Source code: Lib/cgi.py
Support module for Common Gateway Interface (CGI) 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
Most often, CGI scripts live in the server’s special
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>")
Using the cgi module#
Begin by writing
When you write a new script, consider adding these lines:
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:
It’s very helpful to use this feature during script development. The reports
cgitb provide information that can save you a lot of time in
tracking down bugs. You can always remove the
cgitb line later when you
have tested your script and are confident that it works correctly.
To get at submitted form data, use the
FieldStorage class. 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
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.
FieldStorage instance can be indexed like a Python dictionary.
It allows membership testing with the
in operator, and also supports
the standard dictionary method
keys() and the built-in function
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
For instance, the following code (which assumes that the
Content-Type header and blank line have already been printed)
checks that the fields
addr are both set to a non-empty
form = cgi.FieldStorage()
if "name" not in form or "addr" not in form:
print("Please fill in the name and addr fields.")
...further form processing here...
Here the fields, accessed through
form[key], are themselves instances of
MiniFieldStorage, depending on the form
value attribute of the instance yields
the string value of the field. The
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
MiniFieldStorage instance but a list of such instances. Similarly, in
form.getvalue(key) would return a list of strings. If you
expect this possibility (when your HTML form contains multiple fields with the
same name), use the
getlist() 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
value attribute or the
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
filename attribute or the
attribute. You can then read the data from the
attribute before it is automatically closed as part of the garbage collection of
readline() methods will
fileitem = form["userfile"]
# It's an uploaded file; count lines
linecount = 0
line = fileitem.file.readline()
if not line: break
linecount = linecount + 1
FieldStorage objects also support being used in a
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
done attribute of the
object for the field will be set to the value -1.
The file upload draft standard entertains the possibility of uploading multiple
files from one field (using a recursive multipart/* encoding).
When this occurs, the item will be a dictionary-like
This can be determined by testing its
type attribute, which should be
multipart/form-data (or perhaps another MIME type matching
multipart/*). In this case, it can be iterated over recursively
just like the top-level form object.
When a form is submitted in the “old” format (as the query string or as a single
data part of type application/x-www-form-urlencoded), the items will
actually be instances of the class
MiniFieldStorage. In this case, the
filename attributes are always
A form submitted via POST that also has a query string will contain both
Changed in version 3.4: The
file attribute is automatically closed upon the
garbage collection of the creating
Changed in version 3.5: Added support for the context management protocol to the
Higher Level Interface#
The previous section explains how to read CGI form data using the
FieldStorage class. This section describes a higher level interface
which was added to this class to allow one to do it in a more readable and
intuitive way. The interface doesn’t make the techniques described in previous
sections obsolete — they are still useful to process file uploads efficiently,
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.
# 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
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
upper() method on a list is not valid
(since lists do not have a method of this name) and results in an
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.
- FieldStorage.getfirst(name, default=None)#
This method always returns only one value associated with form field name. The method returns only the first value in case that more values were posted under such name. Please note that the order in which the values are received may vary from browser to browser and should not be counted on. 1 If no such form field or value exists then the method returns the value specified by the optional parameter default. This parameter defaults to
Noneif not specified.
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:
form = cgi.FieldStorage()
user = form.getfirst("user", "").upper() # This way it's safe.
for item in form.getlist("item"):
These are useful if you want more control, or if you want to employ some of the algorithms implemented in this module in other circumstances.
- cgi.parse(fp=None, environ=os.environ, keep_blank_values=False, strict_parsing=False, separator='&')#
Parse a query in the environment or from a file (the file defaults to
sys.stdin). The keep_blank_values, strict_parsing and separator parameters are passed to
- cgi.parse_multipart(fp, pdict, encoding='utf-8', errors='replace', separator='&')#
Parse input of type multipart/form-data (for file uploads). Arguments are fp for the input file, pdict for a dictionary containing other parameters in the Content-Type header, and encoding, the request encoding.
Returns a dictionary just like
urllib.parse.parse_qs(): keys are the field names, each value is a list of values for that field. For non-file fields, the value is a list of strings.
This is easy to use but not much good if you are expecting megabytes to be uploaded — in that case, use the
FieldStorageclass instead which is much more flexible.
Changed in version 3.7: Added the encoding and errors parameters. For non-file fields, the value is now a list of strings, not bytes.
Changed in version 3.10: Added the separator parameter.
Parse a MIME header (such as Content-Type) into a main value and a dictionary of parameters.
Robust test CGI script, usable as main program. Writes minimal HTTP headers and formats all information provided to the script in HTML format.
Format the shell environment in HTML.
Format a form in HTML.
Format the current directory in HTML.
Print a list of useful (used by CGI) environment variables in HTML.
Caring about security#
There’s one important rule: if you invoke an external program (via
os.popen() or other functions with similar
functionality), make very sure you don’t pass arbitrary strings received from
the client to the shell. This is a well-known security hole whereby clever
hackers anywhere on the web can exploit a gullible CGI script to invoke
arbitrary shell commands. Even parts of the URL or field names cannot be
trusted, since the request doesn’t have to come from your form!
To be on the safe side, if you must pass a string gotten from a form to a shell command, you should make sure the string contains only alphanumeric characters, dashes, underscores, and periods.
Installing your CGI script on a Unix system#
Read the documentation for your HTTP server and check with your local system
administrator to find the directory where CGI scripts should be installed;
usually this is in a directory
cgi-bin in the server tree.
Make sure that your script is readable and executable by “others”; the Unix file
mode should be
0o755 octal (use
chmod 0755 filename). Make sure that the
first line of the script contains
#! starting in column 1 followed by the
pathname of the Python interpreter, for instance:
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
0o666 for writable. This is because, for security reasons, the
HTTP server executes your script as user “nobody”, without any special
privileges. It can only read (write, execute) files that everybody can read
(write, execute). The current directory at execution time is also different (it
is usually the server’s cgi-bin directory) and the set of environment variables
is also different from what you get when you log in. In particular, don’t count
on the shell’s search path for executables (
PATH) or the Python module
search path (
PYTHONPATH) to be set to anything interesting.
If you need to load modules from a directory which is not on Python’s default module search path, you can change the path in your script, before importing other modules. For example:
(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 format. 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,
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.
This should produce the same results as those gotten from installing the
cgi.py file itself.
When an ordinary Python script raises an unhandled exception (for whatever reason: of a typo in a module name, a file that can’t be opened, etc.), the Python interpreter prints a nice traceback and exits. While the Python interpreter will still do this when your CGI script raises an exception, most likely the traceback will end up in one of the HTTP server’s log files, or be discarded altogether.
Fortunately, once you have managed to get your script to execute some code,
you can easily send tracebacks to the web browser using the
If you haven’t done so already, just add the lines:
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
you can use an even more robust approach (which only uses built-in modules):
sys.stderr = sys.stdout
...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 logfilein a separate window may be useful!)
Always check a script for syntax errors first, by doing something like
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 —
PATHis 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
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.
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.