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 <div class="section" id="module-cgi">
<h1>20.2. <a class="reference internal" href="#module-cgi" title="cgi: Helpers for running Python scripts via the Common Gateway Interface."><tt class="xref py py-mod docutils literal">cgi</tt></a> &#8212; Common Gateway Interface support<a class="headerlink" href="#module-cgi" title="Permalink to this headline">¶</a></h1>
Source code: <a class="reference external" href="http://hg.python.org/cpython/file/2.7/Lib/cgi.py">Lib/cgi.py</a>
<hr class="docutils" />
Support module for Common Gateway Interface (CGI) scripts.
This module defines a number of utilities for use by CGI scripts written in
Python.
<div class="section" id="introduction">
<h2>20.2.1. Introduction<a class="headerlink" href="#introduction" title="Permalink to this headline">¶</a></h2>
A CGI script is invoked by an HTTP server, usually to process user input
submitted through an HTML <tt class="docutils literal">&lt;FORM&gt;</tt> or <tt class="docutils literal">&lt;ISINDEX&gt;</tt> element.
Most often, CGI scripts live in the server&#8217;s special <tt class="file docutils literal">cgi-bin</tt> directory.
The HTTP server places all sorts of information about the request (such as the
client&#8217;s hostname, the requested URL, the query string, and lots of other
goodies) in the script&#8217;s shell environment, executes the script, and sends the
script&#8217;s output back to the client.
The script&#8217;s input is connected to the client too, and sometimes the form data
is read this way; at other times the form data is passed via the &#8220;query string&#8221;
part of the URL. This module is intended to take care of the different cases
and provide a simpler interface to the Python script. It also provides a number
of utilities that help in debugging scripts, and the latest addition is support
for file uploads from a form (if your browser supports it).
The output of a CGI script should consist of two sections, separated by a blank
line. The first section contains a number of headers, telling the client what
kind of data is following. Python code to generate a minimal header section
looks like this:
<div class="highlight-python"><div class="highlight"><pre>print &quot;Content-Type: text/html&quot; # HTML is following
print # blank line, end of headers
</pre></div>
</div>
The second section is usually HTML, which allows the client software to display
nicely formatted text with header, in-line images, etc. Here&#8217;s Python code that
prints a simple piece of HTML:
<div class="highlight-python"><div class="highlight"><pre>print &quot;&lt;TITLE&gt;CGI script output&lt;/TITLE&gt;&quot;
print &quot;&lt;H1&gt;This is my first CGI script&lt;/H1&gt;&quot;
print &quot;Hello, world!&quot;
</pre></div>
</div>
</div>
<div class="section" id="using-the-cgi-module">
<h2>20.2.2. Using the cgi module<a class="headerlink" href="#using-the-cgi-module" title="Permalink to this headline">¶</a></h2>
Begin by writing <tt class="docutils literal">import cgi</tt>. Do not use <tt class="docutils literal">from cgi import *</tt> &#8212; the
module defines all sorts of names for its own use or for backward compatibility
that you don&#8217;t want in your namespace.
When you write a new script, consider adding these lines:
<div class="highlight-python"><div class="highlight"><pre>import cgitb
cgitb.enable()
</pre></div>
</div>
This activates a special exception handler that will display detailed reports in
the Web browser if any errors occur. If you&#8217;d rather not show the guts of your
program to users of your script, you can have the reports saved to files
instead, with code like this:
<div class="highlight-python"><div class="highlight"><pre>import cgitb
cgitb.enable(display=0, logdir=&quot;/path/to/logdir&quot;)
</pre></div>
</div>
It&#8217;s very helpful to use this feature during script development. The reports
produced by <a class="reference internal" href="cgitb.html#module-cgitb" title="cgitb: Configurable traceback handler for CGI scripts."><tt class="xref py py-mod docutils literal">cgitb</tt></a> provide information that can save you a lot of time in
tracking down bugs. You can always remove the <tt class="docutils literal">cgitb</tt> line later when you
have tested your script and are confident that it works correctly.
To get at submitted form data, it&#8217;s best to use the <tt class="xref py py-class docutils literal">FieldStorage</tt> class.
The other classes defined in this module are provided mostly for backward
compatibility. Instantiate it exactly once, without arguments. This reads the
form contents from standard input or the environment (depending on the value of
various environment variables set according to the CGI standard). Since it may
consume standard input, it should be instantiated only once.
The <tt class="xref py py-class docutils literal">FieldStorage</tt> instance can be indexed like a Python dictionary.
It allows membership testing with the <a class="reference internal" href="../reference/expressions.html#in"><tt class="xref std std-keyword docutils literal">in</tt></a> operator, and also supports
the standard dictionary method <tt class="xref py py-meth docutils literal">keys()</tt> and the built-in function
<a class="reference internal" href="functions.html#len" title="len"><tt class="xref py py-func docutils literal">len()</tt></a>. Form fields containing empty strings are ignored and do not appear
in the dictionary; to keep such values, provide a true value for the optional
keep_blank_values keyword parameter when creating the <tt class="xref py py-class docutils literal">FieldStorage</tt>
instance.
For instance, the following code (which assumes that the
Content-Type header and blank line have already been printed)
checks that the fields <tt class="docutils literal">name</tt> and <tt class="docutils literal">addr</tt> are both set to a non-empty
string:
<div class="highlight-python"><div class="highlight"><pre>form = cgi.FieldStorage()
if &quot;name&quot; not in form or &quot;addr&quot; not in form:
 print &quot;&lt;H1&gt;Error&lt;/H1&gt;&quot;
 print &quot;Please fill in the name and addr fields.&quot;
 return
print &quot;&lt;p&gt;name:&quot;, form[&quot;name&quot;].value
print &quot;&lt;p&gt;addr:&quot;, form[&quot;addr&quot;].value
...further form processing here...
</pre></div>
</div>
Here the fields, accessed through <tt class="docutils literal">form[key]</tt>, are themselves instances of
<tt class="xref py py-class docutils literal">FieldStorage</tt> (or <tt class="xref py py-class docutils literal">MiniFieldStorage</tt>, depending on the form
encoding). The <tt class="xref py py-attr docutils literal">value</tt> attribute of the instance yields the string value
of the field. The <tt class="xref py py-meth docutils literal">getvalue()</tt> method returns this string value directly;
it also accepts an optional second argument as a default to return if the
requested key is not present.
If the submitted form data contains more than one field with the same name, the
object retrieved by <tt class="docutils literal">form[key]</tt> is not a <tt class="xref py py-class docutils literal">FieldStorage</tt> or
<tt class="xref py py-class docutils literal">MiniFieldStorage</tt> instance but a list of such instances. Similarly, in
this situation, <tt class="docutils literal">form.getvalue(key)</tt> would return a list of strings. If you
expect this possibility (when your HTML form contains multiple fields with the
same name), use the <tt class="xref py py-func docutils literal">getlist()</tt> function, which always returns a list of
values (so that you do not need to special-case the single item case). For
example, this code concatenates any number of username fields, separated by
commas:
<div class="highlight-python"><div class="highlight"><pre>value = form.getlist(&quot;username&quot;)
usernames = &quot;,&quot;.join(value)
</pre></div>
</div>
If a field represents an uploaded file, accessing the value via the
<tt class="xref py py-attr docutils literal">value</tt> attribute or the <tt class="xref py py-func docutils literal">getvalue()</tt> method reads the entire file in
memory as a string. This may not be what you want. You can test for an uploaded
file by testing either the <tt class="xref py py-attr docutils literal">filename</tt> attribute or the <tt class="xref py py-attr docutils literal">file</tt>
attribute. You can then read the data at leisure from the <tt class="xref py py-attr docutils literal">file</tt>
attribute:
<div class="highlight-python"><div class="highlight"><pre>fileitem = form[&quot;userfile&quot;]
if fileitem.file:
 # It&#39;s an uploaded file; count lines
 linecount = 0
 while 1:
 line = fileitem.file.readline()
 if not line: break
 linecount = linecount + 1
</pre></div>
</div>
If an error is encountered when obtaining the contents of an uploaded file
(for example, when the user interrupts the form submission by clicking on
a Back or Cancel button) the <tt class="xref py py-attr docutils literal">done</tt> attribute of the object for the
field will be set to the value -1.
The file upload draft standard entertains the possibility of uploading multiple
files from one field (using a recursive multipart/* encoding).
When this occurs, the item will be a dictionary-like <tt class="xref py py-class docutils literal">FieldStorage</tt> item.
This can be determined by testing its <tt class="xref py py-attr docutils literal">type</tt> attribute, which should be
multipart/form-data (or perhaps another MIME type matching
multipart/*). In this case, it can be iterated over recursively
just like the top-level form object.
When a form is submitted in the &#8220;old&#8221; format (as the query string or as a single
data part of type application/x-www-form-urlencoded), the items will
actually be instances of the class <tt class="xref py py-class docutils literal">MiniFieldStorage</tt>. In this case, the
<tt class="xref py py-attr docutils literal">list</tt>, <tt class="xref py py-attr docutils literal">file</tt>, and <tt class="xref py py-attr docutils literal">filename</tt> attributes are always <tt class="docutils literal">None</tt>.
A form submitted via POST that also has a query string will contain both
<tt class="xref py py-class docutils literal">FieldStorage</tt> and <tt class="xref py py-class docutils literal">MiniFieldStorage</tt> items.
</div>
<div class="section" id="higher-level-interface">
<h2>20.2.3. Higher Level Interface<a class="headerlink" href="#higher-level-interface" title="Permalink to this headline">¶</a></h2>

New in version 2.2.
The previous section explains how to read CGI form data using the
<tt class="xref py py-class docutils literal">FieldStorage</tt> class. This section describes a higher level interface
which was added to this class to allow one to do it in a more readable and
intuitive way. The interface doesn&#8217;t make the techniques described in previous
sections obsolete &#8212; they are still useful to process file uploads efficiently,
for example.
The interface consists of two simple methods. Using the methods you can process
form data in a generic way, without the need to worry whether only one or more
values were posted under one name.
In the previous section, you learned to write following code anytime you
expected a user to post more than one value under one name:
<div class="highlight-python"><pre>item = form.getvalue("item")
if isinstance(item, list):
 # The user is requesting more than one item.
else:
 # The user is requesting only one item.</pre>
</div>
This situation is common for example when a form contains a group of multiple
checkboxes with the same name:
<div class="highlight-python"><pre>&lt;input type="checkbox" name="item" value="1" /&gt;
&lt;input type="checkbox" name="item" value="2" /&gt;</pre>
</div>
In most situations, however, there&#8217;s only one form control with a particular
name in a form and then you expect and need only one value associated with this
name. So you write a script containing for example this code:
<div class="highlight-python"><div class="highlight"><pre>user = form.getvalue(&quot;user&quot;).upper()
</pre></div>
</div>
The problem with the code is that you should never expect that a client will
provide valid input to your scripts. For example, if a curious user appends
another <tt class="docutils literal">user=foo</tt> pair to the query string, then the script would crash,
because in this situation the <tt class="docutils literal">getvalue(&quot;user&quot;)</tt> method call returns a list
instead of a string. Calling the <a class="reference internal" href="stdtypes.html#str.upper" title="str.upper"><tt class="xref py py-meth docutils literal">upper()</tt></a> method on a list is not valid
(since lists do not have a method of this name) and results in an
<a class="reference internal" href="exceptions.html#exceptions.AttributeError" title="exceptions.AttributeError"><tt class="xref py py-exc docutils literal">AttributeError</tt></a> exception.
Therefore, the appropriate way to read form data values was to always use the
code which checks whether the obtained value is a single value or a list of
values. That&#8217;s annoying and leads to less readable scripts.
A more convenient approach is to use the methods <tt class="xref py py-meth docutils literal">getfirst()</tt> and
<tt class="xref py py-meth docutils literal">getlist()</tt> provided by this higher level interface.
<dl class="method">
<dt id="cgi.FieldStorage.getfirst">
<tt class="descclassname">FieldStorage.</tt><tt class="descname">getfirst</tt><big>(</big>name[, default]<big>)</big><a class="headerlink" href="#cgi.FieldStorage.getfirst" title="Permalink to this definition">¶</a></dt>
<dd>This method always returns only one value associated with form field name.
The method returns only the first value in case that more values were posted
under such name. Please note that the order in which the values are received
may vary from browser to browser and should not be counted on. <a class="footnote-reference" href="#id3" id="id2">[1]</a> If no such
form field or value exists then the method returns the value specified by the
optional parameter default. This parameter defaults to <tt class="docutils literal">None</tt> if not
specified.
</dd></dl>

<dl class="method">
<dt id="cgi.FieldStorage.getlist">
<tt class="descclassname">FieldStorage.</tt><tt class="descname">getlist</tt><big>(</big>name<big>)</big><a class="headerlink" href="#cgi.FieldStorage.getlist" title="Permalink to this definition">¶</a></dt>
<dd>This method always returns a list of values associated with form field name.
The method returns an empty list if no such form field or value exists for
name. It returns a list consisting of one item if only one such value exists.
</dd></dl>

Using these methods you can write nice compact code:
<div class="highlight-python"><div class="highlight"><pre>import cgi
form = cgi.FieldStorage()
user = form.getfirst(&quot;user&quot;, &quot;&quot;).upper() # This way it&#39;s safe.
for item in form.getlist(&quot;item&quot;):
 do_something(item)
</pre></div>
</div>
</div>
<div class="section" id="old-classes">
<h2>20.2.4. Old classes<a class="headerlink" href="#old-classes" title="Permalink to this headline">¶</a></h2>

Deprecated since version 2.6.
<tt class="xref py py-class docutils literal">SvFormContentDict</tt> stores single value form content as dictionary; it
assumes each field name occurs in the form only once.
<tt class="xref py py-class docutils literal">FormContentDict</tt> stores multiple value form content as a dictionary (the
form items are lists of values). Useful if your form contains multiple fields
with the same name.
Other classes (<tt class="xref py py-class docutils literal">FormContent</tt>, <tt class="xref py py-class docutils literal">InterpFormContentDict</tt>) are present
for backwards compatibility with really old applications only.
</div>
<div class="section" id="functions">
<h2>20.2.5. Functions<a class="headerlink" href="#functions" title="Permalink to this headline">¶</a></h2>
These are useful if you want more control, or if you want to employ some of the
algorithms implemented in this module in other circumstances.
<dl class="function">
<dt id="cgi.parse">
<tt class="descclassname">cgi.</tt><tt class="descname">parse</tt><big>(</big>fp[, environ[, keep_blank_values[, strict_parsing]]]<big>)</big><a class="headerlink" href="#cgi.parse" title="Permalink to this definition">¶</a></dt>
<dd>Parse a query in the environment or from a file (the file defaults to
<tt class="docutils literal">sys.stdin</tt> and environment defaults to <tt class="docutils literal">os.environ</tt>). The keep_blank_values and strict_parsing parameters are
passed to <a class="reference internal" href="urlparse.html#urlparse.parse_qs" title="urlparse.parse_qs"><tt class="xref py py-func docutils literal">urlparse.parse_qs()</tt></a> unchanged.
</dd></dl>

<dl class="function">
<dt id="cgi.parse_qs">
<tt class="descclassname">cgi.</tt><tt class="descname">parse_qs</tt><big>(</big>qs[, keep_blank_values[, strict_parsing]]<big>)</big><a class="headerlink" href="#cgi.parse_qs" title="Permalink to this definition">¶</a></dt>
<dd>This function is deprecated in this module. Use <a class="reference internal" href="urlparse.html#urlparse.parse_qs" title="urlparse.parse_qs"><tt class="xref py py-func docutils literal">urlparse.parse_qs()</tt></a>
instead. It is maintained here only for backward compatiblity.
</dd></dl>

<dl class="function">
<dt id="cgi.parse_qsl">
<tt class="descclassname">cgi.</tt><tt class="descname">parse_qsl</tt><big>(</big>qs[, keep_blank_values[, strict_parsing]]<big>)</big><a class="headerlink" href="#cgi.parse_qsl" title="Permalink to this definition">¶</a></dt>
<dd>This function is deprecated in this module. Use <a class="reference internal" href="urlparse.html#urlparse.parse_qsl" title="urlparse.parse_qsl"><tt class="xref py py-func docutils literal">urlparse.parse_qsl()</tt></a>
instead. It is maintained here only for backward compatiblity.
</dd></dl>

<dl class="function">
<dt id="cgi.parse_multipart">
<tt class="descclassname">cgi.</tt><tt class="descname">parse_multipart</tt><big>(</big>fp, pdict<big>)</big><a class="headerlink" href="#cgi.parse_multipart" title="Permalink to this definition">¶</a></dt>
<dd>Parse input of type multipart/form-data (for file uploads).
Arguments are fp for the input file and pdict for a dictionary containing
other parameters in the Content-Type header.
Returns a dictionary just like <a class="reference internal" href="urlparse.html#urlparse.parse_qs" title="urlparse.parse_qs"><tt class="xref py py-func docutils literal">urlparse.parse_qs()</tt></a> keys are the field names, each
value is a list of values for that field. This is easy to use but not much good
if you are expecting megabytes to be uploaded &#8212; in that case, use the
<tt class="xref py py-class docutils literal">FieldStorage</tt> class instead which is much more flexible.
Note that this does not parse nested multipart parts &#8212; use
<tt class="xref py py-class docutils literal">FieldStorage</tt> for that.
</dd></dl>

<dl class="function">
<dt id="cgi.parse_header">
<tt class="descclassname">cgi.</tt><tt class="descname">parse_header</tt><big>(</big>string<big>)</big><a class="headerlink" href="#cgi.parse_header" title="Permalink to this definition">¶</a></dt>
<dd>Parse a MIME header (such as Content-Type) into a main value and a
dictionary of parameters.
</dd></dl>

<dl class="function">
<dt id="cgi.test">
<tt class="descclassname">cgi.</tt><tt class="descname">test</tt><big>(</big><big>)</big><a class="headerlink" href="#cgi.test" title="Permalink to this definition">¶</a></dt>
<dd>Robust test CGI script, usable as main program. Writes minimal HTTP headers and
formats all information provided to the script in HTML form.
</dd></dl>

<dl class="function">
<dt id="cgi.print_environ">
<tt class="descclassname">cgi.</tt><tt class="descname">print_environ</tt><big>(</big><big>)</big><a class="headerlink" href="#cgi.print_environ" title="Permalink to this definition">¶</a></dt>
<dd>Format the shell environment in HTML.
</dd></dl>

<dl class="function">
<dt id="cgi.print_form">
<tt class="descclassname">cgi.</tt><tt class="descname">print_form</tt><big>(</big>form<big>)</big><a class="headerlink" href="#cgi.print_form" title="Permalink to this definition">¶</a></dt>
<dd>Format a form in HTML.
</dd></dl>

<dl class="function">
<dt id="cgi.print_directory">
<tt class="descclassname">cgi.</tt><tt class="descname">print_directory</tt><big>(</big><big>)</big><a class="headerlink" href="#cgi.print_directory" title="Permalink to this definition">¶</a></dt>
<dd>Format the current directory in HTML.
</dd></dl>

<dl class="function">
<dt id="cgi.print_environ_usage">
<tt class="descclassname">cgi.</tt><tt class="descname">print_environ_usage</tt><big>(</big><big>)</big><a class="headerlink" href="#cgi.print_environ_usage" title="Permalink to this definition">¶</a></dt>
<dd>Print a list of useful (used by CGI) environment variables in HTML.
</dd></dl>

<dl class="function">
<dt id="cgi.escape">
<tt class="descclassname">cgi.</tt><tt class="descname">escape</tt><big>(</big>s[, quote]<big>)</big><a class="headerlink" href="#cgi.escape" title="Permalink to this definition">¶</a></dt>
<dd>Convert the characters <tt class="docutils literal">'&amp;'</tt>, <tt class="docutils literal">'&lt;'</tt> and <tt class="docutils literal">'&gt;'</tt> in string s to HTML-safe
sequences. Use this if you need to display text that might contain such
characters in HTML. If the optional flag quote is true, the quotation mark
character (<tt class="docutils literal">&quot;</tt>) is also translated; this helps for inclusion in an HTML
attribute value delimited by double quotes, as in <tt class="docutils literal">&lt;a href=&quot;...&quot;&gt;</tt>. Note
that single quotes are never translated.
If the value to be quoted might include single- or double-quote characters,
or both, consider using the <a class="reference internal" href="xml.sax.utils.html#xml.sax.saxutils.quoteattr" title="xml.sax.saxutils.quoteattr"><tt class="xref py py-func docutils literal">quoteattr()</tt></a> function in the
<a class="reference internal" href="xml.sax.utils.html#module-xml.sax.saxutils" title="xml.sax.saxutils: Convenience functions and classes for use with SAX."><tt class="xref py py-mod docutils literal">xml.sax.saxutils</tt></a> module instead.
</dd></dl>

</div>
<div class="section" id="caring-about-security">
<h2>20.2.6. Caring about security<a class="headerlink" href="#caring-about-security" title="Permalink to this headline">¶</a></h2>
There&#8217;s one important rule: if you invoke an external program (via the
<a class="reference internal" href="os.html#os.system" title="os.system"><tt class="xref py py-func docutils literal">os.system()</tt></a> or <a class="reference internal" href="os.html#os.popen" title="os.popen"><tt class="xref py py-func docutils literal">os.popen()</tt></a> functions. or others with similar
functionality), make very sure you don&#8217;t pass arbitrary strings received from
the client to the shell. This is a well-known security hole whereby clever
hackers anywhere on the Web can exploit a gullible CGI script to invoke
arbitrary shell commands. Even parts of the URL or field names cannot be
trusted, since the request doesn&#8217;t have to come from your form!
To be on the safe side, if you must pass a string gotten from a form to a shell
command, you should make sure the string contains only alphanumeric characters,
dashes, underscores, and periods.
</div>
<div class="section" id="installing-your-cgi-script-on-a-unix-system">
<h2>20.2.7. Installing your CGI script on a Unix system<a class="headerlink" href="#installing-your-cgi-script-on-a-unix-system" title="Permalink to this headline">¶</a></h2>
Read the documentation for your HTTP server and check with your local system
administrator to find the directory where CGI scripts should be installed;
usually this is in a directory <tt class="file docutils literal">cgi-bin</tt> in the server tree.
Make sure that your script is readable and executable by &#8220;others&#8221;; the Unix file
mode should be <tt class="docutils literal">0755</tt> octal (use <tt class="docutils literal">chmod 0755 filename</tt>). Make sure that the
first line of the script contains <tt class="docutils literal">#!</tt> starting in column 1 followed by the
pathname of the Python interpreter, for instance:
<div class="highlight-python"><div class="highlight"><pre>#!/usr/local/bin/python
</pre></div>
</div>
Make sure the Python interpreter exists and is executable by &#8220;others&#8221;.
Make sure that any files your script needs to read or write are readable or
writable, respectively, by &#8220;others&#8221; &#8212; their mode should be <tt class="docutils literal">0644</tt> for
readable and <tt class="docutils literal">0666</tt> for writable. This is because, for security reasons, the
HTTP server executes your script as user &#8220;nobody&#8221;, without any special
privileges. It can only read (write, execute) files that everybody can read
(write, execute). The current directory at execution time is also different (it
is usually the server&#8217;s cgi-bin directory) and the set of environment variables
is also different from what you get when you log in. In particular, don&#8217;t count
on the shell&#8217;s search path for executables (<tt class="xref std std-envvar docutils literal">PATH</tt>) or the Python module
search path (<a class="reference internal" href="../using/cmdline.html#envvar-PYTHONPATH"><tt class="xref std std-envvar docutils literal">PYTHONPATH</tt></a>) to be set to anything interesting.
If you need to load modules from a directory which is not on Python&#8217;s default
module search path, you can change the path in your script, before importing
other modules. For example:
<div class="highlight-python"><div class="highlight"><pre>import sys
sys.path.insert(0, &quot;/usr/home/joe/lib/python&quot;)
sys.path.insert(0, &quot;/usr/local/lib/python&quot;)
</pre></div>
</div>
(This way, the directory inserted last will be searched first!)
Instructions for non-Unix systems will vary; check your HTTP server&#8217;s
documentation (it will usually have a section on CGI scripts).
</div>
<div class="section" id="testing-your-cgi-script">
<h2>20.2.8. Testing your CGI script<a class="headerlink" href="#testing-your-cgi-script" title="Permalink to this headline">¶</a></h2>
Unfortunately, a CGI script will generally not run when you try it from the
command line, and a script that works perfectly from the command line may fail
mysteriously when run from the server. There&#8217;s one reason why you should still
test your script from the command line: if it contains a syntax error, the
Python interpreter won&#8217;t execute it at all, and the HTTP server will most likely
send a cryptic error to the client.
Assuming your script has no syntax errors, yet it does not work, you have no
choice but to read the next section.
</div>
<div class="section" id="debugging-cgi-scripts">
<h2>20.2.9. Debugging CGI scripts<a class="headerlink" href="#debugging-cgi-scripts" title="Permalink to this headline">¶</a></h2>
First of all, check for trivial installation errors &#8212; 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 (<tt class="file docutils literal">cgi.py</tt>) 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&#8217;s
installed in the standard <tt class="file docutils literal">cgi-bin</tt> directory, it should be possible to
send it a request by entering a URL into your browser of the form:
<div class="highlight-python"><pre>http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&amp;addr=At+Home</pre>
</div>
If this gives an error of type 404, the server cannot find the script &#8211; perhaps
you need to install it in a different directory. If it gives another error,
there&#8217;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 &#8220;addr&#8221; with value &#8220;At
Home&#8221; and &#8220;name&#8221; with value &#8220;Joe Blow&#8221;), the <tt class="file docutils literal">cgi.py</tt> 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 <a class="reference internal" href="#module-cgi" title="cgi: Helpers for running Python scripts via the Common Gateway Interface."><tt class="xref py py-mod docutils literal">cgi</tt></a> module&#8217;s <a class="reference internal" href="test.html#module-test" title="test: Regression tests package containing the testing suite for Python."><tt class="xref py py-func docutils literal">test()</tt></a> function
from your script: replace its main code with the single statement
<div class="highlight-python"><div class="highlight"><pre>cgi.test()
</pre></div>
</div>
This should produce the same results as those gotten from installing the
<tt class="file docutils literal">cgi.py</tt> 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&#8217;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&#8217;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 <a class="reference internal" href="cgitb.html#module-cgitb" title="cgitb: Configurable traceback handler for CGI scripts."><tt class="xref py py-mod docutils literal">cgitb</tt></a> module.
If you haven&#8217;t done so already, just add the lines:
<div class="highlight-python"><div class="highlight"><pre>import cgitb
cgitb.enable()
</pre></div>
</div>
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 <a class="reference internal" href="cgitb.html#module-cgitb" title="cgitb: Configurable traceback handler for CGI scripts."><tt class="xref py py-mod docutils literal">cgitb</tt></a> module,
you can use an even more robust approach (which only uses built-in modules):
<div class="highlight-python"><div class="highlight"><pre>import sys
sys.stderr = sys.stdout
print &quot;Content-Type: text/plain&quot;
print
...your code here...
</pre></div>
</div>
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.
</div>
<div class="section" id="common-problems-and-solutions">
<h2>20.2.10. Common problems and solutions<a class="headerlink" href="#common-problems-and-solutions" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li>Most HTTP servers buffer the output from CGI scripts until the script is
completed. This means that it is not possible to display a progress report on
the client&#8217;s display while the script is running.</li>
<li>Check the installation instructions above.</li>
<li>Check the HTTP server&#8217;s log files. (<tt class="docutils literal">tail -f logfile</tt> in a separate window
may be useful!)</li>
<li>Always check a script for syntax errors first, by doing something like
<tt class="docutils literal">python script.py</tt>.</li>
<li>If your script does not have any syntax errors, try adding <tt class="docutils literal">import cgitb;
cgitb.enable()</tt> to the top of the script.</li>
<li>When invoking external programs, make sure they can be found. Usually, this
means using absolute path names &#8212; <tt class="xref std std-envvar docutils literal">PATH</tt> is usually not set to a very
useful value in a CGI script.</li>
<li>When reading or writing external files, make sure they can be read or written
by the userid under which your CGI script will be running: this is typically the
userid under which the web server is running, or some explicitly specified
userid for a web server&#8217;s <tt class="docutils literal">suexec</tt> feature.</li>
<li>Don&#8217;t try to give a CGI script a set-uid mode. This doesn&#8217;t work on most
systems, and is a security liability as well.</li>
</ul>
Footnotes
<table class="docutils footnote" frame="void" id="id3" rules="none">
<colgroup><col class="label" /><col /></colgroup>
<tbody valign="top">
<tr><td class="label"><a class="fn-backref" href="#id2">[1]</a></td><td>Note that some recent versions of the HTML specification do state what order the
field values should be supplied in, but knowing whether a request was
received from a conforming browser, or even from a browser at all, is tedious
and error-prone.</td></tr>
</tbody>
</table>
</div>
</div>

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 <ul>
<li><a class="reference internal" href="#">20.2. <tt class="docutils literal">cgi</tt> &#8212; Common Gateway Interface support</a><ul>
<li><a class="reference internal" href="#introduction">20.2.1. Introduction</a></li>
<li><a class="reference internal" href="#using-the-cgi-module">20.2.2. Using the cgi module</a></li>
<li><a class="reference internal" href="#higher-level-interface">20.2.3. Higher Level Interface</a></li>
<li><a class="reference internal" href="#old-classes">20.2.4. Old classes</a></li>
<li><a class="reference internal" href="#functions">20.2.5. Functions</a></li>
<li><a class="reference internal" href="#caring-about-security">20.2.6. Caring about security</a></li>
<li><a class="reference internal" href="#installing-your-cgi-script-on-a-unix-system">20.2.7. Installing your CGI script on a Unix system</a></li>
<li><a class="reference internal" href="#testing-your-cgi-script">20.2.8. Testing your CGI script</a></li>
<li><a class="reference internal" href="#debugging-cgi-scripts">20.2.9. Debugging CGI scripts</a></li>
<li><a class="reference internal" href="#common-problems-and-solutions">20.2.10. Common problems and solutions</a></li>
</ul>
</li>
</ul>

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