Doc/library/cgi.rst

: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``.

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, use the :class:`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 :class:`FieldStorage` instance can be indexed like a Python dictionary, and
also supports the standard dictionary methods :meth:`__contains__` 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 ("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
: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 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 :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``.


Higher Level Interface
----------------------

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)


.. _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:`parse_qs` unchanged.


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

   Parse a query string given as a string argument (data of type
   :mimetype:`application/x-www-form-urlencoded`).  Data are returned as a
   dictionary.  The dictionary keys are the unique query variable names and the
   values are lists of values for each name.

   The optional argument *keep_blank_values* is a flag indicating whether blank
   values in URL encoded queries should be treated as blank strings.   A true value
   indicates that blanks should be retained as  blank strings.  The default false
   value indicates that blank values are to be ignored and treated as if they were
   not included.

   The optional argument *strict_parsing* is a flag indicating what to do with
   parsing errors.  If false (the default), errors are silently ignored.  If true,
   errors raise a :exc:`ValueError` exception.

   Use the :func:`urllib.urlencode` function to convert such dictionaries into
   query strings.


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

   Parse a query string given as a string argument (data of type
   :mimetype:`application/x-www-form-urlencoded`).  Data are returned as a list of
   name, value pairs.

   The optional argument *keep_blank_values* is a flag indicating whether blank
   values in URL encoded queries should be treated as blank strings.   A true value
   indicates that blanks should be retained as  blank strings.  The default false
   value indicates that blank values are to be ignored and treated as if they were
   not included.

   The optional argument *strict_parsing* is a flag indicating what to do with
   parsing errors.  If false (the default), errors are silently ignored.  If true,
   errors raise a :exc:`ValueError` exception.

   Use the :func:`urllib.urlencode` function to convert such lists of pairs into
   query strings.


.. 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:`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

.. [#] 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.