[Python-ideas] Wild idea: Exception.format

[Yuvgoog Greenle]

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)

On Fri, Sep 18, 2009 at 10:54 PM, ilya <ilya.nikokoshev at gmail.com> wrote:

> How about simplifying creation of a subclass for this pattern::
>
> class NotRegisteredError(LookupError):
>    template = 'Extension {1} not registered'
>
> ...
> raise NotRegisteredError(ext)
>
>
> This is instead of::
>
> class NotRegisteredError(LookupError):
>    def __init__(self, *args):
>        super().__init__('Extension {1} not registered'.format(self,
> *args), *args)
>
>
> Advantages to having a separate subclass:
> (1) it can be reused
> (2) it can be caught separately from LookupError
> (3) you can list possible exceptions in the beginning of module
> (4) you can search for functions that raise a specific exception
>
> What do you think?
>
> Ilya.
>
> On Fri, Sep 18, 2009 at 6:07 PM, Georg Brandl <g.brandl at gmx.net> wrote:
> > To make Exceptions where some object(s) are involved more useful, it is
> > often necessary to put the objects on the exception *in addition to*
> > formatting them into a string representation for the message.
> >
> > This little classmethod would make that easier::
> >
> >  class BaseException:
> >    @classmethod
> >    def format(cls, fmt, *args):
> >      return cls(fmt.format(*args), *args)
> >
> > Example usage::
> >
> >  ext = 'foo'
> >  raise LookupError.format('Extension {0} not registered', ext)
> >
> > 'foo' could then be accessed as ``exc.args[1]``.
> >
> >
> > A similar, but also very useful implementation would be ::
> >
> >  def format(cls, fmt, **kwds):
> >    exc = cls(fmt.format(**kwds))
> >    exc.__dict__.update(kwds)
> >    return exc
> >
> > with example usage being::
> >
> >  raise LookupError.format('Extension {ext} not registered', ext='foo')
> >
> > and 'foo' being accessible as ``exc.ext``.
> >
> > I realize this is probably too obscure for Python core, but I wanted to
> > show it to you anyway, maybe it'll be found useful.
> >
> > Georg
> >
> > --
> > Thus spake the Lord: Thou shalt indent with four spaces. No more, no
> less.
> > Four shall be the number of spaces thou shalt indent, and the number of
> thy
> > indenting shall be four. Eight shalt thou not indent, nor either indent
> thou
> > two, excepting that thou then proceed to four. Tabs are right out.
> >
> > _______________________________________________
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> > Python-ideas at python.org
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> >
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