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First of all, check for trivial installation errors -- reading the
section above on installing your CGI script carefully can save you a
lot of time. If you wonder whether you have understood the
installation procedure correctly, try installing a copy of this module
file (cgi.py) as a CGI script. When invoked as a script, the file
will dump its environment and the contents of the form in HTML form.
Give it the right mode etc, and send it a request. If it's installed
in the standard cgi-bin directory, it should be possible to send it a
request by entering a URL into your browser of the form:
http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home
If this gives an error of type 404, the server cannot find the script
- perhaps you need to install it in a different directory. If it
gives another error, there's an installation problem that
you should fix before trying to go any further. If you get a nicely
formatted listing of the environment and form content (in this
example, the fields should be listed as ``addr'' with value ``At Home''
and ``name'' with value ``Joe Blow''), the cgi.py script has been
installed correctly. If you follow the same procedure for your own
script, you should now be able to debug it.
The next step could be to call the cgi module's
test() function from your script: replace its main code
with the single statement
This should produce the same results as those gotten from installing
the cgi.py file itself.
When an ordinary Python script raises an unhandled exception (for
whatever reason: of a typo in a module name, a file that can't be
opened, etc.), the Python interpreter prints a nice traceback and
exits. While the Python interpreter will still do this when your CGI
script raises an exception, most likely the traceback will end up in
one of the HTTP server's log files, or be discarded altogether.
Fortunately, once you have managed to get your script to execute
some code, you can easily send tracebacks to the Web browser
using the cgitb module. If you haven't done so already,
just add the line:
import cgitb; cgitb.enable()
to the top of your script. Then try running it again; when a
problem occurs, you should see a detailed report that will
likely make apparent the cause of the crash.
If you suspect that there may be a problem in importing the
cgitb module, you can use an even more robust approach
(which only uses built-in modules):
import sys
sys.stderr = sys.stdout
print "Content-Type: text/plain"
print
...your code here...
This relies on the Python interpreter to print the traceback. The
content type of the output is set to plain text, which disables all
HTML processing. If your script works, the raw HTML will be displayed
by your client. If it raises an exception, most likely after the
first two lines have been printed, a traceback will be displayed.
Because no HTML interpretation is going on, the traceback will be
readable.
Release 2.4.5, documentation updated on 18 October 2006.
See About this document... for information on suggesting changes.
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