Mercurial > cpython-lru_cache-weakref
annotate Doc/lib/libcgi.tex @ 8831:00a6bb65ff29 legacy-trunk
Getting rid of this ancient stuff.
| author | Guido van Rossum <guido@python.org> |
|---|---|
| date | Wed, 05 Aug 1998 21:10:09 +0000 |
| parents | 760dcf197048 |
| children | acfffe14628e |
| rev | line source |
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1 \section{Standard Module \module{cgi}} |
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2 \declaremodule{standard}{cgi} |
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3 |
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4 \modulesynopsis{Common Gateway Interface, used to interpret forms in server-side |
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5 scripts.} |
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6 |
| 2449 | 7 \indexii{WWW}{server} |
| 8 \indexii{CGI}{protocol} | |
| 9 \indexii{HTTP}{protocol} | |
| 10 \indexii{MIME}{headers} | |
| 11 \index{URL} | |
| 12 | |
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13 |
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14 Support module for CGI (Common Gateway Interface) scripts.% |
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15 \index{Common Gateway Interface} |
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16 |
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17 This module defines a number of utilities for use by CGI scripts |
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18 written in Python. |
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19 |
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20 \subsection{Introduction} |
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21 \nodename{cgi-intro} |
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22 |
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23 A CGI script is invoked by an HTTP server, usually to process user |
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24 input submitted through an HTML \code{<FORM>} or \code{<ISINPUT>} element. |
| 2449 | 25 |
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26 Most often, CGI scripts live in the server's special \file{cgi-bin} |
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27 directory. The HTTP server places all sorts of information about the |
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28 request (such as the client's hostname, the requested URL, the query |
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29 string, and lots of other goodies) in the script's shell environment, |
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30 executes the script, and sends the script's output back to the client. |
| 2449 | 31 |
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32 The script's input is connected to the client too, and sometimes the |
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33 form data is read this way; at other times the form data is passed via |
| 7663 | 34 the ``query string'' part of the URL. This module is intended |
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35 to take care of the different cases and provide a simpler interface to |
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36 the Python script. It also provides a number of utilities that help |
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37 in debugging scripts, and the latest addition is support for file |
| 7663 | 38 uploads from a form (if your browser supports it --- Grail 0.3 and |
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39 Netscape 2.0 do). |
| 2449 | 40 |
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41 The output of a CGI script should consist of two sections, separated |
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42 by a blank line. The first section contains a number of headers, |
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43 telling the client what kind of data is following. Python code to |
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44 generate a minimal header section looks like this: |
| 2449 | 45 |
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46 \begin{verbatim} |
| 5818 | 47 print "Content-type: text/html" # HTML is following |
| 48 print # blank line, end of headers | |
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49 \end{verbatim} |
| 7663 | 50 |
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51 The second section is usually HTML, which allows the client software |
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52 to display nicely formatted text with header, in-line images, etc. |
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53 Here's Python code that prints a simple piece of HTML: |
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54 |
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55 \begin{verbatim} |
| 5818 | 56 print "<TITLE>CGI script output</TITLE>" |
| 57 print "<H1>This is my first CGI script</H1>" | |
| 58 print "Hello, world!" | |
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59 \end{verbatim} |
| 7663 | 60 |
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61 (It may not be fully legal HTML according to the letter of the |
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62 standard, but any browser will understand it.) |
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63 |
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64 \subsection{Using the cgi module} |
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65 \nodename{Using the cgi module} |
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66 |
| 7663 | 67 Begin by writing \samp{import cgi}. Do not use \samp{from cgi import |
| 68 *} --- the module defines all sorts of names for its own use or for | |
| 69 backward compatibility that you don't want in your namespace. | |
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70 |
| 7663 | 71 It's best to use the \class{FieldStorage} class. The other classes |
| 72 defined in this module are provided mostly for backward compatibility. | |
| 73 Instantiate it exactly once, without arguments. This reads the form | |
| 74 contents from standard input or the environment (depending on the | |
| 75 value of various environment variables set according to the CGI | |
| 76 standard). Since it may consume standard input, it should be | |
| 77 instantiated only once. | |
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78 |
| 7663 | 79 The \class{FieldStorage} instance can be accessed as if it were a Python |
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80 dictionary. For instance, the following code (which assumes that the |
| 7663 | 81 \code{content-type} header and blank line have already been printed) |
| 82 checks that the fields \code{name} and \code{addr} are both set to a | |
| 83 non-empty string: | |
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84 |
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85 \begin{verbatim} |
| 5818 | 86 form = cgi.FieldStorage() |
| 87 form_ok = 0 | |
| 88 if form.has_key("name") and form.has_key("addr"): | |
| 89 if form["name"].value != "" and form["addr"].value != "": | |
| 90 form_ok = 1 | |
| 91 if not form_ok: | |
| 92 print "<H1>Error</H1>" | |
| 93 print "Please fill in the name and addr fields." | |
| 94 return | |
| 95 ...further form processing here... | |
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96 \end{verbatim} |
| 7663 | 97 |
| 98 Here the fields, accessed through \samp{form[\var{key}]}, are | |
| 99 themselves instances of \class{FieldStorage} (or | |
| 100 \class{MiniFieldStorage}, depending on the form encoding). | |
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101 |
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102 If the submitted form data contains more than one field with the same |
| 7663 | 103 name, the object retrieved by \samp{form[\var{key}]} is not a |
| 104 \class{FieldStorage} or \class{MiniFieldStorage} | |
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105 instance but a list of such instances. If you expect this possibility |
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106 (i.e., when your HTML form comtains multiple fields with the same |
| 7663 | 107 name), use the \function{type()} function to determine whether you |
| 108 have a single instance or a list of instances. For example, here's | |
| 109 code that concatenates any number of username fields, separated by | |
| 110 commas: | |
| 2449 | 111 |
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112 \begin{verbatim} |
| 5818 | 113 username = form["username"] |
| 114 if type(username) is type([]): | |
| 115 # Multiple username fields specified | |
| 116 usernames = "" | |
| 117 for item in username: | |
| 118 if usernames: | |
| 119 # Next item -- insert comma | |
| 120 usernames = usernames + "," + item.value | |
| 121 else: | |
| 122 # First item -- don't insert comma | |
| 123 usernames = item.value | |
| 124 else: | |
| 125 # Single username field specified | |
| 126 usernames = username.value | |
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127 \end{verbatim} |
| 7663 | 128 |
| 129 If a field represents an uploaded file, the value attribute reads the | |
| 130 entire file in memory as a string. This may not be what you want. | |
| 131 You can test for an uploaded file by testing either the filename | |
| 132 attribute or the file attribute. You can then read the data at | |
| 133 leasure from the file attribute: | |
| 2449 | 134 |
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135 \begin{verbatim} |
| 5818 | 136 fileitem = form["userfile"] |
| 137 if fileitem.file: | |
| 138 # It's an uploaded file; count lines | |
| 139 linecount = 0 | |
| 140 while 1: | |
| 141 line = fileitem.file.readline() | |
| 142 if not line: break | |
| 143 linecount = linecount + 1 | |
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144 \end{verbatim} |
| 7663 | 145 |
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146 The file upload draft standard entertains the possibility of uploading |
| 7663 | 147 multiple files from one field (using a recursive |
| 148 \mimetype{multipart/*} encoding). When this occurs, the item will be | |
| 149 a dictionary-like \class{FieldStorage} item. This can be determined | |
| 150 by testing its \member{type} attribute, which should be | |
| 151 \mimetype{multipart/form-data} (or perhaps another MIME type matching | |
| 152 \mimetype{multipart/*}). It this case, it can be iterated over | |
| 153 recursively just like the top-level form object. | |
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154 |
| 7663 | 155 When a form is submitted in the ``old'' format (as the query string or |
| 156 as a single data part of type | |
| 157 \mimetype{application/x-www-form-urlencoded}), the items will actually | |
| 158 be instances of the class \class{MiniFieldStorage}. In this case, the | |
| 159 list, file and filename attributes are always \code{None}. | |
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160 |
| 2449 | 161 |
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162 \subsection{Old classes} |
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163 |
| 7663 | 164 These classes, present in earlier versions of the \module{cgi} module, |
| 165 are still supported for backward compatibility. New applications | |
| 166 should use the \class{FieldStorage} class. | |
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167 |
| 7663 | 168 \class{SvFormContentDict} stores single value form content as |
| 169 dictionary; it assumes each field name occurs in the form only once. | |
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170 |
| 7663 | 171 \class{FormContentDict} stores multiple value form content as a |
| 172 dictionary (the form items are lists of values). Useful if your form | |
| 173 contains multiple fields with the same name. | |
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174 |
| 7663 | 175 Other classes (\class{FormContent}, \class{InterpFormContentDict}) are |
| 176 present for backwards compatibility with really old applications only. | |
| 177 If you still use these and would be inconvenienced when they | |
| 178 disappeared from a next version of this module, drop me a note. | |
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179 |
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180 |
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181 \subsection{Functions} |
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182 \nodename{Functions in cgi module} |
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183 |
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184 These are useful if you want more control, or if you want to employ |
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185 some of the algorithms implemented in this module in other |
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186 circumstances. |
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187 |
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188 \begin{funcdesc}{parse}{fp} |
| 7663 | 189 Parse a query in the environment or from a file (default |
| 190 \code{sys.stdin}). | |
| 2449 | 191 \end{funcdesc} |
| 192 | |
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193 \begin{funcdesc}{parse_qs}{qs} |
| 7663 | 194 Parse a query string given as a string argument (data of type |
| 195 \mimetype{application/x-www-form-urlencoded}). | |
| 2449 | 196 \end{funcdesc} |
| 197 | |
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198 \begin{funcdesc}{parse_multipart}{fp, pdict} |
| 7663 | 199 Parse input of type \mimetype{multipart/form-data} (for |
| 200 file uploads). Arguments are \var{fp} for the input file and | |
| 201 \var{pdict} for the dictionary containing other parameters of | |
| 202 \code{content-type} header | |
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203 |
| 7663 | 204 Returns a dictionary just like \function{parse_qs()} keys are the |
| 205 field names, each value is a list of values for that field. This is | |
| 206 easy to use but not much good if you are expecting megabytes to be | |
| 207 uploaded --- in that case, use the \class{FieldStorage} class instead | |
| 208 which is much more flexible. Note that \code{content-type} is the | |
| 209 raw, unparsed contents of the \code{content-type} header. | |
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210 |
| 7663 | 211 Note that this does not parse nested multipart parts --- use |
| 212 \class{FieldStorage} for that. | |
| 2449 | 213 \end{funcdesc} |
| 214 | |
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215 \begin{funcdesc}{parse_header}{string} |
| 7663 | 216 Parse a header like \code{content-type} into a main |
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217 content-type and a dictionary of parameters. |
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218 \end{funcdesc} |
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219 |
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220 \begin{funcdesc}{test}{} |
| 7663 | 221 Robust test CGI script, usable as main program. |
| 222 Writes minimal HTTP headers and formats all information provided to | |
| 223 the script in HTML form. | |
| 2449 | 224 \end{funcdesc} |
| 225 | |
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226 \begin{funcdesc}{print_environ}{} |
| 7663 | 227 Format the shell environment in HTML. |
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228 \end{funcdesc} |
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229 |
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230 \begin{funcdesc}{print_form}{form} |
| 7663 | 231 Format a form in HTML. |
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232 \end{funcdesc} |
| 2449 | 233 |
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234 \begin{funcdesc}{print_directory}{} |
| 7663 | 235 Format the current directory in HTML. |
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236 \end{funcdesc} |
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237 |
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238 \begin{funcdesc}{print_environ_usage}{} |
| 7663 | 239 Print a list of useful (used by CGI) environment variables in |
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240 HTML. |
| 2449 | 241 \end{funcdesc} |
| 242 | |
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243 \begin{funcdesc}{escape}{s\optional{, quote}} |
| 7663 | 244 Convert the characters |
| 245 \character{\&}, \character{<} and \character{>} in string \var{s} to | |
| 246 HTML-safe sequences. Use this if you need to display text that might | |
| 247 contain such characters in HTML. If the optional flag \var{quote} is | |
| 248 true, the double quote character (\character{"}) is also translated; | |
| 249 this helps for inclusion in an HTML attribute value, e.g. in \code{<A | |
| 250 HREF="...">}. | |
| 2449 | 251 \end{funcdesc} |
| 252 | |
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253 |
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254 \subsection{Caring about security} |
| 2449 | 255 |
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256 There's one important rule: if you invoke an external program (e.g. |
| 7663 | 257 via the \function{os.system()} or \function{os.popen()} functions), |
| 258 make very sure you don't pass arbitrary strings received from the | |
| 259 client to the shell. This is a well-known security hole whereby | |
| 260 clever hackers anywhere on the web can exploit a gullible CGI script | |
| 261 to invoke arbitrary shell commands. Even parts of the URL or field | |
| 262 names cannot be trusted, since the request doesn't have to come from | |
| 263 your form! | |
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264 |
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265 To be on the safe side, if you must pass a string gotten from a form |
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266 to a shell command, you should make sure the string contains only |
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267 alphanumeric characters, dashes, underscores, and periods. |
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268 |
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269 |
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270 \subsection{Installing your CGI script on a Unix system} |
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271 |
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272 Read the documentation for your HTTP server and check with your local |
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273 system administrator to find the directory where CGI scripts should be |
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274 installed; usually this is in a directory \file{cgi-bin} in the server tree. |
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275 |
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276 Make sure that your script is readable and executable by ``others''; the |
| 7663 | 277 \UNIX{} file mode should be \code{0755} octal (use \samp{chmod 0755 |
| 278 filename}). Make sure that the first line of the script contains | |
| 279 \code{\#!} starting in column 1 followed by the pathname of the Python | |
| 280 interpreter, for instance: | |
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281 |
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282 \begin{verbatim} |
| 5818 | 283 #!/usr/local/bin/python |
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284 \end{verbatim} |
| 7663 | 285 |
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286 Make sure the Python interpreter exists and is executable by ``others''. |
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287 |
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288 Make sure that any files your script needs to read or write are |
| 7663 | 289 readable or writable, respectively, by ``others'' --- their mode |
| 290 should be \code{0644} for readable and \code{0666} for writable. This | |
| 291 is because, for security reasons, the HTTP server executes your script | |
| 292 as user ``nobody'', without any special privileges. It can only read | |
| 293 (write, execute) files that everybody can read (write, execute). The | |
| 294 current directory at execution time is also different (it is usually | |
| 295 the server's cgi-bin directory) and the set of environment variables | |
| 296 is also different from what you get at login. In particular, don't | |
| 297 count on the shell's search path for executables (\envvar{PATH}) or | |
| 298 the Python module search path (\envvar{PYTHONPATH}) to be set to | |
| 299 anything interesting. | |
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300 |
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301 If you need to load modules from a directory which is not on Python's |
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302 default module search path, you can change the path in your script, |
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303 before importing other modules, e.g.: |
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304 |
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305 \begin{verbatim} |
| 5818 | 306 import sys |
| 307 sys.path.insert(0, "/usr/home/joe/lib/python") | |
| 308 sys.path.insert(0, "/usr/local/lib/python") | |
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309 \end{verbatim} |
| 7663 | 310 |
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311 (This way, the directory inserted last will be searched first!) |
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312 |
| 7086 | 313 Instructions for non-\UNIX{} systems will vary; check your HTTP server's |
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314 documentation (it will usually have a section on CGI scripts). |
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315 |
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316 |
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317 \subsection{Testing your CGI script} |
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318 |
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319 Unfortunately, a CGI script will generally not run when you try it |
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320 from the command line, and a script that works perfectly from the |
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321 command line may fail mysteriously when run from the server. There's |
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322 one reason why you should still test your script from the command |
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323 line: if it contains a syntax error, the Python interpreter won't |
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324 execute it at all, and the HTTP server will most likely send a cryptic |
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325 error to the client. |
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326 |
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327 Assuming your script has no syntax errors, yet it does not work, you |
| 7663 | 328 have no choice but to read the next section. |
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329 |
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330 |
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331 \subsection{Debugging CGI scripts} |
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332 |
| 7663 | 333 First of all, check for trivial installation errors --- reading the |
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334 section above on installing your CGI script carefully can save you a |
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335 lot of time. If you wonder whether you have understood the |
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336 installation procedure correctly, try installing a copy of this module |
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337 file (\file{cgi.py}) as a CGI script. When invoked as a script, the file |
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338 will dump its environment and the contents of the form in HTML form. |
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339 Give it the right mode etc, and send it a request. If it's installed |
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340 in the standard \file{cgi-bin} directory, it should be possible to send it a |
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341 request by entering a URL into your browser of the form: |
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342 |
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343 \begin{verbatim} |
| 5818 | 344 http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home |
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345 \end{verbatim} |
| 7663 | 346 |
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347 If this gives an error of type 404, the server cannot find the script |
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348 -- perhaps you need to install it in a different directory. If it |
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349 gives another error (e.g. 500), there's an installation problem that |
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350 you should fix before trying to go any further. If you get a nicely |
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351 formatted listing of the environment and form content (in this |
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352 example, the fields should be listed as ``addr'' with value ``At Home'' |
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353 and ``name'' with value ``Joe Blow''), the \file{cgi.py} script has been |
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354 installed correctly. If you follow the same procedure for your own |
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355 script, you should now be able to debug it. |
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356 |
| 7663 | 357 The next step could be to call the \module{cgi} module's |
| 358 \function{test()} function from your script: replace its main code | |
| 359 with the single statement | |
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360 |
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361 \begin{verbatim} |
| 5818 | 362 cgi.test() |
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363 \end{verbatim} |
| 7663 | 364 |
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365 This should produce the same results as those gotten from installing |
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366 the \file{cgi.py} file itself. |
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367 |
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368 When an ordinary Python script raises an unhandled exception |
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369 (e.g. because of a typo in a module name, a file that can't be opened, |
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370 etc.), the Python interpreter prints a nice traceback and exits. |
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371 While the Python interpreter will still do this when your CGI script |
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372 raises an exception, most likely the traceback will end up in one of |
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373 the HTTP server's log file, or be discarded altogether. |
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374 |
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375 Fortunately, once you have managed to get your script to execute |
| 7663 | 376 \emph{some} code, it is easy to catch exceptions and cause a traceback |
| 377 to be printed. The \function{test()} function below in this module is | |
| 378 an example. Here are the rules: | |
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379 |
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380 \begin{enumerate} |
| 7663 | 381 \item Import the traceback module before entering the \keyword{try} |
| 382 ... \keyword{except} statement | |
| 383 | |
| 384 \item Assign \code{sys.stderr} to be \code{sys.stdout} | |
| 385 | |
| 386 \item Make sure you finish printing the headers and the blank line | |
| 387 early | |
| 388 | |
| 389 \item Wrap all remaining code in a \keyword{try} ... \keyword{except} | |
| 390 statement | |
| 391 | |
| 392 \item In the except clause, call \function{traceback.print_exc()} | |
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393 \end{enumerate} |
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394 |
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395 For example: |
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396 |
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397 \begin{verbatim} |
| 5818 | 398 import sys |
| 399 import traceback | |
| 400 print "Content-type: text/html" | |
| 401 print | |
| 402 sys.stderr = sys.stdout | |
| 403 try: | |
| 404 ...your code here... | |
| 405 except: | |
| 406 print "\n\n<PRE>" | |
| 407 traceback.print_exc() | |
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408 \end{verbatim} |
| 7663 | 409 |
| 410 Notes: The assignment to \code{sys.stderr} is needed because the | |
| 411 traceback prints to \code{sys.stderr}. | |
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412 The \code{print "{\e}n{\e}n<PRE>"} statement is necessary to |
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413 disable the word wrapping in HTML. |
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414 |
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415 If you suspect that there may be a problem in importing the traceback |
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416 module, you can use an even more robust approach (which only uses |
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417 built-in modules): |
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418 |
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419 \begin{verbatim} |
| 5818 | 420 import sys |
| 421 sys.stderr = sys.stdout | |
| 422 print "Content-type: text/plain" | |
| 423 print | |
| 424 ...your code here... | |
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425 \end{verbatim} |
| 7663 | 426 |
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427 This relies on the Python interpreter to print the traceback. The |
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428 content type of the output is set to plain text, which disables all |
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429 HTML processing. If your script works, the raw HTML will be displayed |
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430 by your client. If it raises an exception, most likely after the |
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431 first two lines have been printed, a traceback will be displayed. |
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432 Because no HTML interpretation is going on, the traceback will |
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433 readable. |
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434 |
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435 |
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436 \subsection{Common problems and solutions} |
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437 |
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438 \begin{itemize} |
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439 \item Most HTTP servers buffer the output from CGI scripts until the |
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440 script is completed. This means that it is not possible to display a |
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441 progress report on the client's display while the script is running. |
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442 |
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443 \item Check the installation instructions above. |
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444 |
| 7663 | 445 \item Check the HTTP server's log files. (\samp{tail -f logfile} in a |
| 446 separate window may be useful!) | |
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447 |
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448 \item Always check a script for syntax errors first, by doing something |
| 7663 | 449 like \samp{python script.py}. |
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450 |
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451 \item When using any of the debugging techniques, don't forget to add |
| 7663 | 452 \samp{import sys} to the top of the script. |
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453 |
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454 \item When invoking external programs, make sure they can be found. |
| 7663 | 455 Usually, this means using absolute path names --- \envvar{PATH} is |
| 456 usually not set to a very useful value in a CGI script. | |
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457 |
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458 \item When reading or writing external files, make sure they can be read |
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459 or written by every user on the system. |
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460 |
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461 \item Don't try to give a CGI script a set-uid mode. This doesn't work on |
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462 most systems, and is a security liability as well. |
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463 \end{itemize} |
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464 |