Excerpted from Python official documents

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 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!")

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 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.

The 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 FieldStorage instance.

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()
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 FieldStorage (or MiniFieldStorage, depending on the form encoding). The value attribute of the instance yields the string value of the field. The 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 FieldStorage or 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 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 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 filename attribute or the file attribute. You can then read the data from the file attribute before it is automatically closed as part of the garbage collection of the FieldStorage instance (the read() and 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

FieldStorage objects also support being used in a 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 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 FieldStorage item. 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 list, file, and filename attributes are always None.

A form submitted via POST that also has a query string will contain both FieldStorage and MiniFieldStorage items.

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, for example.

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 upper() method on a list is not valid (since lists do not have a method of this name) and results in an 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 getfirst() and getlist() provided by this higher level interface.

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 None if not specified.

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)

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 urllib.parse.parse_qs() unchanged.

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 FieldStorage class instead which is much more flexible.

cgi.parse_header(string)

Parse a MIME header (such as Content-Type) into a main value and a dictionary of parameters.

cgi.test()

Robust test CGI script, usable as main program. Writes minimal HTTP headers and formats all information provided to the script in HTML format.

cgi.print_environ()

Format the shell environment in HTML.

cgi.print_form(form)

Format a form in HTML.

cgi.print_directory()

Format the current directory in HTML.

cgi.print_environ_usage()

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

There’s one important rule: if you invoke an external program (via os.system(), 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.