Recommended exception method for modules?

[Fredrik Lundh]

Grant Edwards wrote:
>  3) a class for each type of error with value(s) that further
>     explain what is wrong:
> 
>     class PosixSerialBaudError(Exception):
>         [...]
>          
>     if not validBaud(r):
>         raise PosixSerialBaudError(r)
> 
>     [this last one could be done as a subclass of a general
>      PosixSerialError class, -- that way somebody can catch
>      either all of my exceptions or just individual ones.
>      Right?]

> Does anybody care to share their opinion on the relative merits
> of these?

Don't use string exceptions.  They're evil.  Enough said.

For simple cases, reuse an existing exception (if reasonable),
or use a single exception class, either inherited from Exception
or from a suitable standard exception class (alternative 2).

(in 2.0, the -X option is gone, and built-in exceptions are
always classes)

When it makes sense, use an exception hierarchy (alternative 3).

You can always move from (2) to (3), without breaking existing
code.

</F>