Mercurial > cpython-lru_cache-weakref
annotate Doc/lib/libcgi.tex @ 17581:c1937faabb23 2.1
Added support for optional explanation parameter to the \versionadded
macro.
Refactored do_cmd_versionadded() and do_cmd_versionchanged() to do most
of the work in a helper function, with the do_cmd_*() wrappers just supplying
a portion of the replacement text.
| author | Fred Drake <fdrake@acm.org> |
|---|---|
| date | Wed, 18 Apr 2001 17:29:14 +0000 |
| parents | 41234ae95d2a |
| children | 5668a8550be4 |
| 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 CGI (Common Gateway Interface) 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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There is no <ISINPUT> element, just <ISINDEX>. ;-)
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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 |
| 7663 | 69 It's best to use the \class{FieldStorage} class. The other classes |
| 70 defined in this module are provided mostly for backward compatibility. | |
| 71 Instantiate it exactly once, without arguments. This reads the form | |
| 72 contents from standard input or the environment (depending on the | |
| 73 value of various environment variables set according to the CGI | |
| 74 standard). Since it may consume standard input, it should be | |
| 75 instantiated only once. | |
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76 |
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77 The \class{FieldStorage} instance can be indexed like a Python |
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78 dictionary, and also supports the standard dictionary methods |
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79 \function{has_key()} and \function{keys()}. |
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80 Form fields containing empty strings are ignored |
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81 and do not appear in the dictionary; to keep such values, provide |
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82 the optional \samp{keep_blank_values} argument when creating the |
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83 \class {FieldStorage} instance. |
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84 |
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85 For instance, the following code (which assumes that the |
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86 \code{Content-Type} header and blank line have already been printed) |
| 7663 | 87 checks that the fields \code{name} and \code{addr} are both set to a |
| 88 non-empty string: | |
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89 |
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90 \begin{verbatim} |
| 5818 | 91 form = cgi.FieldStorage() |
| 92 form_ok = 0 | |
| 93 if form.has_key("name") and form.has_key("addr"): | |
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94 form_ok = 1 |
| 5818 | 95 if not form_ok: |
| 96 print "<H1>Error</H1>" | |
| 97 print "Please fill in the name and addr fields." | |
| 98 return | |
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99 print "<p>name:", form["name"].value |
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100 print "<p>addr:", form["addr"].value |
| 5818 | 101 ...further form processing here... |
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102 \end{verbatim} |
| 7663 | 103 |
| 104 Here the fields, accessed through \samp{form[\var{key}]}, are | |
| 105 themselves instances of \class{FieldStorage} (or | |
| 106 \class{MiniFieldStorage}, depending on the form encoding). | |
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107 The \member{value} attribute of the instance yields the string value |
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108 of the field. The \function{getvalue()} method returns this string value |
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109 directly; it also accepts an optional second argument as a default to |
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110 return if the requested key is not present. |
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111 |
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112 If the submitted form data contains more than one field with the same |
| 7663 | 113 name, the object retrieved by \samp{form[\var{key}]} is not a |
| 114 \class{FieldStorage} or \class{MiniFieldStorage} | |
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115 instance but a list of such instances. Similarly, in this situation, |
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116 \samp{form.getvalue(\var{key})} would return a list of strings. |
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117 If you expect this possibility |
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118 (i.e., when your HTML form contains multiple fields with the same |
| 7663 | 119 name), use the \function{type()} function to determine whether you |
| 120 have a single instance or a list of instances. For example, here's | |
| 121 code that concatenates any number of username fields, separated by | |
| 122 commas: | |
| 2449 | 123 |
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124 \begin{verbatim} |
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125 value = form.getvalue("username", "") |
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126 if type(value) is type([]): |
| 5818 | 127 # Multiple username fields specified |
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128 usernames = ",".join(value) |
| 5818 | 129 else: |
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130 # Single or no username field specified |
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131 usernames = value |
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132 \end{verbatim} |
| 7663 | 133 |
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134 If a field represents an uploaded file, accessing the value via the |
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135 \member{value} attribute or the \function{getvalue()} method reads the |
| 7663 | 136 entire file in memory as a string. This may not be what you want. |
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137 You can test for an uploaded file by testing either the \member{filename} |
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138 attribute or the \member{file} attribute. You can then read the data at |
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139 leisure from the \member{file} attribute: |
| 2449 | 140 |
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141 \begin{verbatim} |
| 5818 | 142 fileitem = form["userfile"] |
| 143 if fileitem.file: | |
| 144 # It's an uploaded file; count lines | |
| 145 linecount = 0 | |
| 146 while 1: | |
| 147 line = fileitem.file.readline() | |
| 148 if not line: break | |
| 149 linecount = linecount + 1 | |
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150 \end{verbatim} |
| 7663 | 151 |
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152 The file upload draft standard entertains the possibility of uploading |
| 7663 | 153 multiple files from one field (using a recursive |
| 154 \mimetype{multipart/*} encoding). When this occurs, the item will be | |
| 155 a dictionary-like \class{FieldStorage} item. This can be determined | |
| 156 by testing its \member{type} attribute, which should be | |
| 157 \mimetype{multipart/form-data} (or perhaps another MIME type matching | |
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158 \mimetype{multipart/*}). In this case, it can be iterated over |
| 7663 | 159 recursively just like the top-level form object. |
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160 |
| 7663 | 161 When a form is submitted in the ``old'' format (as the query string or |
| 162 as a single data part of type | |
| 163 \mimetype{application/x-www-form-urlencoded}), the items will actually | |
| 164 be instances of the class \class{MiniFieldStorage}. In this case, the | |
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165 \member{list}, \member{file}, and \member{filename} attributes are |
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166 always \code{None}. |
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167 |
| 2449 | 168 |
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169 \subsection{Old classes} |
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170 |
| 7663 | 171 These classes, present in earlier versions of the \module{cgi} module, |
| 172 are still supported for backward compatibility. New applications | |
| 173 should use the \class{FieldStorage} class. | |
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174 |
| 7663 | 175 \class{SvFormContentDict} stores single value form content as |
| 176 dictionary; it assumes each field name occurs in the form only once. | |
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177 |
| 7663 | 178 \class{FormContentDict} stores multiple value form content as a |
| 179 dictionary (the form items are lists of values). Useful if your form | |
| 180 contains multiple fields with the same name. | |
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181 |
| 7663 | 182 Other classes (\class{FormContent}, \class{InterpFormContentDict}) are |
| 183 present for backwards compatibility with really old applications only. | |
| 184 If you still use these and would be inconvenienced when they | |
| 185 disappeared from a next version of this module, drop me a note. | |
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186 |
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187 |
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188 \subsection{Functions} |
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189 \nodename{Functions in cgi module} |
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190 |
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191 These are useful if you want more control, or if you want to employ |
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192 some of the algorithms implemented in this module in other |
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193 circumstances. |
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194 |
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195 \begin{funcdesc}{parse}{fp} |
| 7663 | 196 Parse a query in the environment or from a file (default |
| 197 \code{sys.stdin}). | |
| 2449 | 198 \end{funcdesc} |
| 199 | |
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200 \begin{funcdesc}{parse_qs}{qs\optional{, keep_blank_values, strict_parsing}} |
| 7663 | 201 Parse a query string given as a string argument (data of type |
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202 \mimetype{application/x-www-form-urlencoded}). Data are |
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203 returned as a dictionary. The dictionary keys are the unique query |
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204 variable names and the values are lists of values for each name. |
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205 |
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206 The optional argument \var{keep_blank_values} is |
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207 a flag indicating whether blank values in |
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208 URL encoded queries should be treated as blank strings. |
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209 A true value indicates that blanks should be retained as |
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210 blank strings. The default false value indicates that |
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211 blank values are to be ignored and treated as if they were |
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212 not included. |
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213 |
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214 The optional argument \var{strict_parsing} is a flag indicating what |
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215 to do with parsing errors. If false (the default), errors |
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216 are silently ignored. If true, errors raise a ValueError |
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217 exception. |
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218 \end{funcdesc} |
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219 |
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220 \begin{funcdesc}{parse_qsl}{qs\optional{, keep_blank_values, strict_parsing}} |
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221 Parse a query string given as a string argument (data of type |
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222 \mimetype{application/x-www-form-urlencoded}). Data are |
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223 returned as a list of name, value pairs. |
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224 |
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225 The optional argument \var{keep_blank_values} is |
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226 a flag indicating whether blank values in |
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227 URL encoded queries should be treated as blank strings. |
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228 A true value indicates that blanks should be retained as |
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229 blank strings. The default false value indicates that |
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230 blank values are to be ignored and treated as if they were |
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231 not included. |
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232 |
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233 The optional argument \var{strict_parsing} is a flag indicating what |
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234 to do with parsing errors. If false (the default), errors |
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235 are silently ignored. If true, errors raise a ValueError |
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236 exception. |
| 2449 | 237 \end{funcdesc} |
| 238 | |
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239 \begin{funcdesc}{parse_multipart}{fp, pdict} |
| 7663 | 240 Parse input of type \mimetype{multipart/form-data} (for |
| 241 file uploads). Arguments are \var{fp} for the input file and | |
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242 \var{pdict} for a dictionary containing other parameters in |
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243 the \code{Content-Type} header. |
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244 |
| 7663 | 245 Returns a dictionary just like \function{parse_qs()} keys are the |
| 246 field names, each value is a list of values for that field. This is | |
| 247 easy to use but not much good if you are expecting megabytes to be | |
| 248 uploaded --- in that case, use the \class{FieldStorage} class instead | |
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249 which is much more flexible. |
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250 |
| 7663 | 251 Note that this does not parse nested multipart parts --- use |
| 252 \class{FieldStorage} for that. | |
| 2449 | 253 \end{funcdesc} |
| 254 | |
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255 \begin{funcdesc}{parse_header}{string} |
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256 Parse a MIME header (such as \code{Content-Type}) into a main |
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257 value and a dictionary of parameters. |
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258 \end{funcdesc} |
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259 |
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260 \begin{funcdesc}{test}{} |
| 7663 | 261 Robust test CGI script, usable as main program. |
| 262 Writes minimal HTTP headers and formats all information provided to | |
| 263 the script in HTML form. | |
| 2449 | 264 \end{funcdesc} |
| 265 | |
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266 \begin{funcdesc}{print_environ}{} |
| 7663 | 267 Format the shell environment in HTML. |
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268 \end{funcdesc} |
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269 |
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270 \begin{funcdesc}{print_form}{form} |
| 7663 | 271 Format a form in HTML. |
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272 \end{funcdesc} |
| 2449 | 273 |
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274 \begin{funcdesc}{print_directory}{} |
| 7663 | 275 Format the current directory in HTML. |
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276 \end{funcdesc} |
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277 |
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278 \begin{funcdesc}{print_environ_usage}{} |
| 7663 | 279 Print a list of useful (used by CGI) environment variables in |
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280 HTML. |
| 2449 | 281 \end{funcdesc} |
| 282 | |
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283 \begin{funcdesc}{escape}{s\optional{, quote}} |
| 7663 | 284 Convert the characters |
| 285 \character{\&}, \character{<} and \character{>} in string \var{s} to | |
| 286 HTML-safe sequences. Use this if you need to display text that might | |
| 287 contain such characters in HTML. If the optional flag \var{quote} is | |
| 288 true, the double quote character (\character{"}) is also translated; | |
| 289 this helps for inclusion in an HTML attribute value, e.g. in \code{<A | |
| 290 HREF="...">}. | |
| 2449 | 291 \end{funcdesc} |
| 292 | |
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293 |
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294 \subsection{Caring about security} |
| 2449 | 295 |
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296 There's one important rule: if you invoke an external program (e.g. |
| 7663 | 297 via the \function{os.system()} or \function{os.popen()} functions), |
| 298 make very sure you don't pass arbitrary strings received from the | |
| 299 client to the shell. This is a well-known security hole whereby | |
| 300 clever hackers anywhere on the web can exploit a gullible CGI script | |
| 301 to invoke arbitrary shell commands. Even parts of the URL or field | |
| 302 names cannot be trusted, since the request doesn't have to come from | |
| 303 your form! | |
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304 |
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305 To be on the safe side, if you must pass a string gotten from a form |
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306 to a shell command, you should make sure the string contains only |
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307 alphanumeric characters, dashes, underscores, and periods. |
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308 |
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309 |
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310 \subsection{Installing your CGI script on a Unix system} |
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311 |
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312 Read the documentation for your HTTP server and check with your local |
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313 system administrator to find the directory where CGI scripts should be |
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314 installed; usually this is in a directory \file{cgi-bin} in the server tree. |
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315 |
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316 Make sure that your script is readable and executable by ``others''; the |
| 7663 | 317 \UNIX{} file mode should be \code{0755} octal (use \samp{chmod 0755 |
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318 \var{filename}}). Make sure that the first line of the script contains |
| 7663 | 319 \code{\#!} starting in column 1 followed by the pathname of the Python |
| 320 interpreter, for instance: | |
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321 |
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322 \begin{verbatim} |
| 5818 | 323 #!/usr/local/bin/python |
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324 \end{verbatim} |
| 7663 | 325 |
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326 Make sure the Python interpreter exists and is executable by ``others''. |
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327 |
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328 Make sure that any files your script needs to read or write are |
| 7663 | 329 readable or writable, respectively, by ``others'' --- their mode |
| 330 should be \code{0644} for readable and \code{0666} for writable. This | |
| 331 is because, for security reasons, the HTTP server executes your script | |
| 332 as user ``nobody'', without any special privileges. It can only read | |
| 333 (write, execute) files that everybody can read (write, execute). The | |
| 334 current directory at execution time is also different (it is usually | |
| 335 the server's cgi-bin directory) and the set of environment variables | |
| 336 is also different from what you get at login. In particular, don't | |
| 337 count on the shell's search path for executables (\envvar{PATH}) or | |
| 338 the Python module search path (\envvar{PYTHONPATH}) to be set to | |
| 339 anything interesting. | |
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340 |
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341 If you need to load modules from a directory which is not on Python's |
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342 default module search path, you can change the path in your script, |
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343 before importing other modules, e.g.: |
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344 |
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345 \begin{verbatim} |
| 5818 | 346 import sys |
| 347 sys.path.insert(0, "/usr/home/joe/lib/python") | |
| 348 sys.path.insert(0, "/usr/local/lib/python") | |
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349 \end{verbatim} |
| 7663 | 350 |
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351 (This way, the directory inserted last will be searched first!) |
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352 |
| 7086 | 353 Instructions for non-\UNIX{} systems will vary; check your HTTP server's |
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354 documentation (it will usually have a section on CGI scripts). |
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355 |
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356 |
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357 \subsection{Testing your CGI script} |
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358 |
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359 Unfortunately, a CGI script will generally not run when you try it |
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360 from the command line, and a script that works perfectly from the |
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361 command line may fail mysteriously when run from the server. There's |
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362 one reason why you should still test your script from the command |
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363 line: if it contains a syntax error, the Python interpreter won't |
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364 execute it at all, and the HTTP server will most likely send a cryptic |
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365 error to the client. |
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366 |
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367 Assuming your script has no syntax errors, yet it does not work, you |
| 7663 | 368 have no choice but to read the next section. |
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369 |
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370 |
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371 \subsection{Debugging CGI scripts} |
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372 |
| 7663 | 373 First of all, check for trivial installation errors --- reading the |
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374 section above on installing your CGI script carefully can save you a |
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375 lot of time. If you wonder whether you have understood the |
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376 installation procedure correctly, try installing a copy of this module |
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377 file (\file{cgi.py}) as a CGI script. When invoked as a script, the file |
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378 will dump its environment and the contents of the form in HTML form. |
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379 Give it the right mode etc, and send it a request. If it's installed |
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380 in the standard \file{cgi-bin} directory, it should be possible to send it a |
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381 request by entering a URL into your browser of the form: |
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382 |
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383 \begin{verbatim} |
| 5818 | 384 http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home |
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385 \end{verbatim} |
| 7663 | 386 |
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387 If this gives an error of type 404, the server cannot find the script |
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388 -- perhaps you need to install it in a different directory. If it |
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389 gives another error (e.g. 500), there's an installation problem that |
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390 you should fix before trying to go any further. If you get a nicely |
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391 formatted listing of the environment and form content (in this |
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392 example, the fields should be listed as ``addr'' with value ``At Home'' |
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393 and ``name'' with value ``Joe Blow''), the \file{cgi.py} script has been |
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394 installed correctly. If you follow the same procedure for your own |
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395 script, you should now be able to debug it. |
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396 |
| 7663 | 397 The next step could be to call the \module{cgi} module's |
| 398 \function{test()} function from your script: replace its main code | |
| 399 with the single statement | |
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400 |
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401 \begin{verbatim} |
| 5818 | 402 cgi.test() |
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403 \end{verbatim} |
| 7663 | 404 |
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405 This should produce the same results as those gotten from installing |
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406 the \file{cgi.py} file itself. |
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407 |
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408 When an ordinary Python script raises an unhandled exception |
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409 (e.g. because of a typo in a module name, a file that can't be opened, |
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410 etc.), the Python interpreter prints a nice traceback and exits. |
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411 While the Python interpreter will still do this when your CGI script |
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412 raises an exception, most likely the traceback will end up in one of |
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413 the HTTP server's log file, or be discarded altogether. |
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414 |
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415 Fortunately, once you have managed to get your script to execute |
| 7663 | 416 \emph{some} code, it is easy to catch exceptions and cause a traceback |
| 417 to be printed. The \function{test()} function below in this module is | |
| 418 an example. Here are the rules: | |
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419 |
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420 \begin{enumerate} |
| 7663 | 421 \item Import the traceback module before entering the \keyword{try} |
| 422 ... \keyword{except} statement | |
| 423 | |
| 424 \item Assign \code{sys.stderr} to be \code{sys.stdout} | |
| 425 | |
| 426 \item Make sure you finish printing the headers and the blank line | |
| 427 early | |
| 428 | |
| 429 \item Wrap all remaining code in a \keyword{try} ... \keyword{except} | |
| 430 statement | |
| 431 | |
| 432 \item In the except clause, call \function{traceback.print_exc()} | |
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433 \end{enumerate} |
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434 |
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435 For example: |
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436 |
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437 \begin{verbatim} |
| 5818 | 438 import sys |
| 439 import traceback | |
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440 print "Content-Type: text/html" |
| 5818 | 441 print |
| 442 sys.stderr = sys.stdout | |
| 443 try: | |
| 444 ...your code here... | |
| 445 except: | |
| 446 print "\n\n<PRE>" | |
| 447 traceback.print_exc() | |
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448 \end{verbatim} |
| 7663 | 449 |
| 450 Notes: The assignment to \code{sys.stderr} is needed because the | |
| 451 traceback prints to \code{sys.stderr}. | |
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452 The \code{print "{\e}n{\e}n<PRE>"} statement is necessary to |
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453 disable the word wrapping in HTML. |
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454 |
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455 If you suspect that there may be a problem in importing the traceback |
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456 module, you can use an even more robust approach (which only uses |
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457 built-in modules): |
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458 |
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459 \begin{verbatim} |
| 5818 | 460 import sys |
| 461 sys.stderr = sys.stdout | |
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462 print "Content-Type: text/plain" |
| 5818 | 463 print |
| 464 ...your code here... | |
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465 \end{verbatim} |
| 7663 | 466 |
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467 This relies on the Python interpreter to print the traceback. The |
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468 content type of the output is set to plain text, which disables all |
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469 HTML processing. If your script works, the raw HTML will be displayed |
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470 by your client. If it raises an exception, most likely after the |
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471 first two lines have been printed, a traceback will be displayed. |
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472 Because no HTML interpretation is going on, the traceback will |
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473 readable. |
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474 |
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475 |
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476 \subsection{Common problems and solutions} |
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477 |
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478 \begin{itemize} |
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479 \item Most HTTP servers buffer the output from CGI scripts until the |
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480 script is completed. This means that it is not possible to display a |
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481 progress report on the client's display while the script is running. |
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482 |
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483 \item Check the installation instructions above. |
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484 |
| 7663 | 485 \item Check the HTTP server's log files. (\samp{tail -f logfile} in a |
| 486 separate window may be useful!) | |
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487 |
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488 \item Always check a script for syntax errors first, by doing something |
| 7663 | 489 like \samp{python script.py}. |
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490 |
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491 \item When using any of the debugging techniques, don't forget to add |
| 7663 | 492 \samp{import sys} to the top of the script. |
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493 |
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494 \item When invoking external programs, make sure they can be found. |
| 7663 | 495 Usually, this means using absolute path names --- \envvar{PATH} is |
| 496 usually not set to a very useful value in a CGI script. | |
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497 |
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498 \item When reading or writing external files, make sure they can be read |
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499 or written by every user on the system. |
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500 |
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501 \item Don't try to give a CGI script a set-uid mode. This doesn't work on |
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502 most systems, and is a security liability as well. |
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503 \end{itemize} |
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504 |