allowed to
> change, so hard-coding an exact expected message makes tests fragile.
> This is a problem with doctest.

I would have assumed that comparing two exceptions leaves out messages 
that are intended for the user, not as part of the API. However, my 
expectations aren't met anyway, because ...

>> This of course requires the exception to be equality-comparable.
> Equality comparison is by id. So this code will not do what you want.

  >>> Exception('foo') == Exception('foo')

Yikes! That was unexpected and completely changes my idea. Any clue 
whether this is intentional? Is identity the fallback when no equality 
is defined for two objects?

Thanks for your feedback!


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