:mod:`cgi` --- Common Gateway Interface support.

================================================

.. module:: cgi

   :synopsis: Helpers for running Python scripts via the Common Gateway Interface.

.. index::

   pair: WWW; server

   pair: CGI; protocol

   pair: HTTP; protocol

   pair: MIME; headers

   single: URL

   single: Common Gateway Interface

Support module for Common Gateway Interface (CGI) scripts.

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

Python.

Introduction

------------

.. _cgi-intro:

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 :file:`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:

Using the cgi module

--------------------

Begin by writing ``import cgi``.  Do not use ``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.

When you write a new script, consider adding the line::

   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 a line 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 :mod:`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, it's best to use the :class:`FieldStorage` class.

The other classes defined 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 :class:`FieldStorage` instance can be indexed like a Python dictionary, and

also supports the standard dictionary methods :meth:`has_key` and :meth:`keys`.

The built-in :func:`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 :class:`FieldStorage` instance.

For instance, the following code (which assumes that the

:mailheader:`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 (form.has_key("name") and form.has_key("addr")):

       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

:class:`FieldStorage` (or :class:`MiniFieldStorage`, depending on the form

encoding). The :attr:`value` attribute of the instance yields the string value

of the field.  The :meth:`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 :class:`FieldStorage` or

:class:`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 :func:`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

:attr:`value` attribute or the :func:`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 :attr:`filename` attribute or the :attr:`file`

attribute.  You can then read the data at leisure from the :attr:`file`

attribute::

   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

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 :attr:`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 :mimetype:`multipart/\*` encoding).

When this occurs, the item will be a dictionary-like :class:`FieldStorage` item.

This can be determined by testing its :attr:`type` attribute, which should be

:mimetype:`multipart/form-data` (or perhaps another MIME type matching

:mimetype:`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 :mimetype:`application/x-www-form-urlencoded`), the items will

actually be instances of the class :class:`MiniFieldStorage`.  In this case, the

:attr:`list`, :attr:`file`, and :attr:`filename` attributes are always ``None``.

A form submitted via POST that also has a query string will contain both

:class:`FieldStorage` and :class:`MiniFieldStorage` items.

Higher Level Interface

----------------------

.. versionadded:: 2.2

The previous section explains how to read CGI form data using the

:class:`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.

.. XXX: Is this true ?

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 :meth:`toupper` method on a list is not valid

(since lists do not have a method of this name) and results in an

:exc:`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 :meth:`getfirst` and

:meth:`getlist` provided by this higher level interface.

.. method:: FieldStorage.getfirst(name[, default])

   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. [#]_  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.

.. method:: 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)

Old classes

-----------

.. deprecated:: 2.6

   These classes, present in earlier versions of the :mod:`cgi` module, are

   still supported for backward compatibility.  New applications should use the

   :class:`FieldStorage` class.

:class:`SvFormContentDict` stores single value form content as dictionary; it

assumes each field name occurs in the form only once.

:class:`FormContentDict` stores multiple value form content as a dictionary (the

form items are lists of values).  Useful if your form contains multiple fields

with the same name.

Other classes (:class:`FormContent`, :class:`InterpFormContentDict`) are present

for backwards compatibility with really old applications only.

.. _functions-in-cgi-module:

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.

.. function:: parse(fp[, keep_blank_values[, strict_parsing]])

   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 :func:`urlparse.parse_qs` unchanged.

.. function:: parse_qs(qs[, keep_blank_values[, strict_parsing]])

   This function is deprecated in this module. Use :func:`urlparse.parse_qs`

   instead. It is maintained here only for backward compatiblity.

.. function:: parse_qsl(qs[, keep_blank_values[, strict_parsing]])

   This function is deprecated in this module. Use :func:`urlparse.parse_qsl`

   instead. It is maintained here only for backward compatiblity.

.. function:: parse_multipart(fp, pdict)

   Parse input of type :mimetype:`multipart/form-data` (for  file uploads).

   Arguments are *fp* for the input file and *pdict* for a dictionary containing

   other parameters in the :mailheader:`Content-Type` header.

   Returns a dictionary just like :func:`urlparse.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

   :class:`FieldStorage` class instead which is much more flexible.

   Note that this does not parse nested multipart parts --- use

   :class:`FieldStorage` for that.

.. function:: parse_header(string)

   Parse a MIME header (such as :mailheader:`Content-Type`) into a main value and a

   dictionary of parameters.

.. function:: test()

   Robust test CGI script, usable as main program. Writes minimal HTTP headers and

   formats all information provided to the script in HTML form.

.. function:: print_environ()

   Format the shell environment in HTML.

.. function:: print_form(form)

   Format a form in HTML.

.. function:: print_directory()

   Format the current directory in HTML.

.. function:: print_environ_usage()

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

.. function:: escape(s[, quote])

   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

   :func:`quoteattr` function in the :mod:`xml.sax.saxutils` module instead.

.. _cgi-security:

Caring about security

---------------------

.. index:: pair: CGI; security

There's one important rule: if you invoke an external program (via the

:func:`os.system` or :func:`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 :file:`cgi-bin` in the server tree.

Make sure that your script is readable and executable by "others"; the Unix file

mode should be ``0755`` 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 ``0644`` for

readable and ``0666`` 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 (:envvar:`PATH`) or the Python module

search path (:envvar:`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

---------------------

.. index:: pair: CGI; debugging

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 (: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 :file:`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 :file:`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 :mod:`cgi` module's :func:`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

:file:`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 :mod:`cgitb` module.

If you haven't done so already, just add the line::

   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 :mod:`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 --- :envvar:`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.

.. rubric:: Footnotes