I think there's a way to define an exception subclass with message as a one-liner:: # Simple syntax to define exception classes. _ = BaseFormattedException class RegisterError(_, LookupError): 'Extension "{0}" not registered.' class ExtensionError(_, ValueError): 'Extension "{0}" returns wrong data.' # Another way to define exception classes --- factory function. MissingError = _.formatted(IndexError, 'No extension named "{0}"') SomeOtherError = _.formatted(Exception)
raise RegisterError('test') Traceback (most recent call last): File "<pyshell#1>", line 1, in <module> raise RegisterError('test') exceptions.RegisterError: Extension "test" not registered.
raise MissingError('test') Traceback (most recent call last): File "<pyshell#2>", line 1, in <module> raise MissingError('test') exceptions.IndexError: No extension named "test"
(Note the difference between two methods above: RegisterError has its separate __name__ while still being a subclass of IndexError, while MissingError doesn't have its own __name__) The trick is in defining the `BaseFormattedException` which harvests (the first line of a) docstring to provide template (I figured out for a small, locally-used exception class there may be no difference between what it prints and its docstring). The whole example is at http://web.mit.edu/~unknot/www/exceptions.py How does that sound? On Sat, Sep 19, 2009 at 5:43 AM, Nick Coghlan <ncoghlan@gmail.com> wrote:
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 ---------------------------------------------------------------