[Python-3000] str.format vs. string.Formatter exceptions

[Eric Smith]

Guido van Rossum wrote:
> Since IndexError and KeyError are conceptually like ValueError but in
> a more narrowly defined context, I think IndexError and KeyError
> actually make sense here (even though they don't inherit from
> ValueError).
> 
> --Guido

Okay, I'll change these to IndexError and KeyError.

Eric.

> 
> On 9/3/07, Eric Smith <eric+python-dev at trueblade.com> wrote:
>> Ron Adam points out some differences in which exceptions are thrown by
>> str.format and string.Formatter.  For example, on a missing positional
>> argument:
>>
>>  >>> "{0}".format()
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>> ValueError: Not enough positional arguments in format string
>>
>>  >>> Formatter().format("{0}")
>> Traceback (most recent call last):
>>    File "<stdin>", line 1, in <module>
>>    File "/shared/src/python/py3k/Lib/string.py", line 201, in format
>>      return self.vformat(format_string, args, kwargs)
>>    File "/shared/src/python/py3k/Lib/string.py", line 220, in vformat
>>      obj, arg_used = self.get_field(field_name, args, kwargs)
>>    File "/shared/src/python/py3k/Lib/string.py", line 278, in get_field
>>      obj = self.get_value(first, args, kwargs)
>>    File "/shared/src/python/py3k/Lib/string.py", line 235, in get_value
>>      return args[key]
>> IndexError: tuple index out of range
>>
>> The PEP says: In general, exceptions generated by the formatter code
>> itself are of the "ValueError" variety -- there is an error in the
>> actual "value" of the format string.
>>
>> I can easily change string.Formatter to make this a ValueError, and I
>> think that's probably the right thing to do.  For example, if the string
>> comes from a translation module, then there might be an extra parameter
>> added by mistake, in which case ValueError seems right to me.
>>
>> But I'd like to hear if anyone else thinks this should be an IndexError,
>> or maybe they both should be some other exception.
>>
>> Similarly "{x}".format()' currently raises ValueError, but
>> 'Formatter().format("{x}")' raises KeyError.
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> 
>