projects / OpenSPARC-T2-DV / blame
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| 54 | ||
| 55 | <H2><A NAME="SECTION0013230000000000000000"> | |
| 56 | 11.2.3 Higher Level Interface</A> | |
| 57 | </H2> | |
| 58 | ||
| 59 | <P> | |
| 60 | ||
| 61 | <span class="versionnote">New in version 2.2.</span> | |
| 62 | ||
| 63 | <P> | |
| 64 | The previous section explains how to read CGI form data using the | |
| 65 | <tt class="class">FieldStorage</tt> class. This section describes a higher level | |
| 66 | interface which was added to this class to allow one to do it in a | |
| 67 | more readable and intuitive way. The interface doesn't make the | |
| 68 | techniques described in previous sections obsolete -- they are still | |
| 69 | useful to process file uploads efficiently, for example. | |
| 70 | ||
| 71 | <P> | |
| 72 | The interface consists of two simple methods. Using the methods | |
| 73 | you can process form data in a generic way, without the need to worry | |
| 74 | whether only one or more values were posted under one name. | |
| 75 | ||
| 76 | <P> | |
| 77 | In the previous section, you learned to write following code anytime | |
| 78 | you expected a user to post more than one value under one name: | |
| 79 | ||
| 80 | <P> | |
| 81 | <div class="verbatim"><pre> | |
| 82 | item = form.getvalue("item") | |
| 83 | if isinstance(item, list): | |
| 84 | # The user is requesting more than one item. | |
| 85 | else: | |
| 86 | # The user is requesting only one item. | |
| 87 | </pre></div> | |
| 88 | ||
| 89 | <P> | |
| 90 | This situation is common for example when a form contains a group of | |
| 91 | multiple checkboxes with the same name: | |
| 92 | ||
| 93 | <P> | |
| 94 | <div class="verbatim"><pre> | |
| 95 | <input type="checkbox" name="item" value="1" /> | |
| 96 | <input type="checkbox" name="item" value="2" /> | |
| 97 | </pre></div> | |
| 98 | ||
| 99 | <P> | |
| 100 | In most situations, however, there's only one form control with a | |
| 101 | particular name in a form and then you expect and need only one value | |
| 102 | associated with this name. So you write a script containing for | |
| 103 | example this code: | |
| 104 | ||
| 105 | <P> | |
| 106 | <div class="verbatim"><pre> | |
| 107 | user = form.getvalue("user").upper() | |
| 108 | </pre></div> | |
| 109 | ||
| 110 | <P> | |
| 111 | The problem with the code is that you should never expect that a | |
| 112 | client will provide valid input to your scripts. For example, if a | |
| 113 | curious user appends another "<tt class="samp">user=foo</tt>" pair to the query string, | |
| 114 | then the script would crash, because in this situation the | |
| 115 | <code>getvalue("user")</code> method call returns a list instead of a | |
| 116 | string. Calling the <tt class="method">toupper()</tt> method on a list is not valid | |
| 117 | (since lists do not have a method of this name) and results in an | |
| 118 | <tt class="exception">AttributeError</tt> exception. | |
| 119 | ||
| 120 | <P> | |
| 121 | Therefore, the appropriate way to read form data values was to always | |
| 122 | use the code which checks whether the obtained value is a single value | |
| 123 | or a list of values. That's annoying and leads to less readable | |
| 124 | scripts. | |
| 125 | ||
| 126 | <P> | |
| 127 | A more convenient approach is to use the methods <tt class="method">getfirst()</tt> | |
| 128 | and <tt class="method">getlist()</tt> provided by this higher level interface. | |
| 129 | ||
| 130 | <P> | |
| 131 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 132 | <td><nobr><b><tt id='l2h-3159' xml:id='l2h-3159' class="method">getfirst</tt></b>(</nobr></td> | |
| 133 | <td><var>name</var><big>[</big><var>, default</var><big>]</big><var></var>)</td></tr></table></dt> | |
| 134 | <dd> | |
| 135 | This method always returns only one value associated with form field | |
| 136 | <var>name</var>. The method returns only the first value in case that | |
| 137 | more values were posted under such name. Please note that the order | |
| 138 | in which the values are received may vary from browser to browser | |
| 139 | and should not be counted on.<A NAME="tex2html53" | |
| 140 | HREF="#foot47232"><SUP>11.1</SUP></A> If no such form | |
| 141 | field or value exists then the method returns the value specified by | |
| 142 | the optional parameter <var>default</var>. This parameter defaults to | |
| 143 | <code>None</code> if not specified. | |
| 144 | </dl> | |
| 145 | ||
| 146 | <P> | |
| 147 | <dl><dt><table cellpadding="0" cellspacing="0"><tr valign="baseline"> | |
| 148 | <td><nobr><b><tt id='l2h-3160' xml:id='l2h-3160' class="method">getlist</tt></b>(</nobr></td> | |
| 149 | <td><var>name</var>)</td></tr></table></dt> | |
| 150 | <dd> | |
| 151 | This method always returns a list of values associated with form | |
| 152 | field <var>name</var>. The method returns an empty list if no such form | |
| 153 | field or value exists for <var>name</var>. It returns a list consisting | |
| 154 | of one item if only one such value exists. | |
| 155 | </dl> | |
| 156 | ||
| 157 | <P> | |
| 158 | Using these methods you can write nice compact code: | |
| 159 | ||
| 160 | <P> | |
| 161 | <div class="verbatim"><pre> | |
| 162 | import cgi | |
| 163 | form = cgi.FieldStorage() | |
| 164 | user = form.getfirst("user", "").upper() # This way it's safe. | |
| 165 | for item in form.getlist("item"): | |
| 166 | do_something(item) | |
| 167 | </pre></div> | |
| 168 | ||
| 169 | <P> | |
| 170 | <BR><HR><H4>Footnotes</H4> | |
| 171 | <DL> | |
| 172 | <DT><A NAME="foot47232">... on.</A><A | |
| 173 | HREF="node472.html#tex2html53"><SUP>11.1</SUP></A></DT> | |
| 174 | <DD>Note that some recent | |
| 175 | versions of the HTML specification do state what order the | |
| 176 | field values should be supplied in, but knowing whether a | |
| 177 | request was received from a conforming browser, or even from a | |
| 178 | browser at all, is tedious and error-prone. | |
| 179 | ||
| 180 | </DD> | |
| 181 | </DL> | |
| 182 | <DIV CLASS="navigation"> | |
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| 184 | <p></p><hr /> | |
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| 188 | href="node471.html"><img src='../icons/previous.png' | |
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| 207 | <b class="navlabel">Previous:</b> | |
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| 213 | </div> | |
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| 216 | <span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span> | |
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