cgi (version 2.2)
Support module for CGI (Common Gateway Interface) scripts.
This module defines a number of utilities for use by CGI scripts
written in Python.
A CGI script is invoked by an HTTP server, usually to process user
input submitted through an HTML <FORM> or <ISINPUT> element.
Most often, CGI scripts live in the server's special cgi-bin
directory. The HTTP server places all sorts of information about the
request (such as the client's hostname, the requested URL, the query
string, and lots of other goodies) in the script's shell environment,
executes the script, and sends the script's output back to the client.
The script's input is connected to the client too, and sometimes the
form data is read this way; at other times the form data is passed via
the "query string" part of the URL. This module (cgi.py) is intended
to take care of the different cases and provide a simpler interface to
the Python script. It also provides a number of utilities that help
in debugging scripts, and the latest addition is support for file
uploads from a form (if your browser supports it -- Grail 0.3 and
Netscape 2.0 do).
The output of a CGI script should consist of two sections, separated
by a blank line. The first section contains a number of headers,
telling the client what kind of data is following. Python code to
generate a minimal header section looks like this:
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!"
It may not be fully legal HTML according to the letter of the
standard, but any browser will understand it.
Using the cgi module
Begin by writing "import cgi". Don't 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.
It's best to use the FieldStorage class. The other classes define in this
module are provided mostly for backward compatibility. Instantiate it
exactly once, without arguments. This reads the form contents from
standard input or the environment (depending on the value of various
environment variables set according to the CGI standard). Since it may
consume standard input, it should be instantiated only once.
The FieldStorage instance can be accessed as if it were a Python
dictionary. For instance, the following code (which assumes that the
Content-type header and blank line have already been printed) checks that
the fields "name" and "addr" are both set to a non-empty string:
form = cgi.FieldStorage()
form_ok = 0
if form.has_key("name") and form.has_key("addr"):
if form["name"].value != "" and form["addr"].value != "":
form_ok = 1
if not form_ok:
print "Please fill in the name and addr fields."
...further form processing here...
Here the fields, accessed through form[key], are themselves instances
of FieldStorage (or MiniFieldStorage, depending on the form encoding).
If the submitted form data contains more than one field with the same
name, the object retrieved by form[key] is not a (Mini)FieldStorage
instance but a list of such instances. If you are expecting this
possibility (i.e., when your HTML form comtains multiple fields with
the same name), use the type() function to determine whether you have
a single instance or a list of instances. For example, here's code
that concatenates any number of username fields, separated by commas:
username = form["username"]
if type(username) is type():
# Multiple username fields specified
usernames = ""
for item in username:
# Next item -- insert comma
usernames = usernames + "," + item.value
# First item -- don't insert comma
usernames = item.value
# Single username field specified
usernames = username.value
If a field represents an uploaded file, the value attribute reads the
entire file in memory as a string. This may not be what you want. You can
test for an uploaded file by testing either the filename attribute or the
file attribute. You can then read the data at leasure from the file
fileitem = form["userfile"]
# It's an uploaded file; count lines
linecount = 0
line = fileitem.file.readline()
if not line: break
linecount = linecount + 1
The file upload draft standard entertains the possibility of uploading
multiple files from one field (using a recursive multipart/*
encoding). When this occurs, the item will be a dictionary-like
FieldStorage item. This can be determined by testing its type
attribute, which should have the value "multipart/form-data" (or
perhaps another string beginning with "multipart/"). It this case, it
can be iterated over recursively just like the top-level form object.
When a form is submitted in the "old" format (as the query string or as a
single data part of type application/x-www-form-urlencoded), the items
will actually be instances of the class MiniFieldStorage. In this case,
the list, file and filename attributes are always None.
These classes, present in earlier versions of the cgi module, are still
supported for backward compatibility. New applications should use the
SvFormContentDict: single value form content as dictionary; assumes each
field name occurs in the form only once.
FormContentDict: multiple value form content as dictionary (the form
items are lists of values). Useful if your form contains multiple
fields with the same name.
Other classes (FormContent, InterpFormContentDict) are present for
backwards compatibility with really old applications only. If you still
use these and would be inconvenienced when they disappeared from a next
version of this module, drop me a note.
These are useful if you want more control, or if you want to employ
some of the algorithms implemented in this module in other
parse(fp, [environ, [keep_blank_values, [strict_parsing]]]): parse a
form into a Python dictionary.
parse_qs(qs, [keep_blank_values, [strict_parsing]]): parse a query
string (data of type 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 vales for each name.
parse_qsl(qs, [keep_blank_values, [strict_parsing]]): parse a query
string (data of type application/x-www-form-urlencoded). Data are
returned as a list of (name, value) pairs.
parse_multipart(fp, pdict): parse input of type multipart/form-data (for
parse_header(string): parse a header like Content-type into a main
value and a dictionary of parameters.
test(): complete test program.
print_environ(): format the shell environment in HTML.
print_form(form): format a form in HTML.
print_environ_usage(): print a list of useful environment variables in
escape(): convert the characters "&", "<" and ">" to HTML-safe
sequences. Use this if you need to display text that might contain
such characters in HTML. To translate URLs for inclusion in the HREF
attribute of an <A> tag, use urllib.quote().
log(fmt, ...): write a line to a log file; see docs for initlog().
Caring about security
There's one important rule: if you invoke an external program (e.g.
via the os.system() or os.popen() functions), make very sure you don't
pass arbitrary strings received from the client to the shell. This is
a well-known security hole whereby clever hackers anywhere on the web
can exploit a gullible CGI script to invoke arbitrary shell commands.
Even parts of the URL or field names cannot be trusted, since the
request doesn't have to come from your form!
To be on the safe side, if you must pass a string gotten from a form
to a shell command, you should make sure the string contains only
alphanumeric characters, dashes, underscores, and periods.
Installing your CGI script on a Unix system
Read the documentation for your HTTP server and check with your local
system administrator to find the directory where CGI scripts should be
installed; usually this is in a directory cgi-bin in the server tree.
Make sure that your script is readable and executable by "others"; the
Unix file mode should be 755 (use "chmod 755 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:
Make sure the Python interpreter exists and is executable by "others".
Note that it's probably not a good idea to use #! /usr/bin/env python
here, since the Python interpreter may not be on the default path
given to CGI scripts!!!
Make sure that any files your script needs to read or write are
readable or writable, respectively, by "others" -- their mode should
be 644 for readable and 666 for writable. This is because, for
security reasons, the HTTP server executes your script as user
"nobody", without any special privileges. It can only read (write,
execute) files that everybody can read (write, execute). The current
directory at execution time is also different (it is usually the
server's cgi-bin directory) and the set of environment variables is
also different from what you get at login. in particular, don't count
on the shell's search path for executables ($PATH) or the Python
module search path ($PYTHONPATH) to be set to anything interesting.
If you need to load modules from a directory which is not on Python's
default module search path, you can change the path in your script,
before importing other modules, e.g.:
This way, the directory inserted last will be searched first!
Instructions for non-Unix systems will vary; check your HTTP server's
documentation (it will usually have a section on CGI scripts).
Testing your CGI script
Unfortunately, a CGI script will generally not run when you try it
from the command line, and a script that works perfectly from the
command line may fail mysteriously when run from the server. There's
one reason why you should still test your script from the command
line: if it contains a syntax error, the python interpreter won't
execute it at all, and the HTTP server will most likely send a cryptic
error to the client.
Assuming your script has no syntax errors, yet it does not work, you
have no choice but to read the next section:
Debugging CGI scripts
First of all, check for trivial installation errors -- reading the
section above on installing your CGI script carefully can save you a
lot of time. If you wonder whether you have understood the
installation procedure correctly, try installing a copy of this module
file (cgi.py) as a CGI script. When invoked as a script, the file
will dump its environment and the contents of the form in HTML form.
Give it the right mode etc, and send it a request. If it's installed
in the standard cgi-bin directory, it should be possible to send it a
request by entering a URL into your browser of the form:
If this gives an error of type 404, the server cannot find the script
-- perhaps you need to install it in a different directory. If it
gives another error (e.g. 500), there's an installation problem that
you should fix before trying to go any further. If you get a nicely
formatted listing of the environment and form content (in this
example, the fields should be listed as "addr" with value "At Home"
and "name" with value "Joe Blow"), the cgi.py script has been
installed correctly. If you follow the same procedure for your own
script, you should now be able to debug it.
The next step could be to call the cgi module's test() function from
your script: replace its main code with the single statement
This should produce the same results as those gotten from installing
the cgi.py file itself.
When an ordinary Python script raises an unhandled exception (e.g.,
because of a typo in a module name, a file that can't be opened,
etc.), the Python interpreter prints a nice traceback and exits.
While the Python interpreter will still do this when your CGI script
raises an exception, most likely the traceback will end up in one of
the HTTP server's log file, or be discarded altogether.
Fortunately, once you have managed to get your script to execute
*some* code, it is easy to catch exceptions and cause a traceback to
be printed. The test() function below in this module is an example.
Here are the rules:
1. Import the traceback module (before entering the
2. Make sure you finish printing the headers and the blank
3. Assign sys.stderr to sys.stdout
3. Wrap all remaining code in a try-except statement
4. In the except clause, call traceback.print_exc()
print "Content-type: text/html"
sys.stderr = sys.stdout
...your code here...
Notes: The assignment to sys.stderr is needed because the traceback
prints to sys.stderr. The print "
<PRE>" statement is necessary to
disable the word wrapping in HTML.
If you suspect that there may be a problem in importing the traceback
module, you can use an even more robust approach (which only uses
sys.stderr = sys.stdout
print "Content-type: text/plain"
...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
When all else fails, you may want to insert calls to log() to your
program or even to a copy of the cgi.py file. Note that this requires
you to set cgi.logfile to the name of a world-writable file before the
first call to log() is made!
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".
- When using any of the debugging techniques, don't forget to add
"import sys" to the top of the script.
- When invoking external programs, make sure they can be found.
Usually, this means using absolute path names -- $PATH is usually not
set to a very useful value in a CGI script.
- When reading or writing external files, make sure they can be read
or written by every user on the system.
- 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.
|__file__ = '/usr/lib/python1.6/cgi.pyc'|
__name__ = 'cgi'
__version__ = '2.2'
logfile = ''
logfp = None
maxlen = 0