Mercurial > cpython-lru_cache-weakref
annotate Doc/lib/libcgi.tex @ 25838:8540887bcec6 2.2
Backport of 2.183:
Fix memory leak in add_subclass() found by NealN with valgrind.
| author | Guido van Rossum <guido@python.org> |
|---|---|
| date | Fri, 18 Oct 2002 13:42:21 +0000 |
| parents | c68311efd346 |
| children | 3ebb4bcebdd7 |
| rev | line source |
|---|---|
| 8918 | 1 \section{\module{cgi} --- |
| 2 Common Gateway Interface support.} | |
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3 \declaremodule{standard}{cgi} |
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4 |
| 8918 | 5 \modulesynopsis{Common Gateway Interface support, used to interpret |
| 6 forms in server-side scripts.} | |
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7 |
| 2449 | 8 \indexii{WWW}{server} |
| 9 \indexii{CGI}{protocol} | |
| 10 \indexii{HTTP}{protocol} | |
| 11 \indexii{MIME}{headers} | |
| 12 \index{URL} | |
| 13 | |
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14 |
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15 Support module for Common Gateway Interface (CGI) scripts.% |
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16 \index{Common Gateway Interface} |
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17 |
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18 This module defines a number of utilities for use by CGI scripts |
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19 written in Python. |
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20 |
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21 \subsection{Introduction} |
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22 \nodename{cgi-intro} |
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23 |
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24 A CGI script is invoked by an HTTP server, usually to process user |
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25 input submitted through an HTML \code{<FORM>} or \code{<ISINDEX>} element. |
| 2449 | 26 |
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27 Most often, CGI scripts live in the server's special \file{cgi-bin} |
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28 directory. The HTTP server places all sorts of information about the |
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29 request (such as the client's hostname, the requested URL, the query |
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30 string, and lots of other goodies) in the script's shell environment, |
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31 executes the script, and sends the script's output back to the client. |
| 2449 | 32 |
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33 The script's input is connected to the client too, and sometimes the |
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34 form data is read this way; at other times the form data is passed via |
| 7663 | 35 the ``query string'' part of the URL. This module is intended |
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36 to take care of the different cases and provide a simpler interface to |
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37 the Python script. It also provides a number of utilities that help |
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38 in debugging scripts, and the latest addition is support for file |
| 7663 | 39 uploads from a form (if your browser supports it --- Grail 0.3 and |
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40 Netscape 2.0 do). |
| 2449 | 41 |
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42 The output of a CGI script should consist of two sections, separated |
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43 by a blank line. The first section contains a number of headers, |
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44 telling the client what kind of data is following. Python code to |
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45 generate a minimal header section looks like this: |
| 2449 | 46 |
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47 \begin{verbatim} |
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48 print "Content-Type: text/html" # HTML is following |
| 5818 | 49 print # blank line, end of headers |
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50 \end{verbatim} |
| 7663 | 51 |
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52 The second section is usually HTML, which allows the client software |
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53 to display nicely formatted text with header, in-line images, etc. |
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54 Here's Python code that prints a simple piece of HTML: |
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55 |
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56 \begin{verbatim} |
| 5818 | 57 print "<TITLE>CGI script output</TITLE>" |
| 58 print "<H1>This is my first CGI script</H1>" | |
| 59 print "Hello, world!" | |
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60 \end{verbatim} |
| 7663 | 61 |
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62 \subsection{Using the cgi module} |
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63 \nodename{Using the cgi module} |
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64 |
| 7663 | 65 Begin by writing \samp{import cgi}. Do not use \samp{from cgi import |
| 66 *} --- the module defines all sorts of names for its own use or for | |
| 67 backward compatibility that you don't want in your namespace. | |
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68 |
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69 When you write a new script, consider adding the line: |
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70 |
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71 \begin{verbatim} |
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72 import cgitb; cgitb.enable() |
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73 \end{verbatim} |
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74 |
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75 This activates a special exception handler that will display detailed |
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76 reports in the Web browser if any errors occur. If you'd rather not |
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77 show the guts of your program to users of your script, you can have |
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78 the reports saved to files instead, with a line like this: |
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79 |
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80 \begin{verbatim} |
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81 import cgitb; cgitb.enable(display=0, logdir="/tmp") |
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82 \end{verbatim} |
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83 |
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84 It's very helpful to use this feature during script development. |
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85 The reports produced by \refmodule{cgitb} provide information that |
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86 can save you a lot of time in tracking down bugs. You can always |
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87 remove the \code{cgitb} line later when you have tested your script |
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88 and are confident that it works correctly. |
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89 |
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90 To get at submitted form data, |
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91 it's best to use the \class{FieldStorage} class. The other classes |
| 7663 | 92 defined in this module are provided mostly for backward compatibility. |
| 93 Instantiate it exactly once, without arguments. This reads the form | |
| 94 contents from standard input or the environment (depending on the | |
| 95 value of various environment variables set according to the CGI | |
| 96 standard). Since it may consume standard input, it should be | |
| 97 instantiated only once. | |
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98 |
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99 The \class{FieldStorage} instance can be indexed like a Python |
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100 dictionary, and also supports the standard dictionary methods |
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101 \method{has_key()} and \method{keys()}. The built-in \function{len()} |
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102 is also supported. Form fields containing empty strings are ignored |
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103 and do not appear in the dictionary; to keep such values, provide |
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104 a true value for the the optional \var{keep_blank_values} keyword |
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105 parameter when creating the \class{FieldStorage} instance. |
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106 |
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107 For instance, the following code (which assumes that the |
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108 \mailheader{Content-Type} header and blank line have already been |
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109 printed) checks that the fields \code{name} and \code{addr} are both |
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110 set to a non-empty string: |
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111 |
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112 \begin{verbatim} |
| 5818 | 113 form = cgi.FieldStorage() |
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114 if not (form.has_key("name") and form.has_key("addr")): |
| 5818 | 115 print "<H1>Error</H1>" |
| 116 print "Please fill in the name and addr fields." | |
| 117 return | |
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118 print "<p>name:", form["name"].value |
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119 print "<p>addr:", form["addr"].value |
| 5818 | 120 ...further form processing here... |
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121 \end{verbatim} |
| 7663 | 122 |
| 123 Here the fields, accessed through \samp{form[\var{key}]}, are | |
| 124 themselves instances of \class{FieldStorage} (or | |
| 125 \class{MiniFieldStorage}, depending on the form encoding). | |
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126 The \member{value} attribute of the instance yields the string value |
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127 of the field. The \method{getvalue()} method returns this string value |
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128 directly; it also accepts an optional second argument as a default to |
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129 return if the requested key is not present. |
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130 |
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131 If the submitted form data contains more than one field with the same |
| 7663 | 132 name, the object retrieved by \samp{form[\var{key}]} is not a |
| 133 \class{FieldStorage} or \class{MiniFieldStorage} | |
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134 instance but a list of such instances. Similarly, in this situation, |
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135 \samp{form.getvalue(\var{key})} would return a list of strings. |
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136 If you expect this possibility |
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137 (when your HTML form contains multiple fields with the same name), use |
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138 the \function{isinstance()} built-in function to determine whether you |
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139 have a single instance or a list of instances. For example, this |
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140 code concatenates any number of username fields, separated by |
| 7663 | 141 commas: |
| 2449 | 142 |
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143 \begin{verbatim} |
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144 value = form.getvalue("username", "") |
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145 if isinstance(value, list): |
| 5818 | 146 # Multiple username fields specified |
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147 usernames = ",".join(value) |
| 5818 | 148 else: |
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149 # Single or no username field specified |
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150 usernames = value |
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151 \end{verbatim} |
| 7663 | 152 |
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153 If a field represents an uploaded file, accessing the value via the |
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154 \member{value} attribute or the \function{getvalue()} method reads the |
| 7663 | 155 entire file in memory as a string. This may not be what you want. |
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156 You can test for an uploaded file by testing either the \member{filename} |
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157 attribute or the \member{file} attribute. You can then read the data at |
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158 leisure from the \member{file} attribute: |
| 2449 | 159 |
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160 \begin{verbatim} |
| 5818 | 161 fileitem = form["userfile"] |
| 162 if fileitem.file: | |
| 163 # It's an uploaded file; count lines | |
| 164 linecount = 0 | |
| 165 while 1: | |
| 166 line = fileitem.file.readline() | |
| 167 if not line: break | |
| 168 linecount = linecount + 1 | |
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169 \end{verbatim} |
| 7663 | 170 |
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171 The file upload draft standard entertains the possibility of uploading |
| 7663 | 172 multiple files from one field (using a recursive |
| 173 \mimetype{multipart/*} encoding). When this occurs, the item will be | |
| 174 a dictionary-like \class{FieldStorage} item. This can be determined | |
| 175 by testing its \member{type} attribute, which should be | |
| 176 \mimetype{multipart/form-data} (or perhaps another MIME type matching | |
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177 \mimetype{multipart/*}). In this case, it can be iterated over |
| 7663 | 178 recursively just like the top-level form object. |
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179 |
| 7663 | 180 When a form is submitted in the ``old'' format (as the query string or |
| 181 as a single data part of type | |
| 182 \mimetype{application/x-www-form-urlencoded}), the items will actually | |
| 183 be instances of the class \class{MiniFieldStorage}. In this case, the | |
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184 \member{list}, \member{file}, and \member{filename} attributes are |
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185 always \code{None}. |
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186 |
| 2449 | 187 |
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188 \subsection{Higher Level Interface} |
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189 |
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190 \versionadded{2.2} % XXX: Is this true ? |
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191 |
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192 The previous section explains how to read CGI form data using the |
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193 \class{FieldStorage} class. This section describes a higher level |
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194 interface which was added to this class to allow one to do it in a |
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195 more readable and intuitive way. The interface doesn't make the |
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196 techniques described in previous sections obsolete --- they are still |
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197 useful to process file uploads efficiently, for example. |
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198 |
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199 The interface consists of two simple methods. Using the methods |
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200 you can process form data in a generic way, without the need to worry |
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201 whether only one or more values were posted under one name. |
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202 |
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203 In the previous section, you learned to write following code anytime |
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204 you expected a user to post more than one value under one name: |
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205 |
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206 \begin{verbatim} |
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207 item = form.getvalue("item") |
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208 if isinstance(item, list): |
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209 # The user is requesting more than one item. |
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210 else: |
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211 # The user is requesting only one item. |
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212 \end{verbatim} |
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213 |
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214 This situation is common for example when a form contains a group of |
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215 multiple checkboxes with the same name: |
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216 |
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217 \begin{verbatim} |
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218 <input type="checkbox" name="item" value="1" /> |
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219 <input type="checkbox" name="item" value="2" /> |
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220 \end{verbatim} |
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221 |
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222 In most situations, however, there's only one form control with a |
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223 particular name in a form and then you expect and need only one value |
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224 associated with this name. So you write a script containing for |
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225 example this code: |
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226 |
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227 \begin{verbatim} |
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228 user = form.getvalue("user").toupper() |
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229 \end{verbatim} |
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230 |
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231 The problem with the code is that you should never expect that a |
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232 client will provide valid input to your scripts. For example, if a |
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233 curious user appends another \samp{user=foo} pair to the query string, |
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234 then the script would crash, because in this situation the |
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235 \code{getvalue("user")} method call returns a list instead of a |
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236 string. Calling the \method{toupper()} method on a list is not valid |
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237 (since lists do not have a method of this name) and results in an |
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238 \exception{AttributeError} exception. |
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239 |
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240 Therefore, the appropriate way to read form data values was to always |
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241 use the code which checks whether the obtained value is a single value |
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242 or a list of values. That's annoying and leads to less readable |
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243 scripts. |
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244 |
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245 A more convenient approach is to use the methods \method{getfirst()} |
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246 and \method{getlist()} provided by this higher level interface. |
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247 |
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248 \begin{methoddesc}[FieldStorage]{getfirst}{name\optional{, default}} |
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249 Thin method always returns only one value associated with form field |
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250 \var{name}. The method returns only the first value in case that |
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251 more values were posted under such name. Please note that the order |
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252 in which the values are received may vary from browser to browser |
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253 and should not be counted on.\footnote{Note that some recent |
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254 versions of the HTML specification do state what order the |
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255 field values should be supplied in, but knowing whether a |
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256 request was received from a conforming browser, or even from a |
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257 browser at all, is tedious and error-prone.} If no such form |
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258 field or value exists then the method returns the value specified by |
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259 the optional parameter \var{default}. This parameter defaults to |
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260 \code{None} if not specified. |
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261 \end{methoddesc} |
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262 |
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263 \begin{methoddesc}[FieldStorage]{getlist}{name} |
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264 This method always returns a list of values associated with form |
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265 field \var{name}. The method returns an empty list if no such form |
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266 field or value exists for \var{name}. It returns a list consisting |
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267 of one item if only one such value exists. |
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268 \end{methoddesc} |
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269 |
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270 Using these methods you can write nice compact code: |
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271 |
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272 \begin{verbatim} |
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273 import cgi |
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274 form = cgi.FieldStorage() |
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275 user = form.getfirst("user", "").toupper() # This way it's safe. |
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276 for item in form.getlist("item"): |
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277 do_something(item) |
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278 \end{verbatim} |
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279 |
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280 |
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281 \subsection{Old classes} |
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282 |
| 7663 | 283 These classes, present in earlier versions of the \module{cgi} module, |
| 284 are still supported for backward compatibility. New applications | |
| 285 should use the \class{FieldStorage} class. | |
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286 |
| 7663 | 287 \class{SvFormContentDict} stores single value form content as |
| 288 dictionary; it assumes each field name occurs in the form only once. | |
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289 |
| 7663 | 290 \class{FormContentDict} stores multiple value form content as a |
| 291 dictionary (the form items are lists of values). Useful if your form | |
| 292 contains multiple fields with the same name. | |
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293 |
| 7663 | 294 Other classes (\class{FormContent}, \class{InterpFormContentDict}) are |
| 295 present for backwards compatibility with really old applications only. | |
| 296 If you still use these and would be inconvenienced when they | |
| 297 disappeared from a next version of this module, drop me a note. | |
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298 |
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299 |
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300 \subsection{Functions} |
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301 \nodename{Functions in cgi module} |
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302 |
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303 These are useful if you want more control, or if you want to employ |
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304 some of the algorithms implemented in this module in other |
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305 circumstances. |
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306 |
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307 \begin{funcdesc}{parse}{fp\optional{, keep_blank_values\optional{, |
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308 strict_parsing}}} |
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309 Parse a query in the environment or from a file (the file defaults |
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310 to \code{sys.stdin}). The \var{keep_blank_values} and |
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311 \var{strict_parsing} parameters are passed to \function{parse_qs()} |
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312 unchanged. |
| 2449 | 313 \end{funcdesc} |
| 314 | |
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315 \begin{funcdesc}{parse_qs}{qs\optional{, keep_blank_values\optional{, |
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316 strict_parsing}}} |
| 7663 | 317 Parse a query string given as a string argument (data of type |
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318 \mimetype{application/x-www-form-urlencoded}). Data are |
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319 returned as a dictionary. The dictionary keys are the unique query |
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320 variable names and the values are lists of values for each name. |
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321 |
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322 The optional argument \var{keep_blank_values} is |
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323 a flag indicating whether blank values in |
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324 URL encoded queries should be treated as blank strings. |
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325 A true value indicates that blanks should be retained as |
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326 blank strings. The default false value indicates that |
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327 blank values are to be ignored and treated as if they were |
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328 not included. |
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329 |
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330 The optional argument \var{strict_parsing} is a flag indicating what |
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331 to do with parsing errors. If false (the default), errors |
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332 are silently ignored. If true, errors raise a ValueError |
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333 exception. |
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334 \end{funcdesc} |
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335 |
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336 \begin{funcdesc}{parse_qsl}{qs\optional{, keep_blank_values\optional{, |
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337 strict_parsing}}} |
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338 Parse a query string given as a string argument (data of type |
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339 \mimetype{application/x-www-form-urlencoded}). Data are |
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340 returned as a list of name, value pairs. |
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341 |
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342 The optional argument \var{keep_blank_values} is |
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343 a flag indicating whether blank values in |
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344 URL encoded queries should be treated as blank strings. |
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345 A true value indicates that blanks should be retained as |
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346 blank strings. The default false value indicates that |
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347 blank values are to be ignored and treated as if they were |
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348 not included. |
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349 |
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350 The optional argument \var{strict_parsing} is a flag indicating what |
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351 to do with parsing errors. If false (the default), errors |
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352 are silently ignored. If true, errors raise a ValueError |
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353 exception. |
| 2449 | 354 \end{funcdesc} |
| 355 | |
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356 \begin{funcdesc}{parse_multipart}{fp, pdict} |
| 7663 | 357 Parse input of type \mimetype{multipart/form-data} (for |
| 358 file uploads). Arguments are \var{fp} for the input file and | |
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359 \var{pdict} for a dictionary containing other parameters in |
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360 the \mailheader{Content-Type} header. |
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361 |
| 7663 | 362 Returns a dictionary just like \function{parse_qs()} keys are the |
| 363 field names, each value is a list of values for that field. This is | |
| 364 easy to use but not much good if you are expecting megabytes to be | |
| 365 uploaded --- in that case, use the \class{FieldStorage} class instead | |
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366 which is much more flexible. |
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367 |
| 7663 | 368 Note that this does not parse nested multipart parts --- use |
| 369 \class{FieldStorage} for that. | |
| 2449 | 370 \end{funcdesc} |
| 371 | |
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372 \begin{funcdesc}{parse_header}{string} |
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373 Parse a MIME header (such as \mailheader{Content-Type}) into a main |
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374 value and a dictionary of parameters. |
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375 \end{funcdesc} |
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376 |
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377 \begin{funcdesc}{test}{} |
| 7663 | 378 Robust test CGI script, usable as main program. |
| 379 Writes minimal HTTP headers and formats all information provided to | |
| 380 the script in HTML form. | |
| 2449 | 381 \end{funcdesc} |
| 382 | |
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383 \begin{funcdesc}{print_environ}{} |
| 7663 | 384 Format the shell environment in HTML. |
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385 \end{funcdesc} |
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386 |
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387 \begin{funcdesc}{print_form}{form} |
| 7663 | 388 Format a form in HTML. |
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389 \end{funcdesc} |
| 2449 | 390 |
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391 \begin{funcdesc}{print_directory}{} |
| 7663 | 392 Format the current directory in HTML. |
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393 \end{funcdesc} |
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394 |
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395 \begin{funcdesc}{print_environ_usage}{} |
| 7663 | 396 Print a list of useful (used by CGI) environment variables in |
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397 HTML. |
| 2449 | 398 \end{funcdesc} |
| 399 | |
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400 \begin{funcdesc}{escape}{s\optional{, quote}} |
| 7663 | 401 Convert the characters |
| 402 \character{\&}, \character{<} and \character{>} in string \var{s} to | |
| 403 HTML-safe sequences. Use this if you need to display text that might | |
| 404 contain such characters in HTML. If the optional flag \var{quote} is | |
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405 true, the double-quote character (\character{"}) is also translated; |
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406 this helps for inclusion in an HTML attribute value, as in \code{<A |
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407 HREF="...">}. If the value to be quoted might include single- or |
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408 double-quote characters, or both, consider using the |
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409 \function{quoteattr()} function in the \refmodule{xml.sax.saxutils} |
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410 module instead. |
| 2449 | 411 \end{funcdesc} |
| 412 | |
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413 |
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414 \subsection{Caring about security \label{cgi-security}} |
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415 |
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416 \indexii{CGI}{security} |
| 2449 | 417 |
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418 There's one important rule: if you invoke an external program (via the |
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419 \function{os.system()} or \function{os.popen()} functions. or others |
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420 with similar functionality), make very sure you don't pass arbitrary |
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421 strings received from the client to the shell. This is a well-known |
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422 security hole whereby clever hackers anywhere on the Web can exploit a |
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423 gullible CGI script to invoke arbitrary shell commands. Even parts of |
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424 the URL or field names cannot be trusted, since the request doesn't |
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425 have to come from your form! |
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426 |
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427 To be on the safe side, if you must pass a string gotten from a form |
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428 to a shell command, you should make sure the string contains only |
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429 alphanumeric characters, dashes, underscores, and periods. |
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430 |
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431 |
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432 \subsection{Installing your CGI script on a \UNIX\ system} |
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433 |
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434 Read the documentation for your HTTP server and check with your local |
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435 system administrator to find the directory where CGI scripts should be |
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436 installed; usually this is in a directory \file{cgi-bin} in the server tree. |
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437 |
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438 Make sure that your script is readable and executable by ``others''; the |
| 7663 | 439 \UNIX{} file mode should be \code{0755} octal (use \samp{chmod 0755 |
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440 \var{filename}}). Make sure that the first line of the script contains |
| 7663 | 441 \code{\#!} starting in column 1 followed by the pathname of the Python |
| 442 interpreter, for instance: | |
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443 |
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444 \begin{verbatim} |
| 5818 | 445 #!/usr/local/bin/python |
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446 \end{verbatim} |
| 7663 | 447 |
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448 Make sure the Python interpreter exists and is executable by ``others''. |
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449 |
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450 Make sure that any files your script needs to read or write are |
| 7663 | 451 readable or writable, respectively, by ``others'' --- their mode |
| 452 should be \code{0644} for readable and \code{0666} for writable. This | |
| 453 is because, for security reasons, the HTTP server executes your script | |
| 454 as user ``nobody'', without any special privileges. It can only read | |
| 455 (write, execute) files that everybody can read (write, execute). The | |
| 456 current directory at execution time is also different (it is usually | |
| 457 the server's cgi-bin directory) and the set of environment variables | |
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458 is also different from what you get when you log in. In particular, don't |
| 7663 | 459 count on the shell's search path for executables (\envvar{PATH}) or |
| 460 the Python module search path (\envvar{PYTHONPATH}) to be set to | |
| 461 anything interesting. | |
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462 |
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463 If you need to load modules from a directory which is not on Python's |
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464 default module search path, you can change the path in your script, |
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465 before importing other modules. For example: |
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466 |
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467 \begin{verbatim} |
| 5818 | 468 import sys |
| 469 sys.path.insert(0, "/usr/home/joe/lib/python") | |
| 470 sys.path.insert(0, "/usr/local/lib/python") | |
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471 \end{verbatim} |
| 7663 | 472 |
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473 (This way, the directory inserted last will be searched first!) |
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474 |
| 7086 | 475 Instructions for non-\UNIX{} systems will vary; check your HTTP server's |
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476 documentation (it will usually have a section on CGI scripts). |
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477 |
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478 |
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479 \subsection{Testing your CGI script} |
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480 |
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481 Unfortunately, a CGI script will generally not run when you try it |
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482 from the command line, and a script that works perfectly from the |
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483 command line may fail mysteriously when run from the server. There's |
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484 one reason why you should still test your script from the command |
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485 line: if it contains a syntax error, the Python interpreter won't |
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486 execute it at all, and the HTTP server will most likely send a cryptic |
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487 error to the client. |
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488 |
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489 Assuming your script has no syntax errors, yet it does not work, you |
| 7663 | 490 have no choice but to read the next section. |
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491 |
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492 |
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493 \subsection{Debugging CGI scripts} \indexii{CGI}{debugging} |
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494 |
| 7663 | 495 First of all, check for trivial installation errors --- reading the |
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496 section above on installing your CGI script carefully can save you a |
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497 lot of time. If you wonder whether you have understood the |
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498 installation procedure correctly, try installing a copy of this module |
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499 file (\file{cgi.py}) as a CGI script. When invoked as a script, the file |
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500 will dump its environment and the contents of the form in HTML form. |
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501 Give it the right mode etc, and send it a request. If it's installed |
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502 in the standard \file{cgi-bin} directory, it should be possible to send it a |
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503 request by entering a URL into your browser of the form: |
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504 |
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505 \begin{verbatim} |
| 5818 | 506 http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home |
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507 \end{verbatim} |
| 7663 | 508 |
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509 If this gives an error of type 404, the server cannot find the script |
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510 -- perhaps you need to install it in a different directory. If it |
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511 gives another error, there's an installation problem that |
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512 you should fix before trying to go any further. If you get a nicely |
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513 formatted listing of the environment and form content (in this |
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514 example, the fields should be listed as ``addr'' with value ``At Home'' |
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515 and ``name'' with value ``Joe Blow''), the \file{cgi.py} script has been |
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516 installed correctly. If you follow the same procedure for your own |
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517 script, you should now be able to debug it. |
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518 |
| 7663 | 519 The next step could be to call the \module{cgi} module's |
| 520 \function{test()} function from your script: replace its main code | |
| 521 with the single statement | |
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522 |
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523 \begin{verbatim} |
| 5818 | 524 cgi.test() |
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525 \end{verbatim} |
| 7663 | 526 |
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527 This should produce the same results as those gotten from installing |
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528 the \file{cgi.py} file itself. |
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529 |
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530 When an ordinary Python script raises an unhandled exception (for |
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531 whatever reason: of a typo in a module name, a file that can't be |
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532 opened, etc.), the Python interpreter prints a nice traceback and |
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533 exits. While the Python interpreter will still do this when your CGI |
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534 script raises an exception, most likely the traceback will end up in |
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535 one of the HTTP server's log files, or be discarded altogether. |
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536 |
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537 Fortunately, once you have managed to get your script to execute |
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538 \emph{some} code, you can easily send tracebacks to the Web browser |
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539 using the \refmodule{cgitb} module. If you haven't done so already, |
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540 just add the line: |
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541 |
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542 \begin{verbatim} |
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543 import cgitb; cgitb.enable() |
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544 \end{verbatim} |
| 7663 | 545 |
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546 to the top of your script. Then try running it again; when a |
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547 problem occurs, you should see a detailed report that will |
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548 likely make apparent the cause of the crash. |
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549 |
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550 If you suspect that there may be a problem in importing the |
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551 \refmodule{cgitb} module, you can use an even more robust approach |
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552 (which only uses built-in modules): |
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553 |
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554 \begin{verbatim} |
| 5818 | 555 import sys |
| 556 sys.stderr = sys.stdout | |
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557 print "Content-Type: text/plain" |
| 5818 | 558 print |
| 559 ...your code here... | |
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560 \end{verbatim} |
| 7663 | 561 |
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562 This relies on the Python interpreter to print the traceback. The |
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563 content type of the output is set to plain text, which disables all |
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564 HTML processing. If your script works, the raw HTML will be displayed |
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565 by your client. If it raises an exception, most likely after the |
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566 first two lines have been printed, a traceback will be displayed. |
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567 Because no HTML interpretation is going on, the traceback will be |
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568 readable. |
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569 |
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570 |
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571 \subsection{Common problems and solutions} |
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572 |
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573 \begin{itemize} |
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574 \item Most HTTP servers buffer the output from CGI scripts until the |
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575 script is completed. This means that it is not possible to display a |
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576 progress report on the client's display while the script is running. |
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577 |
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578 \item Check the installation instructions above. |
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579 |
| 7663 | 580 \item Check the HTTP server's log files. (\samp{tail -f logfile} in a |
| 581 separate window may be useful!) | |
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582 |
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583 \item Always check a script for syntax errors first, by doing something |
| 7663 | 584 like \samp{python script.py}. |
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585 |
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586 \item If your script does not have any syntax errors, try adding |
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587 \samp{import cgitb; cgitb.enable()} to the top of the script. |
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588 |
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589 \item When invoking external programs, make sure they can be found. |
| 7663 | 590 Usually, this means using absolute path names --- \envvar{PATH} is |
| 591 usually not set to a very useful value in a CGI script. | |
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592 |
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593 \item When reading or writing external files, make sure they can be read |
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594 or written by every user on the system. |
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595 |
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596 \item Don't try to give a CGI script a set-uid mode. This doesn't work on |
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597 most systems, and is a security liability as well. |
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598 \end{itemize} |
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599 |