catching exceptions

[Steven D'Aprano]

On Sat, 16 Dec 2006 03:54:52 -0800, jm.suresh at no.spam.gmail.com wrote:

> Hi, In the following program, I have a class Test which has a property
> x. Its setx function gets a string value and converts it into a float
> and stores into it.

[snip code]

Python isn't Java. Are you sure you need properties?


> I am looking for a way to call func1's exception handler also.
> Basically I want to have the class's error handler as the basic text
> based one, and in the calling side, If I have gui, I will pop-up a
> window and say the error message.
> One solution is to remove the exception handling inside the class and
> leave the entire thing to the caller (software people call this
> client?)  side -- if the caller has access to gui it will use gui or
> else will print the message. Any other way?

The exception is consumed by the try...except block inside the class,
so func1 never sees the exception. It might as well not exist.

Generally, you should keep your class as simple as possible. It shouldn't
try to manage any exception it can't recover from. In your case, the class
can't recover from a failure of float(strvalue), so it shouldn't consume
the exception. Two ways of doing that:

def _setx(self, strvalue):
    self._x = float(strvalue)  # just let the exception propagate

or

def _setx(self, strvalue):
    try:
        self._x = float(strvalue)
    except ValueError:
        raise SomeError('could not set x attribute to %s' % strvalue)

where SomeError should be either ValueError or possibly some custom
exception (say, MyClassException).

In either case, the error handling is separate from the object that
generates the error. And that is as it should be. Now your class can
remain the same, no matter how the rest of your program handles the
exception.

By the way, if you want your class to generate warnings, perhaps you
should investigate the Warnings module:

import warnings
help(warnings)



-- 
Steven.