Doc/libcgi.tex

\section{Standard Module \sectcode{cgi}}
\label{module-cgi}
\stmodindex{cgi}
\indexii{WWW}{server}
\indexii{CGI}{protocol}
\indexii{HTTP}{protocol}
\indexii{MIME}{headers}
\index{URL}

\renewcommand{\indexsubitem}{(in module cgi)}

Support module for CGI (Common Gateway Interface) scripts.

This module defines a number of utilities for use by CGI scripts
written in Python.

\subsection{Introduction}
\nodename{Introduction to the CGI module}

A CGI script is invoked by an HTTP server, usually to process user
input submitted through an HTML \code{<FORM>} or \code{<ISINPUT>} element.

Most often, CGI scripts live in the server's special \code{cgi-bin}
directory.  The HTTP server places all sorts of information about the
request (such as the client's hostname, the requested URL, the query
string, and lots of other goodies) in the script's shell environment,
executes the script, and sends the script's output back to the client.

The script's input is connected to the client too, and sometimes the
form data is read this way; at other times the form data is passed via
the ``query string'' part of the URL.  This module (\code{cgi.py}) is intended
to take care of the different cases and provide a simpler interface to
the Python script.  It also provides a number of utilities that help
in debugging scripts, and the latest addition is support for file
uploads from a form (if your browser supports it -- Grail 0.3 and
Netscape 2.0 do).

The output of a CGI script should consist of two sections, separated
by a blank line.  The first section contains a number of headers,
telling the client what kind of data is following.  Python code to
generate a minimal header section looks like this:

\bcode\begin{verbatim}
print "Content-type: text/html"     # HTML is following
print                               # blank line, end of headers
\end{verbatim}\ecode
%
The second section is usually HTML, which allows the client software
to display nicely formatted text with header, in-line images, etc.
Here's Python code that prints a simple piece of HTML:

\bcode\begin{verbatim}
print "<TITLE>CGI script output</TITLE>"
print "<H1>This is my first CGI script</H1>"
print "Hello, world!"
\end{verbatim}\ecode
%
(It may not be fully legal HTML according to the letter of the
standard, but any browser will understand it.)

\subsection{Using the cgi module}
\nodename{Using the cgi module}

Begin by writing \code{import cgi}.  Don't use \code{from cgi import *} -- the
module defines all sorts of names for its own use or for backward 
compatibility that you don't want in your namespace.

It's best to use the \code{FieldStorage} class.  The other classes define in this 
module are provided mostly for backward compatibility.  Instantiate it 
exactly once, without arguments.  This reads the form contents from 
standard input or the environment (depending on the value of various 
environment variables set according to the CGI standard).  Since it may 
consume standard input, it should be instantiated only once.

The \code{FieldStorage} instance can be accessed as if it were a Python 
dictionary.  For instance, the following code (which assumes that the 
\code{Content-type} header and blank line have already been printed) checks that 
the fields \code{name} and \code{addr} are both set to a non-empty string:

\bcode\begin{verbatim}
form = cgi.FieldStorage()
form_ok = 0
if form.has_key("name") and form.has_key("addr"):
    if form["name"].value != "" and form["addr"].value != "":
        form_ok = 1
if not form_ok:
    print "<H1>Error</H1>"
    print "Please fill in the name and addr fields."
    return
...further form processing here...
\end{verbatim}\ecode
%
Here the fields, accessed through \code{form[key]}, are themselves instances
of \code{FieldStorage} (or \code{MiniFieldStorage}, depending on the form encoding).

If the submitted form data contains more than one field with the same
name, the object retrieved by \code{form[key]} is not a \code{(Mini)FieldStorage}
instance but a list of such instances.  If you expect this possibility
(i.e., when your HTML form comtains multiple fields with the same
name), use the \code{type()} function to determine whether you have a single
instance or a list of instances.  For example, here's code that
concatenates any number of username fields, separated by commas:

\bcode\begin{verbatim}
username = form["username"]
if type(username) is type([]):
    # Multiple username fields specified
    usernames = ""
    for item in username:
        if usernames:
            # Next item -- insert comma
            usernames = usernames + "," + item.value
        else:
            # First item -- don't insert comma
            usernames = item.value
else:
    # Single username field specified
    usernames = username.value
\end{verbatim}\ecode
%
If a field represents an uploaded file, the value attribute reads the 
entire file in memory as a string.  This may not be what you want.  You can 
test for an uploaded file by testing either the filename attribute or the 
file attribute.  You can then read the data at leasure from the file 
attribute:

\bcode\begin{verbatim}
fileitem = form["userfile"]
if fileitem.file:
    # It's an uploaded file; count lines
    linecount = 0
    while 1:
        line = fileitem.file.readline()
        if not line: break
        linecount = linecount + 1
\end{verbatim}\ecode
%
The file upload draft standard entertains the possibility of uploading
multiple files from one field (using a recursive \code{multipart/*}
encoding).  When this occurs, the item will be a dictionary-like
FieldStorage item.  This can be determined by testing its type
attribute, which should have the value \code{multipart/form-data} (or
perhaps another string beginning with \code{multipart/}  It this case, it
can be iterated over recursively just like the top-level form object.

When a form is submitted in the ``old'' format (as the query string or as a 
single data part of type \code{application/x-www-form-urlencoded}), the items 
will actually be instances of the class \code{MiniFieldStorage}.  In this case,
the list, file and filename attributes are always \code{None}.


\subsection{Old classes}

These classes, present in earlier versions of the \code{cgi} module, are still 
supported for backward compatibility.  New applications should use the
FieldStorage class.

\code{SvFormContentDict}: single value form content as dictionary; assumes each 
field name occurs in the form only once.

\code{FormContentDict}: multiple value form content as dictionary (the form
items are lists of values).  Useful if your form contains multiple
fields with the same name.

Other classes (\code{FormContent}, \code{InterpFormContentDict}) are present for
backwards compatibility with really old applications only.  If you still 
use these and would be inconvenienced when they disappeared from a next 
version of this module, drop me a note.


\subsection{Functions}
\nodename{Functions in cgi module}

These are useful if you want more control, or if you want to employ
some of the algorithms implemented in this module in other
circumstances.

\begin{funcdesc}{parse}{fp}: Parse a query in the environment or from a file (default \code{sys.stdin}).
\end{funcdesc}

\begin{funcdesc}{parse_qs}{qs}: parse a query string given as a string argument (data of type 
\code{application/x-www-form-urlencoded}).
\end{funcdesc}

\begin{funcdesc}{parse_multipart}{fp\, pdict}: parse input of type \code{multipart/form-data} (for 
file uploads).  Arguments are \code{fp} for the input file and 
    \code{pdict} for the dictionary containing other parameters of \code{content-type} header

    Returns a dictionary just like \code{parse_qs()}: keys are the field names, each 
    value is a list of values for that field.  This is easy to use but not 
    much good if you are expecting megabytes to be uploaded -- in that case, 
    use the \code{FieldStorage} class instead which is much more flexible.  Note 
    that \code{content-type} is the raw, unparsed contents of the \code{content-type} 
    header.

    Note that this does not parse nested multipart parts -- use \code{FieldStorage} for 
    that.
\end{funcdesc}

\begin{funcdesc}{parse_header}{string}: parse a header like \code{Content-type} into a main
content-type and a dictionary of parameters.
\end{funcdesc}

\begin{funcdesc}{test}{}: robust test CGI script, usable as main program.
    Writes minimal HTTP headers and formats all information provided to
    the script in HTML form.
\end{funcdesc}

\begin{funcdesc}{print_environ}{}: format the shell environment in HTML.
\end{funcdesc}

\begin{funcdesc}{print_form}{form}: format a form in HTML.
\end{funcdesc}

\begin{funcdesc}{print_directory}{}: format the current directory in HTML.
\end{funcdesc}

\begin{funcdesc}{print_environ_usage}{}: print a list of useful (used by CGI) environment variables in
HTML.
\end{funcdesc}

\begin{funcdesc}{escape}{s\optional{\, quote}}: convert the characters
``\code{\&}'', ``\code{<}'' and ``\code{>}'' in string \var{s} to HTML-safe
sequences.  Use this if you need to display text that might contain
such characters in HTML.  If the optional flag \var{quote} is true,
the double quote character (\code{"}) is also translated; this helps
for inclusion in an HTML attribute value, e.g. in ``\code{<A HREF="...">}''.
\end{funcdesc}


\subsection{Caring about security}

There's one important rule: if you invoke an external program (e.g.
via the \code{os.system()} or \code{os.popen()} functions), make very sure you don't
pass arbitrary strings received from the client to the shell.  This is
a well-known security hole whereby clever hackers anywhere on the web
can exploit a gullible CGI script to invoke arbitrary shell commands.
Even parts of the URL or field names cannot be trusted, since the
request doesn't have to come from your form!

To be on the safe side, if you must pass a string gotten from a form
to a shell command, you should make sure the string contains only
alphanumeric characters, dashes, underscores, and periods.


\subsection{Installing your CGI script on a Unix system}

Read the documentation for your HTTP server and check with your local
system administrator to find the directory where CGI scripts should be
installed; usually this is in a directory \code{cgi-bin} in the server tree.

Make sure that your script is readable and executable by ``others''; the
Unix file mode should be 755 (use \code{chmod 755 filename}).  Make sure
that the first line of the script contains \code{\#!} starting in column 1
followed by the pathname of the Python interpreter, for instance:

\bcode\begin{verbatim}
#!/usr/local/bin/python
\end{verbatim}\ecode
%
Make sure the Python interpreter exists and is executable by ``others''.

Make sure that any files your script needs to read or write are
readable or writable, respectively, by ``others'' -- their mode should
be 644 for readable and 666 for writable.  This is because, for
security reasons, the HTTP server executes your script as user
``nobody'', without any special privileges.  It can only read (write,
execute) files that everybody can read (write, execute).  The current
directory at execution time is also different (it is usually the
server's cgi-bin directory) and the set of environment variables is
also different from what you get at login.  in particular, don't count
on the shell's search path for executables (\code{\$PATH}) or the Python
module search path (\code{\$PYTHONPATH}) to be set to anything interesting.

If you need to load modules from a directory which is not on Python's
default module search path, you can change the path in your script,
before importing other modules, e.g.:

\bcode\begin{verbatim}
import sys
sys.path.insert(0, "/usr/home/joe/lib/python")
sys.path.insert(0, "/usr/local/lib/python")
\end{verbatim}\ecode
%
(This way, the directory inserted last will be searched first!)

Instructions for non-Unix systems will vary; check your HTTP server's
documentation (it will usually have a section on CGI scripts).


\subsection{Testing your CGI script}

Unfortunately, a CGI script will generally not run when you try it
from the command line, and a script that works perfectly from the
command line may fail mysteriously when run from the server.  There's
one reason why you should still test your script from the command
line: if it contains a syntax error, the python interpreter won't
execute it at all, and the HTTP server will most likely send a cryptic
error to the client.

Assuming your script has no syntax errors, yet it does not work, you
have no choice but to read the next section:


\subsection{Debugging CGI scripts}

First of all, check for trivial installation errors -- reading the
section above on installing your CGI script carefully can save you a
lot of time.  If you wonder whether you have understood the
installation procedure correctly, try installing a copy of this module
file (\code{cgi.py}) as a CGI script.  When invoked as a script, the file
will dump its environment and the contents of the form in HTML form.
Give it the right mode etc, and send it a request.  If it's installed
in the standard \code{cgi-bin} directory, it should be possible to send it a
request by entering a URL into your browser of the form:

\bcode\begin{verbatim}
http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home
\end{verbatim}\ecode
%
If this gives an error of type 404, the server cannot find the script
-- perhaps you need to install it in a different directory.  If it
gives another error (e.g.  500), there's an installation problem that
you should fix before trying to go any further.  If you get a nicely
formatted listing of the environment and form content (in this
example, the fields should be listed as ``addr'' with value ``At Home''
and ``name'' with value ``Joe Blow''), the \code{cgi.py} script has been
installed correctly.  If you follow the same procedure for your own
script, you should now be able to debug it.

The next step could be to call the \code{cgi} module's test() function from
your script: replace its main code with the single statement

\bcode\begin{verbatim}
cgi.test()
\end{verbatim}\ecode
%
This should produce the same results as those gotten from installing
the \code{cgi.py} file itself.

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 interpreter prints a nice traceback and exits.
While the Python interpreter will still do this when your CGI script
raises an exception, most likely the traceback will end up in one of
the HTTP server's log file, or be discarded altogether.

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 \code{test()} function below in this module is an example.
Here are the rules:

\begin{enumerate}
	\item Import the traceback module (before entering the
	   try-except!)
	
	\item Make sure you finish printing the headers and the blank
	   line early
	
	\item Assign \code{sys.stderr} to \code{sys.stdout}
	
	\item Wrap all remaining code in a try-except statement
	
	\item In the except clause, call \code{traceback.print_exc()}
\end{enumerate}

For example:

\bcode\begin{verbatim}
import sys
import traceback
print "Content-type: text/html"
print
sys.stderr = sys.stdout
try:
    ...your code here...
except:
    print "\n\n<PRE>"
    traceback.print_exc()
\end{verbatim}\ecode
%
Notes: The assignment to \code{sys.stderr} is needed because the traceback
prints to \code{sys.stderr}.  The \code{print "$\backslash$n$\backslash$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):

\bcode\begin{verbatim}
import sys
sys.stderr = sys.stdout
print "Content-type: text/plain"
print
...your code here...
\end{verbatim}\ecode
%
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.


\subsection{Common problems and solutions}

\begin{itemize}
\item 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.

\item Check the installation instructions above.

\item Check the HTTP server's log files.  (\code{tail -f logfile} in a separate
window may be useful!)

\item Always check a script for syntax errors first, by doing something
like \code{python script.py}.

\item When using any of the debugging techniques, don't forget to add
\code{import sys} to the top of the script.

\item When invoking external programs, make sure they can be found.
Usually, this means using absolute path names -- \code{\$PATH} is usually not
set to a very useful value in a CGI script.

\item When reading or writing external files, make sure they can be read
or written by every user on the system.

\item 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.
\end{itemize}