11.2.8 Debugging CGI scripts

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:


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 (e.g. 500), 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 (e.g. because 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 file, or be discarded altogether.

Fortunately, once you have managed to get your script to execute some code, it is easy to catch exceptions and cause a traceback to be printed. The test() function below in this module is an example. Here are the rules:

  1. Import the traceback module before entering the try ... except statement

  2. Assign sys.stderr to be sys.stdout

  3. Make sure you finish printing the headers and the blank line early

  4. Wrap all remaining code in a try ... except statement

  5. In the except clause, call traceback.print_exc()

For example:

import sys
import traceback
print "Content-Type: text/html"
sys.stderr = sys.stdout
    ...your code here...
    print "\n\n<PRE>"

Notes: The assignment to sys.stderr is needed because the traceback prints to sys.stderr. The print "\n\n<PRE>" statement is necessary to disable the word wrapping in HTML.

If you suspect that there may be a problem in importing the traceback 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"
...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 readable.

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