[Python-ideas] Arguments to exceptions

[David Mertz]

If a method, why not a property?

On Jul 4, 2017 2:41 PM, "Terry Reedy" <tjreedy at udel.edu> wrote:

> On 7/4/2017 3:32 PM, David Mertz wrote:
>
>> I don't see the usefulness rich exception data as at all as limited as
>> this. Here's some toy code that shows a use:
>>
>>
>> ----
>>
>> # For some reason, imports might not be ready immediately
>> # Maybe flaky network drive, maybe need to install modules, etc
>> # The overall program can do things too make them available
>> lazy_import("foo", "bar", "baz", "blat")
>>
>> while True:
>>      try:
>>          x = foo(1) * bar(2) + baz(3)**blat(4)
>>          break
>>      except NameError as err:
>>          lazy_import(err.name <http://err.name>)
>>          sleep(1)
>>
>
> Alternate proposal: give the NameError class a .name instance method that
> extracts the name from the message.  This should not increase the time to
> create an instance.  You would then write 'err.name()' instead of '
> err.name'. For 3.6
>
> def name(self):
>     msg = self.args[0]
>     return msg[6:msg.rindex("'")]
>
> # current test
>
> try: xyz
> except NameError as e:
>     print(name(e) == 'xyz')
>
> # Exceptions unittest to ensure that the method
> # stays synchronized with future versions of instances
>
> def test_nameerror_name(self):
>     try:
>         xyz
>     except NameError as e:
>         self.assertEqual(e.name(), 'xyz')
>
> Generalize to other exceptions.
>
> Further only-partially baked idea: Since exceptions are (in cpython) coded
> in C, I wonder if C data could be inexpensively attached to the instance to
> be retrieved and converted to python objects by methods when needed.
>
> --
> Terry Jan Reedy
>
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