json.loads() should return a more specific error

Terry Reedy tjreedy at udel.edu
Wed Jun 27 19:57:43 CEST 2012

On 6/27/2012 8:45 AM, Roy Smith wrote:
> Before I go open an enhancement request, what do people think of the
> idea that json.load() should return something more specific than
> ValueError?

I do not know of any written policy about when to create custom error 
classes in the stdlib. I know there are some that are just empty 
catchall renamings.

"class ModException(Exception): pass"

This does not seem to have much use except your use us scanning logs, 
and does have some cost.

> I've got some code that looks like
>      try:
>          response = requests.get(url)
>      except RequestException as ex:
>          logger.exception(ex)
>          return []
>      data = response.text
>      try:
>          events = json.loads(data)
>      except ValueError as ex:
>          logger.error("%s: %r", ex, data)
>          return []

You could solve your immediate problem by starting the string with 
something like 'JSON: '. One might want the url included in the log, 
which would never be part of an exception from json. Given that data can 
be arbitrarily long, logging data does not seem like that good an idea.

To me, the important question is not the exception name, which you can 
replace, but whether the message part of the exception gives information 
about the actual problem. If it just says something redundant and 
useless like 'bad input', then improving that would be a good enhancement.

> This would be so much neater if json would return something I could
> identify as a json error.  It would all just collapse into:
>      try:
>          events = requests.get(url).json

Would not this be
     events = json.loads(requests.get(url).text)
Either way, 'events' is the only new binding that persists.

>      except (RequestException, JSONDecodeError) as ex:

Just using ValueError would work if you condition the logging on the 
value of ex.

>          logger.exception(ex)

This would only be the equivalent of your first code if the arbitrarily 
large input data were attached to the JSONDecodeError -- and thereby 
kept alive when it could otherwise be deleted/collected (and the custom 
class had its own __str__ method). I do not think this a good idea. The 
exception should only contain extracted bits that show the problem.

>          return []
> We could make JSONDecodeError a subclass of ValueError so existing code
> would continue to work.

Bottom line: I do not think you should expect exception instances to 
necessarily have all the info you would want logged for reading out of 
context (as opposed to reading interactively). On the other hand, 
exceptions should contain specific information about the problem that 
the raising code knows and that is hard to get otherwise.

Terry Jan Reedy

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