catching object

[Igor V. Rafienko]

Hi,


I was wondering if someone could help me explain this situation:

h[1] >>> import inspect
h[1] >>> inspect.getmro(ValueError)
(<type 'exceptions.ValueError'>, <type 'exceptions.StandardError'>,
<type 'exceptions.Exception'>, <type 'exceptions.BaseException'>,
<type 'object'>)
h[2] >>> try:
raise ValueError("argh")
except object:
print "why not?"

Traceback (most recent call last):
File "<stdin>", line 2, in <module>
ValueError: argh

The question is, why isn't ValueError caught? It *does* inherit from
object (albeit indirectly), and my understanding of the wording of
CPython docs is that this guarantees "compatibility" (between what is
being raised and what is being caught).

So, why doesn't object match ValueError (or any other exception for
that matter). I am aware of "except:", but in my particular situation
it is eh... unsuitable.

Any hints/pointers are appreciated.





ivr

 

[Stargaming]

Hi,


I was wondering if someone could help me explain this situation:

h[1] >>> import inspect
h[1] >>> inspect.getmro(ValueError)
(<type 'exceptions.ValueError'>, <type 'exceptions.StandardError'>,
<type 'exceptions.Exception'>, <type 'exceptions.BaseException'>, <type
'object'>)
h[2] >>> try:
raise ValueError("argh")
except object:
print "why not?"

Traceback (most recent call last):
File "<stdin>", line 2, in <module>
ValueError: argh

The question is, why isn't ValueError caught? It *does* inherit from
object (albeit indirectly), and my understanding of the wording of
CPython docs is that this guarantees "compatibility" (between what is
being raised and what is being caught).

I think inheritance is meant in reference to the Exception tree here. So,
the uppermost class is `BaseException`. Python 3 gives this exception
when trying to handle `object`::

TypeError: catching classes that do not inherit from BaseException is
not allowed

You are right, the wording is quite confusing here but that might be due
to the crude situation of exceptions: All old-style classes (deprecated
themselves) can be raised. Strings can still be raised but this is
deprecated. New-style classes can be raised but only if they inherit (at
least indirectly) from BaseException. I'd say: feel free to contribute
patches.

So, why doesn't object match ValueError (or any other exception for that
matter). I am aware of "except:", but in my particular situation it is
eh... unsuitable.

Why so?

The only possible solution me and my crystal ball could come up with is
that you're having a lot of old-style exceptions raised from some library
you cannot modify. You cannot handle any common base class and don't want
to handle every of the exceptions. If that's the case, just use a bare
`except` and utilize `sys.exc_info`.

HTH,

 

[Igor V. Rafienko]

[ (e-mail address removed) ]


[ ... ]

I think inheritance is meant in reference to the Exception tree here. So,
the uppermost class is `BaseException`. Python 3 gives this exception
when trying to handle `object`::

TypeError: catching classes that do not inherit from BaseException is
not allowed

I realise that exceptions should inherit from Exception, but the
question is more "why isn't an instance of ValueError a match for
object, when ValueError is a subclass of object (at least in 2.5)

[ ... ]

Why so?

I am writing a function that takes a function and an exception list
arguments and returns a new function that calls the original while
trapping the specified arguments. One of the applications is to make a
wrapper that ignores all of the exceptions. I thought that specifying
the exception list containing object would suffice to cover that.
Alas, no joy.

[ ... ]





ivr