[EMAIL PROTECTED] wrote:
> O/S: Windows XP Service Pack 2
> Python version: 2.4
>
> Unable to understand how to build a class to handle an exception.
>
> Contents of sample01.py:
> import exceptions
> class SampleMain:
> try:
> def __init__(self):
> print 'in SampleMain constructor'
>
> def Allowed(self):
> print 'in allowed'
>
> def NotYetAllowed(self):
> UCError = UnderConstructionError('not yet ready')
> raise UCError
>
> except UnderConstructionError, e:
> print e.msg
>
> class Error(exceptions.Exception):
> def __init__(self):
> print 'in base class constructor'
>
> class UnderConstructionError(Error):
> def __init__(self, message):
> print 'in UnderConstructionError constructor'
> self.msg = message
! def __str__(self):
! return self.msg
>
> Copy/paste of interactive window:
> PythonWin 2.4 (#60, Nov 30 2004, 09:34:21) [MSC v.1310 32 bit
(Intel)]
> on win32.
> Portions Copyright 1994-2004 Mark Hammond ([EMAIL PROTECTED])
-
> see 'Help/About PythonWin' for further copyright information.
> >>> import sample01
> >>> x = sample01.SampleMain()
> in SampleMain constructor
> >>> x.NotYetAllowed()
> in UnderConstructionError constructor
> Traceback (most recent call last):
> File "<interactive input>", line 1, in ?
> File "C:\Python24\sample01.py", line 12, in NotYetAllowed
> raise UCError
> UnderConstructionError: <unprintable instance object>
> >>>
>
> My questions are:
> 1) What is causing the error described in the Traceback?
The text <unprintable instance object> was printed because there was no
mechanism to print the error object.
> 2) Given that what I want to happen when the NotYetAllowed() method
is
> called is:
> a) an exception to be raised
Happening already.
> b) the exception results in a message getting printed; the
message
> should come from the place where the exception was raised, and it
> should be passed to the exception class as a string object; so in
this
> case the message that should be printed is 'not yet ready'
Insert __str__ method as per above.
> c) the exception gets handled with the try/except within the
> SampleMain class
>
Now that would be difficult. That 'try' statement is executed when the
class is *loaded*. Put a print statement just before the try statement
and you'll see what I mean. It's not a declaration that encompasses all
calls to methods of instances of the class.
Let's take a step back: Why do you think you need to catch and *handle*
such exceptions? You have some methods that aren't fit to be seen in
public yet? Don't tell anyone about them. Don't put your code in a
callable place. Alternatively, put "raise NotImplementedError" --
that's a standard built-in exception -- as the first line of each
method. A bit of explanation from you might be useful so that the help
can be tailored.
And if all you really want to do is just print a message, that's what
happens by default e.g.
>>> raise NotImplementedError, 'come back after lunch'
Traceback (most recent call last):
File "<stdin>", line 1, in ?
NotImplementedError: come back after lunch
>>> class Mistake(Exception):
... pass
...
>>> def foo():
... raise Mistake, 'somebody goofed'
...
>>> foo()
Traceback (most recent call last):
File "<stdin>", line 1, in ?
File "<stdin>", line 2, in foo
__main__.Mistake: somebody goofed
>>>
HTH,
John
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