
``cgi`` --- Common Gateway Interface support
********************************************

Support module for Common Gateway Interface (CGI) scripts.

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


Introduction
============

A CGI script is invoked by an HTTP server, usually to process user
input submitted through an HTML ``<FORM>`` or ``<ISINDEX>`` element.

Most often, CGI scripts live in the server's special ``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 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).

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:

   print("Content-Type: text/html")    # HTML is following
   print()                             # blank line, end of headers

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:

   print("<TITLE>CGI script output</TITLE>")
   print("<H1>This is my first CGI script</H1>")
   print("Hello, world!")


Using the cgi module
====================

Begin by writing ``import cgi``.

When you write a new script, consider adding these lines:

   import cgitb
   cgitb.enable()

This activates a special exception handler that will display detailed
reports in the Web browser if any errors occur.  If you'd rather not
show the guts of your program to users of your script, you can have
the reports saved to files instead, with code like this:

   import cgitb
   cgitb.enable(display=0, logdir="/tmp")

It's very helpful to use this feature during script development. The
reports produced by ``cgitb`` provide information that can save you a
lot of time in tracking down bugs.  You can always remove the
``cgitb`` line later when you have tested your script and are
confident that it works correctly.

To get at submitted form data, use the ``FieldStorage`` class.
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 ``FieldStorage`` instance can be indexed like a Python dictionary,
and also supports the standard dictionary methods ``__contains__()``
and ``keys()``.  The built-in ``len()`` is also supported.  Form
fields containing empty strings are ignored and do not appear in the
dictionary; to keep such values, provide a true value for the optional
*keep_blank_values* keyword parameter when creating the
``FieldStorage`` instance.

For instance, the following code (which assumes that the *Content-
Type* header and blank line have already been printed) checks that the
fields ``name`` and ``addr`` are both set to a non-empty string:

   form = cgi.FieldStorage()
   if not ("name" in form and "addr" in form):
       print("<H1>Error</H1>")
       print("Please fill in the name and addr fields.")
       return
   print("<p>name:", form["name"].value)
   print("<p>addr:", form["addr"].value)
   ...further form processing here...

Here the fields, accessed through ``form[key]``, are themselves
instances of ``FieldStorage`` (or ``MiniFieldStorage``, depending on
the form encoding). The ``value`` attribute of the instance yields the
string value of the field.  The ``getvalue()`` method returns this
string value directly; it also accepts an optional second argument as
a default to return if the requested key is not present.

If the submitted form data contains more than one field with the same
name, the object retrieved by ``form[key]`` is not a ``FieldStorage``
or ``MiniFieldStorage`` instance but a list of such instances.
Similarly, in this situation, ``form.getvalue(key)`` would return a
list of strings. If you expect this possibility (when your HTML form
contains multiple fields with the same name), use the ``getlist()``
function, which always returns a list of values (so that you do not
need to special-case the single item case).  For example, this code
concatenates any number of username fields, separated by commas:

   value = form.getlist("username")
   usernames = ",".join(value)

If a field represents an uploaded file, accessing the value via the
``value`` attribute or the ``getvalue()`` method 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 leisure from
the ``file`` attribute:

   fileitem = form["userfile"]
   if fileitem.file:
       # It's an uploaded file; count lines
       linecount = 0
       while True:
           line = fileitem.file.readline()
           if not line: break
           linecount = linecount + 1

If an error is encountered when obtaining the contents of an uploaded
file (for example, when the user interrupts the form submission by
clicking on a Back or Cancel button) the ``done`` attribute of the
object for the field will be set to the value -1.

The file upload draft standard entertains the possibility of uploading
multiple files from one field (using a recursive *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 be *multipart/form-data* (or perhaps another
MIME type matching *multipart/**).  In 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 *application/x-www-form-urlencoded*),
the items will actually be instances of the class
``MiniFieldStorage``.  In this case, the ``list``, ``file``, and
``filename`` attributes are always ``None``.

A form submitted via POST that also has a query string will contain
both ``FieldStorage`` and ``MiniFieldStorage`` items.


Higher Level Interface
======================

The previous section explains how to read CGI form data using the
``FieldStorage`` class.  This section describes a higher level
interface which was added to this class to allow one to do it in a
more readable and intuitive way.  The interface doesn't make the
techniques described in previous sections obsolete --- they are still
useful to process file uploads efficiently, for example.

The interface consists of two simple methods. Using the methods you
can process form data in a generic way, without the need to worry
whether only one or more values were posted under one name.

In the previous section, you learned to write following code anytime
you expected a user to post more than one value under one name:

   item = form.getvalue("item")
   if isinstance(item, list):
       # The user is requesting more than one item.
   else:
       # The user is requesting only one item.

This situation is common for example when a form contains a group of
multiple checkboxes with the same name:

   <input type="checkbox" name="item" value="1" />
   <input type="checkbox" name="item" value="2" />

In most situations, however, there's only one form control with a
particular name in a form and then you expect and need only one value
associated with this name.  So you write a script containing for
example this code:

   user = form.getvalue("user").upper()

The problem with the code is that you should never expect that a
client will provide valid input to your scripts.  For example, if a
curious user appends another ``user=foo`` pair to the query string,
then the script would crash, because in this situation the
``getvalue("user")`` method call returns a list instead of a string.
Calling the ``toupper()`` method on a list is not valid (since lists
do not have a method of this name) and results in an
``AttributeError`` exception.

Therefore, the appropriate way to read form data values was to always
use the code which checks whether the obtained value is a single value
or a list of values.  That's annoying and leads to less readable
scripts.

A more convenient approach is to use the methods ``getfirst()`` and
``getlist()`` provided by this higher level interface.

FieldStorage.getfirst(name, default=None)

   This method always returns only one value associated with form
   field *name*. The method returns only the first value in case that
   more values were posted under such name.  Please note that the
   order in which the values are received may vary from browser to
   browser and should not be counted on. [1]  If no such form field or
   value exists then the method returns the value specified by the
   optional parameter *default*.  This parameter defaults to ``None``
   if not specified.

FieldStorage.getlist(name)

   This method always returns a list of values associated with form
   field *name*. The method returns an empty list if no such form
   field or value exists for *name*.  It returns a list consisting of
   one item if only one such value exists.

Using these methods you can write nice compact code:

   import cgi
   form = cgi.FieldStorage()
   user = form.getfirst("user", "").upper()    # This way it's safe.
   for item in form.getlist("item"):
       do_something(item)


Functions
=========

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.

cgi.parse(fp=None, environ=os.environ, keep_blank_values=False, strict_parsing=False)

   Parse a query in the environment or from a file (the file defaults
   to ``sys.stdin``).  The *keep_blank_values* and *strict_parsing*
   parameters are passed to ``urllib.parse.parse_qs()`` unchanged.

cgi.parse_qs(qs, keep_blank_values=False, strict_parsing=False)

   This function is deprecated in this module. Use
   ``urllib.parse.parse_qs()`` instead. It is maintained here only for
   backward compatibility.

cgi.parse_qsl(qs, keep_blank_values=False, strict_parsing=False)

   This function is deprecated in this module. Use
   ``urllib.parse.parse_qs()`` instead. It is maintained here only for
   backward compatibility.

cgi.parse_multipart(fp, pdict)

   Parse input of type *multipart/form-data* (for  file uploads).
   Arguments are *fp* for the input file and *pdict* for a dictionary
   containing other parameters in the *Content-Type* header.

   Returns a dictionary just like ``urllib.parse.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 ``FieldStorage``
   class instead which is much more flexible.

   Note that this does not parse nested multipart parts --- use
   ``FieldStorage`` for that.

cgi.parse_header(string)

   Parse a MIME header (such as *Content-Type*) into a main value and
   a dictionary of parameters.

cgi.test()

   Robust test CGI script, usable as main program. Writes minimal HTTP
   headers and formats all information provided to the script in HTML
   form.

cgi.print_environ()

   Format the shell environment in HTML.

cgi.print_form(form)

   Format a form in HTML.

cgi.print_directory()

   Format the current directory in HTML.

cgi.print_environ_usage()

   Print a list of useful (used by CGI) environment variables in HTML.

cgi.escape(s, quote=False)

   Convert the characters ``'&'``, ``'<'`` and ``'>'`` in string *s*
   to HTML-safe sequences.  Use this if you need to display text that
   might contain such characters in HTML.  If the optional flag
   *quote* is true, the quotation mark character (``'"'``) is also
   translated; this helps for inclusion in an HTML attribute value, as
   in ``<A HREF="...">``.  If the value to be quoted might include
   single- or double-quote characters, or both, consider using the
   ``quoteattr()`` function in the ``xml.sax.saxutils`` module
   instead.


Caring about security
=====================

There's one important rule: if you invoke an external program (via the
``os.system()`` or ``os.popen()`` functions. or others with similar
functionality), 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.


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 ``cgi-bin`` in the server
tree.

Make sure that your script is readable and executable by "others"; the
Unix file mode should be ``0o755`` octal (use ``chmod 0755
filename``).  Make sure that the first line of the script contains
``#!`` starting in column 1 followed by the pathname of the Python
interpreter, for instance:

   #!/usr/local/bin/python

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 ``0o644`` for readable and ``0o666`` 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 when you log in.  In particular,
don't count on the shell's search path for executables (**PATH**) or
the Python module search path (**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.  For example:

   import sys
   sys.path.insert(0, "/usr/home/joe/lib/python")
   sys.path.insert(0, "/usr/local/lib/python")

(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).


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.


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 (``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 ``cgi-bin`` directory, it should be possible to send
it a request by entering a URL into your browser of the form:

   http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home

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, 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 ``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 ``cgi`` module's ``test()``
function from your script: replace its main code with the single
statement

   cgi.test()

This should produce the same results as those gotten from installing
the ``cgi.py`` file itself.

When an ordinary Python script raises an unhandled exception (for
whatever reason: 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 files, or be discarded altogether.

Fortunately, once you have managed to get your script to execute
*some* code, you can easily send tracebacks to the Web browser using
the ``cgitb`` module. If you haven't done so already, just add the
lines:

   import cgitb
   cgitb.enable()

to the top of your script.  Then try running it again; when a problem
occurs, you should see a detailed report that will likely make
apparent the cause of the crash.

If you suspect that there may be a problem in importing the ``cgitb``
module, you can use an even more robust approach (which only uses
built-in modules):

   import sys
   sys.stderr = sys.stdout
   print("Content-Type: text/plain")
   print()
   ...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 be
readable.


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.

* Check the installation instructions above.

* 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``.

* If your script does not have any syntax errors, try adding ``import
  cgitb; cgitb.enable()`` 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 reading or writing external files, make sure they can be read
  or written by the userid under which your CGI script will be
  running: this is typically the userid under which the web server is
  running, or some explicitly specified userid for a web server's
  ``suexec`` feature.

* 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.

-[ Footnotes ]-

[1] Note that some recent versions of the HTML specification do state
    what order the field values should be supplied in, but knowing
    whether a request was received from a conforming browser, or even
    from a browser at all, is tedious and error-prone.
