Only a message at the highest exception

[Cecil Westerhof]

Cameron Simpson <cs at cskk.id.au> writes:

> On 28Jun2019 12:17, Cecil Westerhof <Cecil at decebal.nl> wrote:
>>Chris Angelico <rosuav at gmail.com> writes:
>>> On Fri, Jun 28, 2019 at 7:33 PM Cecil Westerhof <Cecil at decebal.nl> wrote:
>>>> I have a tkinter program where I have a function generate_report
>>>> which
>>>> in a try block calls the function append_row. This function has also a
>>>> try block. When they get an exception they give message. But when
>>>> append_row has already given a message then generate_report should
>>>> not. To implement this I use the following class:
>>>>
>>>>     class AlreadyHandledException(Exception):
>>>>         pass
>>>>
>>>> Then in append_row I have:
>>>>     except Exception as err:
>>>>         messagebox.showerror(error_str,
>>>>                              error_append + '\n\n\n\n' + str(err))
>>>>         raise AlreadyHandledException()
>>>>
>>>> And in generate_report I have:
>>>>     except Exception as err:
>>>>         if type(err).__name__ != "AlreadyHandledException":
>>>>             messagebox.showerror(error_str,
>>>>                                  error_generate + '\n\n\n\n' + str(err))
>>>>         progress.pack_forget()
>>>>
>>>> Is this an acceptable way, or should I do it differently?
>>>
>>> Well, first off, I would use two separate except clauses, rather than
>>> a type check.
>>
>>You are completely right.
>>
>>> But are you able to just NOT catch the exception inside
>>> append_row? Let it be handled at the top level only.
>>
>>I am giving different messages. I changed the top part to:
>>    except AlreadyHandledException:
>>        pass
>>    except Exception as err:
>>        messagebox.showerror(error_str, error_generate + '\n\n\n\n' + str(err))
>>    progress.lower()
>
> For contrast, another approach.
>
> By catching AlreadyHandledException and just going "pass" you're
> effectively swalling that exception. For your purpose, that works.
>
> However, in a since, why raise an exception you _know_ you're going to
> deliberately ignore?

I am not. I am using it to quit generating the report. Just ignoring
that I could not append a record does not sound correct to me. Yes
they got a message that something went wrong. But if there is still a
generated report, things will go wrong.


> What if you have append_row return a Boolean? True on success, False on
> failure but situation handled (i.e. append_row has shown an error, or
> whatever recovery you adopt). And keep the uncaught exceptions for the
> _unhandled_ situation. The "exceptional" situation.
>
> Example:
>
>  def append_row(...):
>      ...
>      try:
>          ...stuff...
>      except ExpectedExceptionHere as err:
>          messagebox.showerror(....)
>          ...maybe some cleanup...
>          return False
>      ...
>      return True
>
> Then your generate_report code goes:
>
>    try:
>        appended = append_row(....)
>    except Exception as err:
>        messagebox.showerror(....)
>    else:
>        if appended:
>            hooray!
>        else:
>            unhappy, but continuing

And that is exactly what I not want to do.

-- 
Cecil Westerhof
Senior Software Engineer
LinkedIn: http://www.linkedin.com/in/cecilwesterhof