Unexpected parameter handling in python Exceptions

Question

While creating a custom Exception class, I encountered an unexpected situation how the base Exception class handles parameters. Specifically, with how it sets the 'message' attribute.

When you pass more than one parameter to the Exception.__init__(), it does not initialize the message attribute. For example, this works

>>> e = Exception('msg')
>>> e.message
'msg'

BUT This does NOT set the message attribute

>>> e = Exception('msg', 'extra')
>>> e.message
''

It does of course store all the parameters in the args attribute:

>>> e = Exception('msg', 'extra')
>>> e.args
('msg', 'extra')

Can anyone shed light on this? I've scoured the Exception docs, but I'm at a loss to understand why the Exception class does this. In case it matters, this is python 2.7

wim · Accepted Answer · 2014-11-06 15:00:53Z

5

Use the source, Luke!

BaseException_init(PyBaseExceptionObject *self, PyObject *args, PyObject *kwds)
{
    if (!_PyArg_NoKeywords(Py_TYPE(self)->tp_name, kwds))
        return -1;

    Py_DECREF(self->args);
    self->args = args;
    Py_INCREF(self->args);

    if (PyTuple_GET_SIZE(self->args) == 1) {
        Py_CLEAR(self->message);
        self->message = PyTuple_GET_ITEM(self->args, 0);
        Py_INCREF(self->message);
    }
    return 0;
}

So message is only set if the args is length 1.

The reason for this behaviour is for backwards compatibility. Exception.message is deprecated since python 2.6, and the implementation for BaseException.__str__ doesn't look at message at all - it only uses the args tuple.

edited Nov 6, 2014 at 15:00

answered Nov 6, 2014 at 13:23

wim

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3 Comments

user590028 Over a year ago

Thanks for the code -- but I'm curious about why this behavior is true?

user590028 Over a year ago

Thanks...I'll accept this as the correct answer (need to wait 6 minutes for SO to allow this). Do you have a reference to the deprecated statement? I'd like to read up.

chepner Over a year ago

See PEP-352. The message attribute was actually a short-lived addition to the BaseException class, added in 2.5 but immediately deprecated in 2.6.

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