Re: [Python-ideas] Wild idea: Exception.format

Sept. 19, 2009

      Yuvgoog Greenle wrote:
...
I fail to see why you can't have both args and kwargs...
class BaseException:
    @classmethod
    def format(cls, fmt, *args, **kwargs):
        return cls(fmt.format(*args, **kwargs), *args, **kwargs)
And concerning Gerog's actual question, I think this is a not uncommon
pattern, but
    raise Exception.format(...)
doesn't sound good. It's not really as readable as
    raise Exception(fmt.format(ext), ext)
ilya suggested an almost ok direction but he doesn't define formats at
runtime. So maybe instead we can make a factory for formatted exceptions.
def formatted_exception(exception):
    def format_and_return(fmt, *args, **kwargs):
        return exception(fmt.format(*args, **kwargs), *args, **kwargs)
    return format_and_return
@formatted_exception
class NotRegisteredError(LookupError): pass
then the usage becomes:
raise NotRegisteredError("Extension {0} not registered.", ext)
I like the class decorator idea (to avoid namespace conflicts on the
exception objects - cf. the dramas with ex.message). However, the above
would break exception handling since NotRegisteredError would refer to
the factory function instead of the exception that is actually thrown.

Combining it with Ilya's idea (by having the class decorator return a
new subclass rather than a factory function) gives something that should
work in practice:

def formatted_exception(exc):
  class FormattedExc(exc):
    def __init__(*args, **kwds):
      self, fmt, args = args[0], args[1], args[2:]
      super(FormattedExc, self).__init__(
            fmt.format(*args, **kwds), *args, **kwds)
    def __str__(self):
      return self.args[0]
    __name__ = exc.__name__
    __doc__ = exc.__doc__
    __module__ = exc.__module__
  return FormattedExc

@formatted_exception
class ExampleError(Exception): pass
...
...
...
try:
...   raise ExampleError("Format: {}", "Value")
... except ExampleError as ex:
...   saved = ex
...
saved
FormattedExc('Format: Value', 'Value')
print(saved)
Format: Value
saved.args[1]
'Value'
saved.__name__
'ExampleError'
Cheers,
Nick.

-- 
Nick Coghlan   |   ncoghlan@gmail.com   |   Brisbane, Australia
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