This is my first CGI script

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name:", form["name"].value) print("

addr:", form["addr"].value) ...further form processing here... Here the fields, accessed through ``form[key]``, are themselves instances of :class:`FieldStorage` (or :class:`MiniFieldStorage`, depending on the form encoding). The :attr:`~FieldStorage.value` attribute of the instance yields the string value of the field. The :meth:`~FieldStorage.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 :class:`FieldStorage` or :class:`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 :meth:`~FieldStorage.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 :attr:`~FieldStorage.value` attribute or the :meth:`~FieldStorage.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 :attr:`~FieldStorage.filename` attribute or the :attr:`~FieldStorage.file` attribute. You can then read the data from the :attr:`!file` attribute before it is automatically closed as part of the garbage collection of the :class:`FieldStorage` instance (the :func:`~io.RawIOBase.read` and :func:`~io.IOBase.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 :class:`FieldStorage` objects also support being used in a :keyword:`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 :attr:`~FieldStorage.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 :mimetype:`multipart/\*` encoding). When this occurs, the item will be a dictionary-like :class:`FieldStorage` item. This can be determined by testing its :attr:`!type` attribute, which should be :mimetype:`multipart/form-data` (or perhaps another MIME type matching :mimetype:`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 :mimetype:`application/x-www-form-urlencoded`), the items will actually be instances of the class :class:`MiniFieldStorage`. In this case, the :attr:`!list`, :attr:`!file`, and :attr:`filename` attributes are always ``None``. A form submitted via POST that also has a query string will contain both :class:`FieldStorage` and :class:`MiniFieldStorage` items. .. versionchanged:: 3.4 The :attr:`~FieldStorage.file` attribute is automatically closed upon the garbage collection of the creating :class:`FieldStorage` instance. .. versionchanged:: 3.5 Added support for the context management protocol to the :class:`FieldStorage` class. Higher Level Interface ---------------------- The previous section explains how to read CGI form data using the :class:`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. .. XXX: Is this true ? 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:: 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 :meth:`~str.upper` method on a list is not valid (since lists do not have a method of this name) and results in an :exc:`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 :meth:`~FieldStorage.getfirst` and :meth:`~FieldStorage.getlist` provided by this higher level interface. .. method:: 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. [#]_ 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. .. method:: 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) .. _functions-in-cgi-module: Functions --------- 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. .. function:: parse(fp=None, environ=os.environ, keep_blank_values=False, strict_parsing=False) Parse a query in the environment or from a file (the file defaults to ``sys.stdin``). The *keep_blank_values* and *strict_parsing* parameters are passed to :func:`urllib.parse.parse_qs` unchanged. .. function:: parse_qs(qs, keep_blank_values=False, strict_parsing=False) This function is deprecated in this module. Use :func:`urllib.parse.parse_qs` instead. It is maintained here only for backward compatibility. .. function:: parse_qsl(qs, keep_blank_values=False, strict_parsing=False) This function is deprecated in this module. Use :func:`urllib.parse.parse_qsl` instead. It is maintained here only for backward compatibility. .. function:: parse_multipart(fp, pdict) Parse input of type :mimetype:`multipart/form-data` (for file uploads). Arguments are *fp* for the input file and *pdict* for a dictionary containing other parameters in the :mailheader:`Content-Type` header. Returns a dictionary just like :func:`urllib.parse.parse_qs` keys are the field names, each value is a list of values for that field. This is easy to use but not much good if you are expecting megabytes to be uploaded --- in that case, use the :class:`FieldStorage` class instead which is much more flexible. Note that this does not parse nested multipart parts --- use :class:`FieldStorage` for that. .. function:: parse_header(string) Parse a MIME header (such as :mailheader:`Content-Type`) into a main value and a dictionary of parameters. .. function:: test() Robust test CGI script, usable as main program. Writes minimal HTTP headers and formats all information provided to the script in HTML form. .. function:: print_environ() Format the shell environment in HTML. .. function:: print_form(form) Format a form in HTML. .. function:: print_directory() Format the current directory in HTML. .. function:: print_environ_usage() Print a list of useful (used by CGI) environment variables in HTML. .. function:: escape(s, quote=False) Convert the characters ``'&'``, ``'<'`` and ``'>'`` in string *s* to HTML-safe sequences. Use this if you need to display text that might contain such characters in HTML. If the optional flag *quote* is true, the quotation mark character (``"``) is also translated; this helps for inclusion in an HTML attribute value delimited by double quotes, as in ````. Note that single quotes are never translated. .. deprecated:: 3.2 This function is unsafe because *quote* is false by default, and therefore deprecated. Use :func:`html.escape` instead. .. _cgi-security: Caring about security --------------------- .. index:: pair: CGI; security There's one important rule: if you invoke an external program (via the :func:`os.system` or :func:`os.popen` functions. or others 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 :file:`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:: #!/usr/local/bin/python 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 ``0o644`` for readable and ``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 (:envvar:`PATH`) or the Python module search path (:envvar:`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:: import sys sys.path.insert(0, "/usr/home/joe/lib/python") sys.path.insert(0, "/usr/local/lib/python") (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 --------------------- .. index:: pair: CGI; debugging 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 (: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 :file:`cgi-bin` directory, it should be possible to send it a request by entering a URL into your browser of the form: .. code-block:: none http://yourhostname/cgi-bin/cgi.py?name=Joe+Blow&addr=At+Home 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 :file:`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 :mod:`cgi` module's :func:`test` function from your script: replace its main code with the single statement :: cgi.test() This should produce the same results as those gotten from installing the :file:`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 :mod:`cgitb` module. If you haven't done so already, just add the lines:: import cgitb cgitb.enable() 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 :mod:`cgitb` module, you can use an even more robust approach (which only uses built-in modules):: import sys sys.stderr = sys.stdout print("Content-Type: text/plain") print() ...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 logfile`` in a separate window may be useful!) * Always check a script for syntax errors first, by doing something like ``python script.py``. * 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 --- :envvar:`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 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 ``suexec`` feature. * 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. .. rubric:: Footnotes .. [#] 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.