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[Python-ideas] Comparable exceptions

Ionel Cristian Mărieș contact at ionelmc.ro
Wed Feb 25 11:46:57 CET 2015 
What if we make the hash from .args and .__dict__? That would cover most
cases.

For me it makes sense because they look like containers. There might be
cases where a user doesn't want __eq__ or __hash__ for exception instances
- how about  we enable those methods just for the builtin types? And then
the user can explicitly enable them for custom exceptions (there could be a
special base ComparableException class that he could inherit from).


Thanks,
-- Ionel Cristian Mărieș, blog.ionelmc.ro

On Wed, Feb 25, 2015 at 1:55 AM, Guido van Rossum <guido at python.org> wrote:

> Actually, they don't have a __hash__ either -- they just inherit the
> default __hash__ from object, just as they inherit __eq__ from object (and
> both just use the address of the object). I can see many concerns with
> trying to define __eq__ for exceptions in general -- often there are a
> variety of fields, not all of which perhaps should be considered when
> comparing, and then there are user-defined exceptions which may have other
> attributes. I don't know what your use case was, but if, as you say, the
> exceptions you care about already have `args` attributes, maybe you should
> just compare that (and the type).
>
> On Tue, Feb 24, 2015 at 3:43 PM, Ionel Cristian Mărieș <contact at ionelmc.ro
> > wrote:
>
>> Hey everyone,
>>
>> I was writing a test the other day and I wanted to assert that some
>> function raises some exception, but with some exact value. Turns out you
>> cannot compare OSError("foobar") to OSError("foobar") as it doesn't have
>> any __eq__. (it's the same for all exceptions?)
>>
>> I think exceptions are great candidates to have an __eq__ method - they
>> already have a __hash__ (that's computed from the contained values) and
>> they are already containers in a way (they all have the `args` attribute).
>>
>> Comparing exceptions in a test assertion seems like a good usecase for
>> this.
>>
>> Thanks,
>> -- Ionel Cristian Mărieș, blog.ionelmc.ro
>>
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>
>
>
> --
> --Guido van Rossum (python.org/~guido)
>
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