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| 55 | <H2><A NAME="SECTION0013220000000000000000"> |
| 56 | 11.2.2 Using the cgi module</A> |
| 57 | </H2> |
| 58 | <A NAME="Using_the_cgi_module"></A> |
| 59 | <P> |
| 60 | Begin by writing "<tt class="samp">import cgi</tt>". Do not use "<tt class="samp">from cgi import |
| 61 | *</tt>" -- the module defines all sorts of names for its own use or for |
| 62 | backward compatibility that you don't want in your namespace. |
| 63 | |
| 64 | <P> |
| 65 | When you write a new script, consider adding the line: |
| 66 | |
| 67 | <P> |
| 68 | <div class="verbatim"><pre> |
| 69 | import cgitb; cgitb.enable() |
| 70 | </pre></div> |
| 71 | |
| 72 | <P> |
| 73 | This activates a special exception handler that will display detailed |
| 74 | reports in the Web browser if any errors occur. If you'd rather not |
| 75 | show the guts of your program to users of your script, you can have |
| 76 | the reports saved to files instead, with a line like this: |
| 77 | |
| 78 | <P> |
| 79 | <div class="verbatim"><pre> |
| 80 | import cgitb; cgitb.enable(display=0, logdir="/tmp") |
| 81 | </pre></div> |
| 82 | |
| 83 | <P> |
| 84 | It's very helpful to use this feature during script development. |
| 85 | The reports produced by <tt class="module"><a href="module-cgitb.html">cgitb</a></tt> provide information that |
| 86 | can save you a lot of time in tracking down bugs. You can always |
| 87 | remove the <code>cgitb</code> line later when you have tested your script |
| 88 | and are confident that it works correctly. |
| 89 | |
| 90 | <P> |
| 91 | To get at submitted form data, |
| 92 | it's best to use the <tt class="class">FieldStorage</tt> class. The other classes |
| 93 | defined in this module are provided mostly for backward compatibility. |
| 94 | Instantiate it exactly once, without arguments. This reads the form |
| 95 | contents from standard input or the environment (depending on the |
| 96 | value of various environment variables set according to the CGI |
| 97 | standard). Since it may consume standard input, it should be |
| 98 | instantiated only once. |
| 99 | |
| 100 | <P> |
| 101 | The <tt class="class">FieldStorage</tt> instance can be indexed like a Python |
| 102 | dictionary, and also supports the standard dictionary methods |
| 103 | <tt class="method">has_key()</tt> and <tt class="method">keys()</tt>. The built-in <tt class="function">len()</tt> |
| 104 | is also supported. Form fields containing empty strings are ignored |
| 105 | and do not appear in the dictionary; to keep such values, provide |
| 106 | a true value for the optional <var>keep_blank_values</var> keyword |
| 107 | parameter when creating the <tt class="class">FieldStorage</tt> instance. |
| 108 | |
| 109 | <P> |
| 110 | For instance, the following code (which assumes that the |
| 111 | <span class="mailheader">Content-Type:</span> header and blank line have already been |
| 112 | printed) checks that the fields <code>name</code> and <code>addr</code> are both |
| 113 | set to a non-empty string: |
| 114 | |
| 115 | <P> |
| 116 | <div class="verbatim"><pre> |
| 117 | form = cgi.FieldStorage() |
| 118 | if not (form.has_key("name") and form.has_key("addr")): |
| 119 | print "<H1>Error</H1>" |
| 120 | print "Please fill in the name and addr fields." |
| 121 | return |
| 122 | print "<p>name:", form["name"].value |
| 123 | print "<p>addr:", form["addr"].value |
| 124 | ...further form processing here... |
| 125 | </pre></div> |
| 126 | |
| 127 | <P> |
| 128 | Here the fields, accessed through "<tt class="samp">form[<var>key</var>]</tt>", are |
| 129 | themselves instances of <tt class="class">FieldStorage</tt> (or |
| 130 | <tt class="class">MiniFieldStorage</tt>, depending on the form encoding). |
| 131 | The <tt class="member">value</tt> attribute of the instance yields the string value |
| 132 | of the field. The <tt class="method">getvalue()</tt> method returns this string value |
| 133 | directly; it also accepts an optional second argument as a default to |
| 134 | return if the requested key is not present. |
| 135 | |
| 136 | <P> |
| 137 | If the submitted form data contains more than one field with the same |
| 138 | name, the object retrieved by "<tt class="samp">form[<var>key</var>]</tt>" is not a |
| 139 | <tt class="class">FieldStorage</tt> or <tt class="class">MiniFieldStorage</tt> |
| 140 | instance but a list of such instances. Similarly, in this situation, |
| 141 | "<tt class="samp">form.getvalue(<var>key</var>)</tt>" would return a list of strings. |
| 142 | If you expect this possibility |
| 143 | (when your HTML form contains multiple fields with the same name), use |
| 144 | the <tt class="function">getlist()</tt> function, which always returns a list of values (so that you |
| 145 | do not need to special-case the single item case). For example, this |
| 146 | code concatenates any number of username fields, separated by |
| 147 | commas: |
| 148 | |
| 149 | <P> |
| 150 | <div class="verbatim"><pre> |
| 151 | value = form.getlist("username") |
| 152 | usernames = ",".join(value) |
| 153 | </pre></div> |
| 154 | |
| 155 | <P> |
| 156 | If a field represents an uploaded file, accessing the value via the |
| 157 | <tt class="member">value</tt> attribute or the <tt class="function">getvalue()</tt> method reads the |
| 158 | entire file in memory as a string. This may not be what you want. |
| 159 | You can test for an uploaded file by testing either the <tt class="member">filename</tt> |
| 160 | attribute or the <tt class="member">file</tt> attribute. You can then read the data at |
| 161 | leisure from the <tt class="member">file</tt> attribute: |
| 162 | |
| 163 | <P> |
| 164 | <div class="verbatim"><pre> |
| 165 | fileitem = form["userfile"] |
| 166 | if fileitem.file: |
| 167 | # It's an uploaded file; count lines |
| 168 | linecount = 0 |
| 169 | while 1: |
| 170 | line = fileitem.file.readline() |
| 171 | if not line: break |
| 172 | linecount = linecount + 1 |
| 173 | </pre></div> |
| 174 | |
| 175 | <P> |
| 176 | The file upload draft standard entertains the possibility of uploading |
| 177 | multiple files from one field (using a recursive |
| 178 | <span class="mimetype">multipart/*</span> encoding). When this occurs, the item will be |
| 179 | a dictionary-like <tt class="class">FieldStorage</tt> item. This can be determined |
| 180 | by testing its <tt class="member">type</tt> attribute, which should be |
| 181 | <span class="mimetype">multipart/form-data</span> (or perhaps another MIME type matching |
| 182 | <span class="mimetype">multipart/*</span>). In this case, it can be iterated over |
| 183 | recursively just like the top-level form object. |
| 184 | |
| 185 | <P> |
| 186 | When a form is submitted in the ``old'' format (as the query string or |
| 187 | as a single data part of type |
| 188 | <span class="mimetype">application/x-www-form-urlencoded</span>), the items will actually |
| 189 | be instances of the class <tt class="class">MiniFieldStorage</tt>. In this case, the |
| 190 | <tt class="member">list</tt>, <tt class="member">file</tt>, and <tt class="member">filename</tt> attributes are |
| 191 | always <code>None</code>. |
| 192 | |
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