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55 | <H2><A NAME="SECTION0013290000000000000000"> | |
56 | 11.2.9 Debugging CGI scripts</A> | |
57 | </H2> <a id='l2h-3175' xml:id='l2h-3175'></a> | |
58 | <P> | |
59 | First of all, check for trivial installation errors -- reading the | |
60 | section above on installing your CGI script carefully can save you a | |
61 | lot of time. If you wonder whether you have understood the | |
62 | installation procedure correctly, try installing a copy of this module | |
63 | file (<span class="file">cgi.py</span>) as a CGI script. When invoked as a script, the file | |
64 | will dump its environment and the contents of the form in HTML form. | |
65 | Give it the right mode etc, and send it a request. If it's installed | |
66 | in the standard <span class="file">cgi-bin</span> directory, it should be possible to send it a | |
67 | request by entering a URL into your browser of the form: | |
68 | ||
69 | <P> | |
70 | <div class="verbatim"><pre> | |
71 | http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home | |
72 | </pre></div> | |
73 | ||
74 | <P> | |
75 | If this gives an error of type 404, the server cannot find the script | |
76 | - perhaps you need to install it in a different directory. If it | |
77 | gives another error, there's an installation problem that | |
78 | you should fix before trying to go any further. If you get a nicely | |
79 | formatted listing of the environment and form content (in this | |
80 | example, the fields should be listed as ``addr'' with value ``At Home'' | |
81 | and ``name'' with value ``Joe Blow''), the <span class="file">cgi.py</span> script has been | |
82 | installed correctly. If you follow the same procedure for your own | |
83 | script, you should now be able to debug it. | |
84 | ||
85 | <P> | |
86 | The next step could be to call the <tt class="module">cgi</tt> module's | |
87 | <tt class="function">test()</tt> function from your script: replace its main code | |
88 | with the single statement | |
89 | ||
90 | <P> | |
91 | <div class="verbatim"><pre> | |
92 | cgi.test() | |
93 | </pre></div> | |
94 | ||
95 | <P> | |
96 | This should produce the same results as those gotten from installing | |
97 | the <span class="file">cgi.py</span> file itself. | |
98 | ||
99 | <P> | |
100 | When an ordinary Python script raises an unhandled exception (for | |
101 | whatever reason: of a typo in a module name, a file that can't be | |
102 | opened, etc.), the Python interpreter prints a nice traceback and | |
103 | exits. While the Python interpreter will still do this when your CGI | |
104 | script raises an exception, most likely the traceback will end up in | |
105 | one of the HTTP server's log files, or be discarded altogether. | |
106 | ||
107 | <P> | |
108 | Fortunately, once you have managed to get your script to execute | |
109 | <em>some</em> code, you can easily send tracebacks to the Web browser | |
110 | using the <tt class="module"><a href="module-cgitb.html">cgitb</a></tt> module. If you haven't done so already, | |
111 | just add the line: | |
112 | ||
113 | <P> | |
114 | <div class="verbatim"><pre> | |
115 | import cgitb; cgitb.enable() | |
116 | </pre></div> | |
117 | ||
118 | <P> | |
119 | to the top of your script. Then try running it again; when a | |
120 | problem occurs, you should see a detailed report that will | |
121 | likely make apparent the cause of the crash. | |
122 | ||
123 | <P> | |
124 | If you suspect that there may be a problem in importing the | |
125 | <tt class="module"><a href="module-cgitb.html">cgitb</a></tt> module, you can use an even more robust approach | |
126 | (which only uses built-in modules): | |
127 | ||
128 | <P> | |
129 | <div class="verbatim"><pre> | |
130 | import sys | |
131 | sys.stderr = sys.stdout | |
132 | print "Content-Type: text/plain" | |
133 | ||
134 | ...your code here... | |
135 | </pre></div> | |
136 | ||
137 | <P> | |
138 | This relies on the Python interpreter to print the traceback. The | |
139 | content type of the output is set to plain text, which disables all | |
140 | HTML processing. If your script works, the raw HTML will be displayed | |
141 | by your client. If it raises an exception, most likely after the | |
142 | first two lines have been printed, a traceback will be displayed. | |
143 | Because no HTML interpretation is going on, the traceback will be | |
144 | readable. | |
145 | ||
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