[Tutor] Help with 500 error on cgi script

Sun, 18 Mar 2001 18:08:45 -0800

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    > From: Sheila King <sheila@spamcop.net>
    > Subject: [Tutor] Help with 500 error on cgi script
    > Date: Sun, 18 Mar 2001 01:33:57 -0800
    >
    > ... [snip] ...
    > if form.has_key("subscriberEmail") and form.has_key("command"):
    >     form_ok = 1
    > if not form_ok:
    > 	output = '''Content-type: text/html\n\n
    > <html>
    > ... [snip] ...
    > </body>\n</html>
    > '''
    > 	print output
    > else:
    > 	for k in form.keys():
    >     	print k, " : ", form[k]
    > 	print "All done!"

sheila,

as others have pointed out in earlier posts, there were 2 basic
problems with the script, that's it!  everything else was fine.

you probably already know this, but playing with CGI is a 3-way game.
the (1) user or web client makes a call to the (2) web server for some data.
becuse the web server is too "dumb" to be able to comprehend form data,
it must ask (3) a 3rd party application (via the Common Gateway Interface)
to do the real work and just give it back some HTML to return to the client.
a web server only knows about the (MIME) header it needs to send back and
raw HTML (whether from a static .html file or from a CGI application).

(1) Error 500s or ISEs (Internal Server Errors) occur when the web
    server gets either a bad header or invalid HTML.  it pukes and
    sends a 500 back to the web client.  so whenever you get these,
    it's a sign that somehow your CGI app messed up somehow.

    in your code above, altho you handle the error situation just
    fine, your "it's ok" code does not send back a header, i.e.
    "Content-Type:"... nor does it send back valid HTML.  instead,
    it does:
   	'print k, " : ", form[k]'

    ... which is not HTML at all, out comes a 500; and once you fix
    *that* bug, you'll get another 500 with:

	print "All done!"

    ... which is also not HTML.  Surround your output with the ap-
    propriate HTML doo-dads and you'll be okay.

(2) daniel also pointed out that you should not mix spaces and TABs
    and that is also true.  in fact, just avoiding use of TABs works
    out best.  TABs are represented with a different number of spaces
    on each system so source files are not easily read on other
    systems.  sticking with pure spaces will allow you to edit the
    file without inconsistency across different platforms.

one way you can avoid problems such as (1) is to make your output
interface consistent.  here's one way:

# whether okay or error, same header is returned
output = 'Content-type: text/html\n\n'

# okay output
if form.has_key("subscriberEmail") and form.has_key("command"):

    # all okay output has this header
    output += '<HTML><BODY><H1>YEAH!!</H1>\n'

    # build form item list
    for k in form.keys():
	output += "%s : %s<BR>\n" % (k, form[k])

    # end okay output
    output += 'All done!</BODY></HTML>'

# error output
else:
    # use error string output
    output += '<HTML><BODY><H1>YOU GOOFED!</BODY></HTML>'

# whether okay or error, send the output (same variable!)
print output

---

hope this helps!

-wesley

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
Silicon Valley-SF Bay Area Python users group:  http://www.baypiggies.org

"Core Python Programming", Prentice Hall PTR, December 2000
    http://www.phptr.com/ptrbooks/ptr_0130260363.html

wesley.j.chun :: wesc@baypiggies.org
cyberweb.consulting :: silicon.valley, ca
http://www.roadkill.com/~wesc/cyberweb/