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Lib/cgi.py
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Lib/cgi.py
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@ -9,57 +9,58 @@
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Introduction
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------------
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A CGI script is invoked by an HTTP server, usually to process user input
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submitted through an HTML <FORM> or <ISINPUT> element.
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A CGI script is invoked by an HTTP server, usually to process user
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input submitted through an HTML <FORM> or <ISINPUT> element.
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Most often, CGI scripts live in the server's special cgi-bin directory.
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The HTTP server places all sorts of information about the request (such as
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the client's hostname, the requested URL, the query string, and lots of
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other goodies) in the script's shell environment, executes the script, and
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sends the script's output back to the client.
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Most often, CGI scripts live in the server's special cgi-bin
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directory. The HTTP server places all sorts of information about the
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request (such as the client's hostname, the requested URL, the query
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string, and lots of other goodies) in the script's shell environment,
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executes the script, and sends the script's output back to the client.
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The script's input is connected to the client too, and sometimes the form
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data is read this way; at other times the form data is passed via the
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"query string" part of the URL. This module (cgi.py) is intended to take
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care of the different cases and provide a simpler interface to the Python
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script. It also provides a number of utilities that help in debugging
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scripts, and the latest addition is support for file uploads from a form
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(if your browser supports it -- Grail 0.3 and Netscape 2.0 do).
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The script's input is connected to the client too, and sometimes the
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form data is read this way; at other times the form data is passed via
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the "query string" part of the URL. This module (cgi.py) is intended
|
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to take care of the different cases and provide a simpler interface to
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the Python script. It also provides a number of utilities that help
|
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in debugging scripts, and the latest addition is support for file
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uploads from a form (if your browser supports it -- Grail 0.3 and
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Netscape 2.0 do).
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The output of a CGI script should consist of two sections, separated by a
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blank line. The first section contains a number of headers, telling the
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client what kind of data is following. Python code to generate a minimal
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header section looks like this:
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The output of a CGI script should consist of two sections, separated
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by a blank line. The first section contains a number of headers,
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telling the client what kind of data is following. Python code to
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generate a minimal header section looks like this:
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print "Content-type: text/html" # HTML is following
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print # blank line, end of headers
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The second section is usually HTML, which allows the client software to
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display nicely formatted text with header, in-line images, etc. Here's
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Python code that prints a simple piece of HTML:
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The second section is usually HTML, which allows the client software
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to display nicely formatted text with header, in-line images, etc.
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Here's Python code that prints a simple piece of HTML:
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print "<TITLE>CGI script output</TITLE>"
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print "<H1>This is my first CGI script</H1>"
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print "Hello, world!"
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(It may not be fully legal HTML according to the letter of the standard,
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but any browser will understand it.)
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(It may not be fully legal HTML according to the letter of the
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standard, but any browser will understand it.)
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Using the cgi module
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--------------------
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Begin by writing "import cgi". Don't use "from cgi import *" -- the module
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defines all sorts of names for its own use that you don't want in your
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namespace.
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Begin by writing "import cgi". Don't use "from cgi import *" -- the
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module defines all sorts of names for its own use that you don't want
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in your namespace.
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If you have a standard form, it's best to use the SvFormContentDict class.
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Instantiate the SvFormContentDict class exactly once: it consumes any input
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on standard input, which can't be wound back (it's a network connection,
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not a disk file).
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If you have a standard form, it's best to use the SvFormContentDict
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class. Instantiate the SvFormContentDict class exactly once: it
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consumes any input on standard input, which can't be wound back (it's
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a network connection, not a disk file).
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The SvFormContentDict instance can be accessed as if it were a Python
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dictionary. For instance, the following code checks that the fields
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The SvFormContentDict instance can be accessed as if it were a Python
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dictionary. For instance, the following code checks that the fields
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"name" and "addr" are both set to a non-empty string:
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form = SvFormContentDict()
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@ -73,40 +74,41 @@
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return
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...actual form processing here...
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If you have an input item of type "file" in your form and the client
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supports file uploads, the value for that field, if present in the form,
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is not a string but a tuple of (filename, content-type, data).
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If you have an input item of type "file" in your form and the client
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supports file uploads, the value for that field, if present in the
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form, is not a string but a tuple of (filename, content-type, data).
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Overview of classes
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-------------------
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SvFormContentDict: single value form content as dictionary; described
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SvFormContentDict: single value form content as dictionary; described
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above.
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FormContentDict: multiple value form content as dictionary (the form items
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are lists of values). Useful if your form contains multiple fields with
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the same name.
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FormContentDict: multiple value form content as dictionary (the form
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items are lists of values). Useful if your form contains multiple
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fields with the same name.
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Other classes (FormContent, InterpFormContentDict) are present for
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Other classes (FormContent, InterpFormContentDict) are present for
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backwards compatibility only.
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Overview of functions
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---------------------
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These are useful if you want more control, or if you want to employ some
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of the algorithms implemented in this module in other circumstances.
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These are useful if you want more control, or if you want to employ
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some of the algorithms implemented in this module in other
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circumstances.
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parse(): parse a form into a Python dictionary.
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parse_qs(qs): parse a query string.
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parse_multipart(...): parse input of type multipart/form-data (for file
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uploads).
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parse_multipart(...): parse input of type multipart/form-data (for
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file uploads).
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parse_header(string): parse a header like Content-type into a main value
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and a dictionary of parameters.
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parse_header(string): parse a header like Content-type into a main
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value and a dictionary of parameters.
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test(): complete test program.
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@ -114,58 +116,62 @@
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print_form(form): format a form in HTML.
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print_environ_usage(): print a list of useful environment variables in HTML.
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print_environ_usage(): print a list of useful environment variables in
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HTML.
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escape(): convert the characters "&", "<" and ">" to HTML-safe sequences.
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escape(): convert the characters "&", "<" and ">" to HTML-safe
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sequences. Use this if you need to display text that might contain
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such characters in HTML. To translate URLs for inclusion in the HREF
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attribute of an <A> tag, use urllib.quote().
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Caring about security
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---------------------
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There's one important rule: if you invoke an external program (e.g. via
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the os.system() or os.popen() functions), make very sure you don't pass
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arbitrary strings received from the client to the shell. This is a
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well-known security hole whereby clever hackers anywhere on the web can
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exploit a gullible CGI script to invoke arbitrary shell commands. Even
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parts of the URL or field names cannot be trusted, since the request
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doesn't have to come from your form!
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There's one important rule: if you invoke an external program (e.g.
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via the os.system() or os.popen() functions), make very sure you don't
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pass arbitrary strings received from the client to the shell. This is
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a well-known security hole whereby clever hackers anywhere on the web
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can exploit a gullible CGI script to invoke arbitrary shell commands.
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Even parts of the URL or field names cannot be trusted, since the
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request doesn't have to come from your form!
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To be on the safe side, if you must pass a string gotten from a form to a
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shell command, you should make sure the string contains only alphanumeric
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characters, dashes, underscores, and periods.
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To be on the safe side, if you must pass a string gotten from a form
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to a shell command, you should make sure the string contains only
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alphanumeric characters, dashes, underscores, and periods.
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Installing your CGI script on a Unix system
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-------------------------------------------
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Read the documentation for your HTTP server and check with your local
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system administrator to find the directory where CGI scripts should be
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Read the documentation for your HTTP server and check with your local
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system administrator to find the directory where CGI scripts should be
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installed; usually this is in a directory cgi-bin in the server tree.
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Make sure that your script is readable and executable by "others"; the Unix
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file mode should be 755 (use "chmod 755 filename"). Make sure that the
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first line of the script contains "#!" starting in column 1 followed by the
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pathname of the Python interpreter, for instance:
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Make sure that your script is readable and executable by "others"; the
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Unix file mode should be 755 (use "chmod 755 filename"). Make sure
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that the first line of the script contains "#!" starting in column 1
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followed by the pathname of the Python interpreter, for instance:
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#!/usr/local/bin/python
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Make sure the Python interpreter exists and is executable by "others".
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Make sure the Python interpreter exists and is executable by "others".
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Make sure that any files your script needs to read or write are readable or
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writable, respectively, by "others" -- their mode should be 644 for
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readable and 666 for writable. This is because, for security reasons, the
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HTTP server executes your script as user "nobody", without any special
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privileges. It can only read (write, execute) files that everybody can
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read (write, execute). The current directory at execution time is also
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different (it is usually the server's cgi-bin directory) and the set of
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environment variables is also different from what you get at login. in
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particular, don't count on the shell's search path for executables ($PATH)
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or the Python module search path ($PYTHONPATH) to be set to anything
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interesting.
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Make sure that any files your script needs to read or write are
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readable or writable, respectively, by "others" -- their mode should
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be 644 for readable and 666 for writable. This is because, for
|
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security reasons, the HTTP server executes your script as user
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"nobody", without any special privileges. It can only read (write,
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execute) files that everybody can read (write, execute). The current
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directory at execution time is also different (it is usually the
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server's cgi-bin directory) and the set of environment variables is
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also different from what you get at login. in particular, don't count
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on the shell's search path for executables ($PATH) or the Python
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module search path ($PYTHONPATH) to be set to anything interesting.
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If you need to load modules from a directory which is not on Python's
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default module search path, you can change the path in your script, before
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importing other modules, e.g.:
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If you need to load modules from a directory which is not on Python's
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default module search path, you can change the path in your script,
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before importing other modules, e.g.:
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import sys
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sys.path.insert(0, "/usr/home/joe/lib/python")
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@ -173,71 +179,75 @@
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(This way, the directory inserted last will be searched first!)
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Instructions for non-Unix systems will vary; check your HTTP server's
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Instructions for non-Unix systems will vary; check your HTTP server's
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documentation (it will usually have a section on CGI scripts).
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Testing your CGI script
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-----------------------
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Unfortunately, a CGI script will generally not run when you try it from the
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command line, and a script that works perfectly from the command line may
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fail mysteriously when run from the server. There's one reason why you
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should still test your script from the command line: if it contains a
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syntax error, the python interpreter won't execute it at all, and the HTTP
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server will most likely send a cryptic error to the client.
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Unfortunately, a CGI script will generally not run when you try it
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from the command line, and a script that works perfectly from the
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command line may fail mysteriously when run from the server. There's
|
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one reason why you should still test your script from the command
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line: if it contains a syntax error, the python interpreter won't
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execute it at all, and the HTTP server will most likely send a cryptic
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error to the client.
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Assuming your script has no syntax errors, yet it does not work, you have
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no choice but to read the next section:
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Assuming your script has no syntax errors, yet it does not work, you
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have no choice but to read the next section:
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Debugging CGI scripts
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---------------------
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First of all, check for trivial installation errors -- reading the section
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above on installing your CGI script carefully can save you a lot of time.
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If you wonder whether you have understood the installation procedure
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correctly, try installing a copy of this module file (cgi.py) as a CGI
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script. When invoked as a script, the file will dump its environment and
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the contents of the form in HTML form. Give it the right mode etc, and
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send it a request. If it's installed in the standard cgi-bin directory, it
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should be possible to send it a request by entering a URL into your browser
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of the form:
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First of all, check for trivial installation errors -- reading the
|
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section above on installing your CGI script carefully can save you a
|
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lot of time. If you wonder whether you have understood the
|
||||
installation procedure correctly, try installing a copy of this module
|
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file (cgi.py) as a CGI script. When invoked as a script, the file
|
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will dump its environment and the contents of the form in HTML form.
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Give it the right mode etc, and send it a request. If it's installed
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in the standard cgi-bin directory, it should be possible to send it a
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request by entering a URL into your browser of the form:
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http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home
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If this gives an error of type 404, the server cannot find the script --
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perhaps you need to install it in a different directory. If it gives
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another error (e.g. 500), there's an installation problem that you should
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fix before trying to go any further. If you get a nicely formatted listing
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of the environment and form content (in this example, the fields should be
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listed as "addr" with value "At Home" and "name" with value "Joe Blow"),
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the cgi.py script has been installed correctly. If you follow the same
|
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procedure for your own script, you should now be able to debug it.
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If this gives an error of type 404, the server cannot find the script
|
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-- perhaps you need to install it in a different directory. If it
|
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gives another error (e.g. 500), there's an installation problem that
|
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you should fix before trying to go any further. If you get a nicely
|
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formatted listing of the environment and form content (in this
|
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example, the fields should be listed as "addr" with value "At Home"
|
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and "name" with value "Joe Blow"), the cgi.py script has been
|
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installed correctly. If you follow the same procedure for your own
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script, you should now be able to debug it.
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The next step could be to call the cgi module's test() function from your
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script: replace its main code with the single statement
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The next step could be to call the cgi module's test() function from
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your script: replace its main code with the single statement
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cgi.test()
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This should produce the same results as those gotten from installing the
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cgi.py file itself.
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This should produce the same results as those gotten from installing
|
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the cgi.py file itself.
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|
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When an ordinary Python script raises an unhandled exception (e.g. because
|
||||
of a typo in a module name, a file that can't be opened, etc.), the Python
|
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interpreter prints a nice traceback and exits. While the Python
|
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interpreter will still do this when your CGI script raises an exception,
|
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most likely the traceback will end up in one of the HTTP server's log
|
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file, or be discarded altogether.
|
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When an ordinary Python script raises an unhandled exception
|
||||
(e.g. because of a typo in a module name, a file that can't be opened,
|
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etc.), the Python interpreter prints a nice traceback and exits.
|
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While the Python interpreter will still do this when your CGI script
|
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raises an exception, most likely the traceback will end up in one of
|
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the HTTP server's log file, or be discarded altogether.
|
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|
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Fortunately, once you have managed to get your script to execute *some*
|
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code, it is easy to catch exceptions and cause a traceback to be printed.
|
||||
The test() function below in this module is an example. Here are the
|
||||
rules:
|
||||
Fortunately, once you have managed to get your script to execute
|
||||
*some* code, it is easy to catch exceptions and cause a traceback to
|
||||
be printed. The test() function below in this module is an example.
|
||||
Here are the rules:
|
||||
|
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1. Import the traceback module (before entering the try-except!)
|
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1. Import the traceback module (before entering the
|
||||
try-except!)
|
||||
|
||||
2. Make sure you finish printing the headers and the blank line early
|
||||
2. Make sure you finish printing the headers and the blank
|
||||
line early
|
||||
|
||||
3. Assign sys.stderr to sys.stdout
|
||||
|
||||
|
|
@ -258,13 +268,13 @@
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print "\n\n<PRE>"
|
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traceback.print_exc()
|
||||
|
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Notes: The assignment to sys.stderr is needed because the traceback prints
|
||||
to sys.stderr. The print "\n\n<PRE>" statement is necessary to disable the
|
||||
word wrapping in HTML.
|
||||
Notes: The assignment to sys.stderr is needed because the traceback
|
||||
prints to sys.stderr. The print "\n\n<PRE>" statement is necessary to
|
||||
disable the word wrapping in HTML.
|
||||
|
||||
If you suspect that there may be a problem in importing the traceback
|
||||
module, you can use an even more robust approach (which only uses built-in
|
||||
modules):
|
||||
If you suspect that there may be a problem in importing the traceback
|
||||
module, you can use an even more robust approach (which only uses
|
||||
built-in modules):
|
||||
|
||||
import sys
|
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sys.stderr = sys.stdout
|
||||
|
|
@ -272,12 +282,13 @@
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print
|
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...your code here...
|
||||
|
||||
This relies on the Python interpreter to print the traceback. The content
|
||||
type of the output is set to plain text, which disables all HTML
|
||||
processing. If your script works, the raw HTML will be displayed by your
|
||||
client. If it raises an exception, most likely after the first two lines
|
||||
have been printed, a traceback will be displayed. Because no HTML
|
||||
interpretation is going on, the traceback will readable.
|
||||
This relies on the Python interpreter to print the traceback. The
|
||||
content type of the output is set to plain text, which disables all
|
||||
HTML processing. If your script works, the raw HTML will be displayed
|
||||
by your client. If it raises an exception, most likely after the
|
||||
first two lines have been printed, a traceback will be displayed.
|
||||
Because no HTML interpretation is going on, the traceback will
|
||||
readable.
|
||||
|
||||
Good luck!
|
||||
|
||||
|
|
@ -285,40 +296,40 @@
|
|||
Common problems and solutions
|
||||
-----------------------------
|
||||
|
||||
- Most HTTP servers buffer the output from CGI scripts until the script is
|
||||
completed. This means that it is not possible to display a progress report
|
||||
on the client's display while the script is running.
|
||||
- Most HTTP servers buffer the output from CGI scripts until the
|
||||
script is completed. This means that it is not possible to display a
|
||||
progress report on the client's display while the script is running.
|
||||
|
||||
- Check the installation instructions above.
|
||||
|
||||
- Check the HTTP server's log files. ("tail -f logfile" in a separate
|
||||
- Check the HTTP server's log files. ("tail -f logfile" in a separate
|
||||
window may be useful!)
|
||||
|
||||
- Always check a script for syntax errors first, by doing something like
|
||||
"python script.py".
|
||||
- Always check a script for syntax errors first, by doing something
|
||||
like "python script.py".
|
||||
|
||||
- When using any of the debugging techniques, don't forget to add
|
||||
"import sys" to the top of the script.
|
||||
|
||||
- When invoking external programs, make sure they can be found. Usually,
|
||||
this means using absolute path names -- $PATH is usually not set to a
|
||||
very useful value in a CGI script.
|
||||
- When invoking external programs, make sure they can be found.
|
||||
Usually, this means using absolute path names -- $PATH is usually not
|
||||
set to a very useful value in a CGI script.
|
||||
|
||||
- When reading or writing external files, make sure they can be read or
|
||||
written by every user on the system.
|
||||
- When reading or writing external files, make sure they can be read
|
||||
or written by every user on the system.
|
||||
|
||||
- Don't try to give a CGI script a set-uid mode. This doesn't work on most
|
||||
systems, and is a security liability as well.
|
||||
- Don't try to give a CGI script a set-uid mode. This doesn't work on
|
||||
most systems, and is a security liability as well.
|
||||
|
||||
|
||||
History
|
||||
-------
|
||||
|
||||
Michael McLay started this module. Steve Majewski changed the interface to
|
||||
SvFormContentDict and FormContentDict. The multipart parsing was inspired
|
||||
by code submitted by Andreas Paepcke. Guido van Rossum rewrote,
|
||||
reformatted and documented the module and is currently responsible for its
|
||||
maintenance.
|
||||
Michael McLay started this module. Steve Majewski changed the
|
||||
interface to SvFormContentDict and FormContentDict. The multipart
|
||||
parsing was inspired by code submitted by Andreas Paepcke. Guido van
|
||||
Rossum rewrote, reformatted and documented the module and is currently
|
||||
responsible for its maintenance.
|
||||
|
||||
"""
|
||||
|
||||
|
|
@ -376,7 +387,7 @@ def parse_qs(qs):
|
|||
if len(nv) != 2:
|
||||
continue
|
||||
name = nv[0]
|
||||
value = urllib.unquote(regsub.gsub('+',' ',nv[1]))
|
||||
value = urllib.unquote(regsub.gsub('+', ' ', nv[1]))
|
||||
if len(value):
|
||||
if dict.has_key (name):
|
||||
dict[name].append(value)
|
||||
|
|
@ -528,13 +539,13 @@ def __len__( self ):
|
|||
class SvFormContentDict(FormContentDict):
|
||||
"""Strict single-value expecting form content as dictionary.
|
||||
|
||||
IF you only expect a single value for each field, then form[key]
|
||||
will return that single value.
|
||||
It will raise an IndexError if that expectation is not true.
|
||||
IF you expect a field to have possible multiple values, than you
|
||||
can use form.getlist(key) to get all of the values.
|
||||
values() and items() are a compromise: they return single strings
|
||||
where there is a single value, and lists of strings otherwise.
|
||||
IF you only expect a single value for each field, then
|
||||
form[key] will return that single value. It will raise an
|
||||
IndexError if that expectation is not true. IF you expect a
|
||||
field to have possible multiple values, than you can use
|
||||
form.getlist(key) to get all of the values. values() and
|
||||
items() are a compromise: they return single strings where
|
||||
there is a single value, and lists of strings otherwise.
|
||||
|
||||
"""
|
||||
def __getitem__(self, key):
|
||||
|
|
@ -627,7 +638,7 @@ def test():
|
|||
print_environ()
|
||||
print_form(FormContentDict())
|
||||
print
|
||||
print "<H3>Current Working Directory</H3>"
|
||||
print "<H3>Current Working Directory:</H3>"
|
||||
try:
|
||||
pwd = os.getcwd()
|
||||
except os.error, msg:
|
||||
|
|
|
|||
Loading…
Add table
Add a link
Reference in a new issue