Weird exception in my, um, exception class constructor

[Joel Koltner]

I have a generic (do nothing) exception class that's coded like this:

class MyError(excepti ons.Exception):
def __init__(self,a rgs=None):
self.args = args

When I attempt to raise this exception via 'raise MyError' I get an exception
within the MyError constructor __init__ as follows:

Traceback (most recent call last):
[lines deleted]
File "c:\Documen ts and Settings\Joel.K olstad\My Documents\Pytho n\
MyStuff.py", line 7, in __init__
self.args = args
TypeError: 'NoneType' object is not iterable

Ummm... why should 'args' have to be iterable anyway? I don't understand
what's going on here? Could someone help me with this?

I've found empirically that I can write:

if args self.args = args

....and everything *appears* to work OK, but I'd really like to know what the
core problem is.

Thank you,
---Joel

[Arnaud Delobelle]

Re: Weird exception in my, um, exception class constructor

"Joel Koltner" <zapwireDASHgro ups@yahoo.comwr ites:

I have a generic (do nothing) exception class that's coded like this:
>
class MyError(excepti ons.Exception):
def __init__(self,a rgs=None):
self.args = args
>
When I attempt to raise this exception via 'raise MyError' I get an
exception within the MyError constructor __init__ as follows:
>
Traceback (most recent call last):
[lines deleted]
File "c:\Documen ts and Settings\Joel.K olstad\My Documents\Pytho n\
MyStuff.py", line 7, in __init__
self.args = args
TypeError: 'NoneType' object is not iterable

That's because the class 'Exception' defines a descriptor 'args' which
has to be a sequence. Just call 'args' something else, and it will
work. E.g.

>>class MyError(Excepti on):

.... def __init__(self, args=None):
.... self.myargs = args
....

>>MyError()

MyError()


OTOH you could just take advantage of Exception.args:

>>class MyError(Excepti on): pass

....

>>MyError()

MyError()

>>MyError(1, 2, 3)

MyError(1, 2, 3)

(Details in the documentation are a bit scant, but see
http://docs.python.org/lib/module-exceptions.html)

HTH

--
Arnaud

[Paul Hankin]

Re: Weird exception in my, um, exception class constructor

On May 27, 9:21 pm, "Joel Koltner" <zapwireDASHgro ...@yahoo.com>
wrote:

I have a generic (do nothing) exception class that's coded like this:
>
class MyError(excepti ons.Exception):
    def __init__(self,a rgs=None):
        self.args = args
>
When I attempt to raise this exception via 'raise MyError' I get an exception
within the MyError constructor __init__ as follows:
>
Traceback (most recent call last):
[lines deleted]
  File "c:\Documen ts and Settings\Joel.K olstad\My Documents\Pytho n\
MyStuff.py", line 7, in __init__
    self.args = args
TypeError: 'NoneType' object is not iterable
>
Ummm... why should 'args' have to be iterable anyway?  I don't understand
what's going on here?  Could someone help me with this?

Did you actually write self,args = args?

--
Paul Hankin

[Joel Koltner]

Re: Weird exception in my, um, exception class constructor

Hi Arnaud,

"Arnaud Delobelle" <arnodel@google mail.comwrote in message
news:m2wslf4haq .fsf@googlemail .com...

That's because the class 'Exception' defines a descriptor 'args' which
has to be a sequence.

Ah, thanks. I was following the example in Beazley's book and should have dug
into the actual documentation a bit more.

OTOH you could just take advantage of Exception.args:

>>>class MyError(Excepti on): pass

Sounds good to me. I take it that, if I don't inherit from Exception, various
expected behaviors will break? (This is what Beazley suggests...)

---Joel

[Joel Koltner]

Re: Weird exception in my, um, exception class constructor

"Paul Hankin" <paul.hankin@gm ail.comwrote in message
news:0ea08237-c9f9-441b-a744-7410114b5ae9@t5 4g2000hsg.googl egroups.com...
"Did you actually write self,args = args?"

(looks at source code)

$#@#%#$ Why, yes, yes I did! Thanks for catching that...

[Terry Reedy]

Re: Weird exception in my, um, exception class constructor


"Joel Koltner" <zapwireDASHgro ups@yahoo.comwr ote in message
news:Kq__j.1770 27$ng7.15947@en-nntp-05.dc1.easynews .com...
| Sounds good to me. I take it that, if I don't inherit from Exception,
various
| expected behaviors will break? (This is what Beazley suggests...)

All builtin exceptions have been in the builtin namespace for a while. The
duplicate exceptions module is gone in 3.0, In 3.0, if not before, all
exceptions must subclass from one of the builtin exceptions. Exception is
the usual choice.

tjr

[Arnaud Delobelle]

Re: Weird exception in my, um, exception class constructor

"Joel Koltner" <zapwireDASHgro ups@yahoo.comwr ites:

"Paul Hankin" <paul.hankin@gm ail.comwrote in message
news:0ea08237-c9f9-441b-a744-7410114b5ae9@t5 4g2000hsg.googl egroups.com...
"Did you actually write self,args = args?"
>
(looks at source code)
>
$#@#%#$ Why, yes, yes I did! Thanks for catching that...

This is odd, because you should get this error even if writing
self.args=args, witness this copy-paste from the interactive prompt:

>>class MyError(Excepti on):

.... def __init__(self, args=None):
.... self.args = args
....

>>MyError()

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 3, in __init__
TypeError: 'NoneType' object is not iterable

--
Arnaud