/usr/share/doc/python3-docs/html/_sources/library/

/usr/share/doc/python3-docs/html/_sources/library

Name	Size	Mode	Actions
2to3.rst.txt	15898	0644	edit dl rm
abc.rst.txt	11805	0644	edit dl rm
aifc.rst.txt	7503	0644	edit dl rm
allos.rst.txt	678	0644	edit dl rm
archiving.rst.txt	440	0644	edit dl rm
argparse.rst.txt	77480	0644	edit dl rm
array.rst.txt	10812	0644	edit dl rm
ast.rst.txt	10159	0644	edit dl rm
asynchat.rst.txt	8528	0644	edit dl rm
asyncio-dev.rst.txt	13398	0644	edit dl rm
asyncio-eventloop.rst.txt	33701	0644	edit dl rm
asyncio-eventloops.rst.txt	7660	0644	edit dl rm
asyncio-protocol.rst.txt	25170	0644	edit dl rm
asyncio-queue.rst.txt	4328	0644	edit dl rm
asyncio-stream.rst.txt	14363	0644	edit dl rm
asyncio-subprocess.rst.txt	15544	0644	edit dl rm
asyncio-sync.rst.txt	9078	0644	edit dl rm
asyncio-task.rst.txt	25406	0644	edit dl rm
asyncio.rst.txt	2272	0644	edit dl rm
asyncore.rst.txt	13624	0644	edit dl rm
atexit.rst.txt	3669	0644	edit dl rm
audioop.rst.txt	10712	0644	edit dl rm
base64.rst.txt	10371	0644	edit dl rm
bdb.rst.txt	12985	0644	edit dl rm
binary.rst.txt	654	0644	edit dl rm
binascii.rst.txt	6539	0644	edit dl rm
binhex.rst.txt	1709	0644	edit dl rm
bisect.rst.txt	5398	0644	edit dl rm
builtins.rst.txt	1465	0644	edit dl rm
bz2.rst.txt	8825	0644	edit dl rm
calendar.rst.txt	10988	0644	edit dl rm
cgi.rst.txt	22650	0644	edit dl rm
cgitb.rst.txt	2918	0644	edit dl rm
chunk.rst.txt	5094	0644	edit dl rm
cmath.rst.txt	9326	0644	edit dl rm
cmd.rst.txt	13795	0644	edit dl rm
code.rst.txt	7817	0644	edit dl rm
codecs.rst.txt	75564	0644	edit dl rm
codeop.rst.txt	3021	0644	edit dl rm
collections.abc.rst.txt	12861	0644	edit dl rm
collections.rst.txt	46063	0644	edit dl rm
colorsys.rst.txt	1820	0644	edit dl rm
compileall.rst.txt	8975	0644	edit dl rm
concurrency.rst.txt	673	0644	edit dl rm
concurrent.futures.rst.txt	16910	0644	edit dl rm
concurrent.rst.txt	171	0644	edit dl rm
configparser.rst.txt	48756	0644	edit dl rm
constants.rst.txt	3331	0644	edit dl rm
contextlib.rst.txt	28175	0644	edit dl rm
copy.rst.txt	3378	0644	edit dl rm
copyreg.rst.txt	2182	0644	edit dl rm
crypt.rst.txt	5122	0644	edit dl rm
crypto.rst.txt	411	0644	edit dl rm
csv.rst.txt	19902	0644	edit dl rm
ctypes.rst.txt	88613	0644	edit dl rm
curses.ascii.rst.txt	8920	0644	edit dl rm
curses.panel.rst.txt	2764	0644	edit dl rm
curses.rst.txt	76397	0644	edit dl rm
custominterp.rst.txt	569	0644	edit dl rm
datatypes.rst.txt	751	0644	edit dl rm
datetime.rst.txt	90524	0644	edit dl rm
dbm.rst.txt	14293	0644	edit dl rm
debug.rst.txt	471	0644	edit dl rm
decimal.rst.txt	74869	0644	edit dl rm
development.rst.txt	704	0644	edit dl rm
difflib.rst.txt	29861	0644	edit dl rm
dis.rst.txt	32567	0644	edit dl rm
distribution.rst.txt	452	0644	edit dl rm
distutils.rst.txt	1974	0644	edit dl rm
doctest.rst.txt	71743	0644	edit dl rm
dummy_threading.rst.txt	784	0644	edit dl rm
email.charset.rst.txt	9293	0644	edit dl rm
email.compat32-message.rst.txt	33493	0644	edit dl rm
email.contentmanager.rst.txt	9112	0644	edit dl rm
email.encoders.rst.txt	2715	0644	edit dl rm
email.errors.rst.txt	4792	0644	edit dl rm
email.examples.rst.txt	1915	0644	edit dl rm
email.generator.rst.txt	13637	0644	edit dl rm
email.header.rst.txt	9188	0644	edit dl rm
email.headerregistry.rst.txt	17957	0644	edit dl rm
email.iterators.rst.txt	2795	0644	edit dl rm
email.message.rst.txt	33008	0644	edit dl rm
email.mime.rst.txt	11721	0644	edit dl rm
email.parser.rst.txt	14092	0644	edit dl rm
email.policy.rst.txt	27022	0644	edit dl rm
email.rst.txt	6785	0644	edit dl rm
email.util.rst.txt	9214	0644	edit dl rm
ensurepip.rst.txt	4987	0644	edit dl rm
enum.rst.txt	32751	0644	edit dl rm
errno.rst.txt	6811	0644	edit dl rm
exceptions.rst.txt	25570	0644	edit dl rm
faulthandler.rst.txt	6136	0644	edit dl rm
fcntl.rst.txt	7152	0644	edit dl rm
filecmp.rst.txt	5623	0644	edit dl rm
fileformats.rst.txt	287	0644	edit dl rm
fileinput.rst.txt	8051	0644	edit dl rm
filesys.rst.txt	961	0644	edit dl rm
fnmatch.rst.txt	3103	0644	edit dl rm
formatter.rst.txt	13242	0644	edit dl rm
fpectl.rst.txt	4180	0644	edit dl rm
fractions.rst.txt	6364	0644	edit dl rm
frameworks.rst.txt	391	0644	edit dl rm
ftplib.rst.txt	17552	0644	edit dl rm
functional.rst.txt	365	0644	edit dl rm
functions.rst.txt	71621	0644	edit dl rm
functools.rst.txt	18503	0644	edit dl rm
gc.rst.txt	9694	0644	edit dl rm
getopt.rst.txt	6557	0644	edit dl rm
getpass.rst.txt	1882	0644	edit dl rm
gettext.rst.txt	25588	0644	edit dl rm
glob.rst.txt	3508	0644	edit dl rm
grp.rst.txt	2417	0644	edit dl rm
gzip.rst.txt	8301	0644	edit dl rm
hashlib.rst.txt	26523	0644	edit dl rm
heapq.rst.txt	13402	0644	edit dl rm
hmac.rst.txt	4020	0644	edit dl rm
html.entities.rst.txt	1319	0644	edit dl rm
html.parser.rst.txt	11275	0644	edit dl rm
html.rst.txt	1304	0644	edit dl rm
http.client.rst.txt	18870	0644	edit dl rm
http.cookiejar.rst.txt	27832	0644	edit dl rm
http.cookies.rst.txt	8645	0644	edit dl rm
http.rst.txt	7096	0644	edit dl rm
http.server.rst.txt	17639	0644	edit dl rm
i18n.rst.txt	408	0644	edit dl rm
idle.rst.txt	25834	0644	edit dl rm
imaplib.rst.txt	20367	0644	edit dl rm
imghdr.rst.txt	2938	0644	edit dl rm
imp.rst.txt	15548	0644	edit dl rm
importlib.rst.txt	53563	0644	edit dl rm
index.rst.txt	2258	0644	edit dl rm
inspect.rst.txt	53928	0644	edit dl rm
internet.rst.txt	992	0644	edit dl rm
intro.rst.txt	2645	0644	edit dl rm
io.rst.txt	40328	0644	edit dl rm
ipaddress.rst.txt	32340	0644	edit dl rm
ipc.rst.txt	649	0644	edit dl rm
itertools.rst.txt	38459	0644	edit dl rm
json.rst.txt	27876	0644	edit dl rm
keyword.rst.txt	617	0644	edit dl rm
language.rst.txt	522	0644	edit dl rm
linecache.rst.txt	2408	0644	edit dl rm
locale.rst.txt	25470	0644	edit dl rm
logging.config.rst.txt	33886	0644	edit dl rm
logging.handlers.rst.txt	42497	0644	edit dl rm
logging.rst.txt	60479	0644	edit dl rm
lzma.rst.txt	17361	0644	edit dl rm
macpath.rst.txt	707	0644	edit dl rm
mailbox.rst.txt	63055	0644	edit dl rm
mailcap.rst.txt	3691	0644	edit dl rm
markup.rst.txt	679	0644	edit dl rm
marshal.rst.txt	4855	0644	edit dl rm
math.rst.txt	14375	0644	edit dl rm
mimetypes.rst.txt	9839	0644	edit dl rm
misc.rst.txt	247	0644	edit dl rm
mm.rst.txt	431	0644	edit dl rm
mmap.rst.txt	11232	0644	edit dl rm
modulefinder.rst.txt	3240	0644	edit dl rm
modules.rst.txt	355	0644	edit dl rm
msilib.rst.txt	18612	0644	edit dl rm
msvcrt.rst.txt	4392	0644	edit dl rm
multiprocessing.rst.txt	104494	0644	edit dl rm
netdata.rst.txt	339	0644	edit dl rm
netrc.rst.txt	3007	0644	edit dl rm
nis.rst.txt	1993	0644	edit dl rm
nntplib.rst.txt	21821	0644	edit dl rm
numbers.rst.txt	8089	0644	edit dl rm
numeric.rst.txt	696	0644	edit dl rm
operator.rst.txt	18989	0644	edit dl rm
optparse.rst.txt	76996	0644	edit dl rm
os.path.rst.txt	16419	0644	edit dl rm
os.rst.txt	137534	0644	edit dl rm
ossaudiodev.rst.txt	17845	0644	edit dl rm
othergui.rst.txt	2822	0644	edit dl rm
parser.rst.txt	15148	0644	edit dl rm
pathlib.rst.txt	30045	0644	edit dl rm
pdb.rst.txt	19454	0644	edit dl rm
persistence.rst.txt	591	0644	edit dl rm
pickle.rst.txt	37440	0644	edit dl rm
pickletools.rst.txt	3729	0644	edit dl rm
pipes.rst.txt	2557	0644	edit dl rm
pkgutil.rst.txt	8730	0644	edit dl rm
platform.rst.txt	9624	0644	edit dl rm
plistlib.rst.txt	7398	0644	edit dl rm
poplib.rst.txt	8197	0644	edit dl rm
posix.rst.txt	3694	0644	edit dl rm
pprint.rst.txt	14274	0644	edit dl rm
profile.rst.txt	27969	0644	edit dl rm
pty.rst.txt	3123	0644	edit dl rm
pwd.rst.txt	2739	0644	edit dl rm
pyclbr.rst.txt	3297	0644	edit dl rm
pydoc.rst.txt	4688	0644	edit dl rm
pyexpat.rst.txt	28620	0644	edit dl rm
python.rst.txt	475	0644	edit dl rm
py_compile.rst.txt	3851	0644	edit dl rm
queue.rst.txt	7268	0644	edit dl rm
quopri.rst.txt	2571	0644	edit dl rm
random.rst.txt	18503	0644	edit dl rm
re.rst.txt	64936	0644	edit dl rm
readline.rst.txt	11943	0644	edit dl rm
reprlib.rst.txt	5162	0644	edit dl rm
resource.rst.txt	12291	0644	edit dl rm
rlcompleter.rst.txt	2294	0644	edit dl rm
runpy.rst.txt	8285	0644	edit dl rm
sched.rst.txt	4842	0644	edit dl rm
secrets.rst.txt	5930	0644	edit dl rm
select.rst.txt	28175	0644	edit dl rm
selectors.rst.txt	8925	0644	edit dl rm
shelve.rst.txt	8390	0644	edit dl rm
shlex.rst.txt	16441	0644	edit dl rm
shutil.rst.txt	26014	0644	edit dl rm
signal.rst.txt	17218	0644	edit dl rm
site.rst.txt	9628	0644	edit dl rm
smtpd.rst.txt	10806	0644	edit dl rm
smtplib.rst.txt	23439	0644	edit dl rm
sndhdr.rst.txt	1994	0644	edit dl rm
socket.rst.txt	68202	0644	edit dl rm
socketserver.rst.txt	23712	0644	edit dl rm
spwd.rst.txt	2958	0644	edit dl rm
sqlite3.rst.txt	39215	0644	edit dl rm
ssl.rst.txt	96021	0644	edit dl rm
stat.rst.txt	9805	0644	edit dl rm
statistics.rst.txt	15301	0644	edit dl rm
stdtypes.rst.txt	179240	0644	edit dl rm
string.rst.txt	35374	0644	edit dl rm
stringprep.rst.txt	4279	0644	edit dl rm
struct.rst.txt	19581	0644	edit dl rm
subprocess.rst.txt	45793	0644	edit dl rm
sunau.rst.txt	7390	0644	edit dl rm
superseded.rst.txt	258	0644	edit dl rm
symbol.rst.txt	975	0644	edit dl rm
symtable.rst.txt	4964	0644	edit dl rm
sys.rst.txt	58273	0644	edit dl rm
sysconfig.rst.txt	8749	0644	edit dl rm
syslog.rst.txt	4302	0644	edit dl rm
tabnanny.rst.txt	2007	0644	edit dl rm
tarfile.rst.txt	31749	0644	edit dl rm
telnetlib.rst.txt	7899	0644	edit dl rm
tempfile.rst.txt	13857	0644	edit dl rm
termios.rst.txt	3751	0644	edit dl rm
test.rst.txt	25764	0644	edit dl rm
text.rst.txt	584	0644	edit dl rm
textwrap.rst.txt	10519	0644	edit dl rm
threading.rst.txt	39257	0644	edit dl rm
time.rst.txt	32967	0644	edit dl rm
timeit.rst.txt	13000	0644	edit dl rm
tk.rst.txt	1639	0644	edit dl rm
tkinter.rst.txt	33027	0644	edit dl rm
tkinter.scrolledtext.rst.txt	1255	0644	edit dl rm
tkinter.tix.rst.txt	22653	0644	edit dl rm
tkinter.ttk.rst.txt	58489	0644	edit dl rm
token.rst.txt	2607	0644	edit dl rm
tokenize.rst.txt	10002	0644	edit dl rm
trace.rst.txt	6914	0644	edit dl rm
traceback.rst.txt	17854	0644	edit dl rm
tracemalloc.rst.txt	22089	0644	edit dl rm
tty.rst.txt	1097	0644	edit dl rm
turtle.rst.txt	71221	0644	edit dl rm
types.rst.txt	9999	0644	edit dl rm
typing.rst.txt	34332	0644	edit dl rm
undoc.rst.txt	780	0644	edit dl rm
unicodedata.rst.txt	5762	0644	edit dl rm
unittest.mock-examples.rst.txt	46206	0644	edit dl rm
unittest.mock.rst.txt	85991	0644	edit dl rm
unittest.rst.txt	93304	0644	edit dl rm
unix.rst.txt	446	0644	edit dl rm
urllib.error.rst.txt	2199	0644	edit dl rm
urllib.parse.rst.txt	27332	0644	edit dl rm
urllib.request.rst.txt	59360	0644	edit dl rm
urllib.robotparser.rst.txt	2969	0644	edit dl rm
urllib.rst.txt	466	0644	edit dl rm
uu.rst.txt	2387	0644	edit dl rm
uuid.rst.txt	8760	0644	edit dl rm
venv.rst.txt	20621	0644	edit dl rm
warnings.rst.txt	20152	0644	edit dl rm
wave.rst.txt	6870	0644	edit dl rm
weakref.rst.txt	21328	0644	edit dl rm
webbrowser.rst.txt	9783	0644	edit dl rm
windows.rst.txt	272	0644	edit dl rm
winreg.rst.txt	24073	0644	edit dl rm
winsound.rst.txt	5134	0644	edit dl rm
wsgiref.rst.txt	33047	0644	edit dl rm
xdrlib.rst.txt	8078	0644	edit dl rm
xml.dom.minidom.rst.txt	10198	0644	edit dl rm
xml.dom.pulldom.rst.txt	5186	0644	edit dl rm
xml.dom.rst.txt	39507	0644	edit dl rm
xml.etree.elementtree.rst.txt	43871	0644	edit dl rm
xml.rst.txt	6071	0644	edit dl rm
xml.sax.handler.rst.txt	15399	0644	edit dl rm
xml.sax.reader.rst.txt	12133	0644	edit dl rm
xml.sax.rst.txt	7159	0644	edit dl rm
xml.sax.utils.rst.txt	3901	0644	edit dl rm
xmlrpc.client.rst.txt	23071	0644	edit dl rm
xmlrpc.rst.txt	475	0644	edit dl rm
xmlrpc.server.rst.txt	14990	0644	edit dl rm
zipapp.rst.txt	17392	0644	edit dl rm
zipfile.rst.txt	24005	0644	edit dl rm
zipimport.rst.txt	5805	0644	edit dl rm
zlib.rst.txt	13912	0644	edit dl rm
_dummy_thread.rst.txt	762	0644	edit dl rm
_thread.rst.txt	6886	0644	edit dl rm
__future__.rst.txt	5245	0644	edit dl rm
__main__.rst.txt	904	0644	edit dl rm

Edit: /usr/share/doc/python3-docs/html/_sources/library/cgi.rst.txt (22650B)

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

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

**Source code:** :source:`Lib/cgi.py`

.. 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 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="/path/to/logdir")

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. If the form
contains non-ASCII characters, use the *encoding* keyword parameter set to the
value of the encoding defined for the document. It is usually contained in the
META tag in the HEAD section of the HTML document or by the
:mailheader:`Content-Type` header).  This reads the form contents from the
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.
It allows membership testing with the :keyword:`in` operator, and also supports
the standard dictionary method :meth:`~dict.keys` and the built-in function
:func:`len`.  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 "name" not in form or "addr" not 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:`~FieldStorage.value` attribute of the instance yields
the string value of the field.  The :meth:`~FieldStorage.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 :meth:`~FieldStorage.getlist` method, 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:`~FieldStorage.value` attribute or the :meth:`~FieldStorage.getvalue`
method reads the entire file in memory as bytes.  This may not be what you
want.  You can test for an uploaded file by testing either the
:attr:`~FieldStorage.filename` attribute or the :attr:`~FieldStorage.file`
attribute.  You can then read the data from the :attr:`!file`
attribute before it is automatically closed as part of the garbage collection of
the :class:`FieldStorage` instance
(the :func:`~io.RawIOBase.read` and :func:`~io.IOBase.readline` methods will
return bytes)::

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

:class:`FieldStorage` objects also support being used in a :keyword:`with`
statement, which will automatically close them when done.

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:`~FieldStorage.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.

.. versionchanged:: 3.4
   The :attr:`~FieldStorage.file` attribute is automatically closed upon the
   garbage collection of the creating :class:`FieldStorage` instance.

.. versionchanged:: 3.5
   Added support for the context management protocol to the
   :class:`FieldStorage` class.

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::

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:`~str.upper` 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:`~FieldStorage.getfirst`
and :meth:`~FieldStorage.getlist` provided by this higher level interface.

.. method:: 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. [#]_  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=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 :func:`urllib.parse.parse_qs` unchanged.

.. function:: parse_qs(qs, keep_blank_values=False, strict_parsing=False)

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

.. function:: parse_qsl(qs, keep_blank_values=False, strict_parsing=False)

This function is deprecated in this module. Use :func:`urllib.parse.parse_qsl`
   instead. It is maintained here only for backward compatibility.

.. 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:`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
   :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=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 delimited by double quotes, as in ``<a href="...">``.  Note
   that single quotes are never translated.

.. deprecated:: 3.2
      This function is unsafe because *quote* is false by default, and therefore
      deprecated.  Use :func:`html.escape` 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 ``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 (: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:

.. code-block:: none

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 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 :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.

:mod:`cgi` --- Common Gateway Interface support =============================================== .. module:: cgi :synopsis: Helpers for running Python scripts via the Common Gateway Interface. **Source code:** :source:`Lib/cgi.py` .. 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 ``

`` or ```` 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("CGI script output") print("

This is my first CGI script

") print("Hello, world!") .. _using-the-cgi-module: 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="/path/to/logdir") 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. If the form contains non-ASCII characters, use the *encoding* keyword parameter set to the value of the encoding defined for the document. It is usually contained in the META tag in the HEAD section of the HTML document or by the :mailheader:`Content-Type` header). This reads the form contents from the 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. It allows membership testing with the :keyword:`in` operator, and also supports the standard dictionary method :meth:`~dict.keys` and the built-in function :func:`len`. 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 "name" not in form or "addr" not in form: print("

Error

") print("Please fill in the name and addr fields.") return print("

name:", form["name"].value) print("

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:`~FieldStorage.value` attribute of the instance yields the string value of the field. The :meth:`~FieldStorage.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 :meth:`~FieldStorage.getlist` method, 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:`~FieldStorage.value` attribute or the :meth:`~FieldStorage.getvalue` method reads the entire file in memory as bytes. This may not be what you want. You can test for an uploaded file by testing either the :attr:`~FieldStorage.filename` attribute or the :attr:`~FieldStorage.file` attribute. You can then read the data from the :attr:`!file` attribute before it is automatically closed as part of the garbage collection of the :class:`FieldStorage` instance (the :func:`~io.RawIOBase.read` and :func:`~io.IOBase.readline` methods will return bytes):: 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 :class:`FieldStorage` objects also support being used in a :keyword:`with` statement, which will automatically close them when done. 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:`~FieldStorage.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. .. versionchanged:: 3.4 The :attr:`~FieldStorage.file` attribute is automatically closed upon the garbage collection of the creating :class:`FieldStorage` instance. .. versionchanged:: 3.5 Added support for the context management protocol to the :class:`FieldStorage` class. 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:: 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:`~str.upper` 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:`~FieldStorage.getfirst` and :meth:`~FieldStorage.getlist` provided by this higher level interface. .. method:: 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. [#]_ 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=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 :func:`urllib.parse.parse_qs` unchanged. .. function:: parse_qs(qs, keep_blank_values=False, strict_parsing=False) This function is deprecated in this module. Use :func:`urllib.parse.parse_qs` instead. It is maintained here only for backward compatibility. .. function:: parse_qsl(qs, keep_blank_values=False, strict_parsing=False) This function is deprecated in this module. Use :func:`urllib.parse.parse_qsl` instead. It is maintained here only for backward compatibility. .. 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:`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 :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=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 delimited by double quotes, as in ````. Note that single quotes are never translated. .. deprecated:: 3.2 This function is unsafe because *quote* is false by default, and therefore deprecated. Use :func:`html.escape` 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 ``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 (: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: .. code-block:: none 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 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 :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.