[Tutor] subject screw [exception handling]

[Kirk Bailey]

Thank you Danny, the script now reads:

#
# Let's extract the subject for later use!
#
try:
	subject= string.strip(Message['Subject'])
except Keyerror:
	subject="(none provided by sender)"
#
# The information is in the dictionary 'Message', each item keyed with
the
# header's name. The BODY of that dictionary is in the string 'msg'. So
are
# any attachments.

This ought to handle it, and when I come back with wife from registering
her car I will install it and test it. I will let the list know how it
works.


Danny Yoo wrote:
> 
> On Wed, 26 Dec 2001, Kirk Bailey wrote:
> 
> > Discovered this when testing TLpost. SOME letters came through, some
> > did not. The bombes, had no subject field. SO I sent a letter to an
> > alias to save the incoming letter as a file, and ran the script
> > locally, redirecting input from the file. This way I could watch error
> > messages on the console screen. Very informative, I got back:
> >
> > ns# ./TLpost.py testlist3 < testfile
> > Traceback (innermost last):
> >   File "./TLpost.py", line 159, in ?
> >     subject= string.strip(Message['Subject'])
> >   File "/usr/local/lib/python1.5/rfc822.py", line 356, in __getitem__
> >     return self.dict[string.lower(name)]
> > KeyError: subject
> > ------------------------------------------------------------------------
> >
> > Apparently when there is NO subject, no kidding we really mean it, it
> > omits this header. When rfc822 receives such a leter, it screws the
> > pooch and crashes, albeit politely.
> 
> Right; the designers of rfc822 felt that missing a header like this
> should be a serious error, so that's why we're seeing the KeyError.
> 
> To recover gracefully from this, we can use "exception handling".  On your
> line:
> 
>     subject = string.strip(Message['Subject'])
> 
> we can place an "exception handler" to take care of wacky situations (such
> as missing Subject lines):
> 
> ###
>     try:
>         subject = string.strip(Message['Subject'])
>     except KeyError:
>         subject = ''
> ###
> 
> which says something like: "Try to assign 'subject' that message headers.
> On the exceptional circumstance of a KeyError, assign 'subject' to the
> empty string instead."
> 
> It might be good to write a function that automates this exception
> handling for us when we grab at headers:
> 
> ###
> def getHeader(msg, header_name):
>     try:
>         header = string.strip(msg[header_name])
>     except KeyError:
>         header = ''
>     return header
> ###
> 
> This getHeader() function then guarantees that we'll at least get an empty
> string out of the situation.
> 
> The Python tutorial gives some good information on the idea of exceptions:
> 
>     http://python.org/doc/current/tut/node10.html
> 
> Good luck to you!

-- 
Respectfully,
             -Kirk D Bailey (C)2001
              Addme! icq #27840081
end


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