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(python2.1-lib.info)Debugging CGI scripts

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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:

     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 (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

     cgi.test()

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"
     print
     sys.stderr = sys.stdout
     try:
         ...your code here...
     except:
         print "\n\n<PRE>"
         traceback.print_exc()

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"
     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 readable.