11.1.2 Using the cgi module

Begin by writing "import cgi". Do not 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 defined 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 "<H1>Error</H1>"
    print "Please fill in the name and addr fields."
    return
...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 FieldStorage or MiniFieldStorage instance but a list of such instances. If you expect 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:
        if usernames:
            # Next item -- insert comma
            usernames = usernames + "," + item.value
        else:
            # First item -- don't insert comma
            usernames = item.value
else:
    # 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 attribute:

fileitem = form["userfile"]
if fileitem.file:
    # It's an uploaded file; count lines
    linecount = 0
    while 1:
        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 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.