cgi.FieldStorage()获取网页间提交的数据《Head First Python》第七章

To get at submitted form data, 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.

FieldStorage

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.

1.1 要获取的“键”是唯一的

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.

如下图所示为第七章中，Generate_list.py向Generating_timing_data.py传递“which_athlete”对应的值的方法：

1.2 要获取的“键”不唯一

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()

function, 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)

1.3 要获取字段代表是要上传的文件

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 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 leisure 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

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.