Accessing Form Data Values in Python http.server Using CGI Module

Question

I am trying to create a Python web server that takes user input and makes a request to a third-party API. The user input is obtained via a simple form I created.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Test Document</title>
</head>
<body>
    <form action="server.py" method="post">
        <label for="firstname">First Name</label>
        <input type="text" name="firstname" id="firstname"><br>
        <label for="lastname">Last Name</label>
        <input type="text" name="lastname" id="lastname"><br>
        <label for="url">URL</label>
        <input type="text" name="url" id="application-url"><br>
        <input type="submit" value="Submit">
    </form>
</body>
</html>

I named this file index.html. My server.py file looks like this:

import cgi
from http.server import HTTPServer, CGIHTTPRequestHandler

class TestServerHandler(CGIHTTPRequestHandler):
    def do_POST(self):
        if self.path == '/':
            self.path = './index.html'

        try:
            form = cgi.FieldStorage()
            firstname = form.getvalue('firstname')
            lastname = form.getvalue('lastname')
            url = form.getvalue('url')
            print(firstname + ' ' + lastname + ' ' + url)
            output=firstname + lastname + url
        except: 
            self.send_error(404, 'Bad request submitted.')

        self.end_headers()
        self.wfile.write(bytes(output, 'utf-8'))


test_server = HTTPServer(('localhost', 8080), TestServerHandler)
test_server.serve_forever()

I then run the server.py file on my terminal and go to the http://localhost:8080/ url in my browser. However, when I submit the form I get an error in the browser. The terminal output shows an error that says 'cannot concatenate a 'str' with a 'nonetype'. Based on this error, the form values aren't being passed to this page. Either that or the HTTP server is passing them as query parameters. In any case, would I be able to use the cgi.FieldStorage class inside my HTTP server to access the form field values?

Eddie · Accepted Answer · 2019-06-07 20:25:46Z

After about a week of looking for a solution to this I found that the http.server Python module is not really compatible with the cgi module. The cgi module is used inside of Python scripts that are passed form values from some HTML document on the web server (i.e. a form on the index.html page of a web server with the "action" attribute set to the Python script in question). However, in order for the cgi module to have access to the form values passed to that script (via the cgi.FieldStorage() call), the script must be running inside a web server. The problem with my code example above is that the server.py script I created IS a web server itself. Specifically, it creates an instance of HTTPServer using a custom request handler class I create (TestServerHandler). My custom class subclasses the CGIHTTPRequestHandler class contained in the http.server module. This class contains the Do_POST method. When implementing this method, any form data passed to the Python script is contained inside the self.rfile instance variable. To access this variable and get to the form data I wrote code similar to this.

content_length = int(self.headers['Content-Length'])
data_input = bytes.decode(self.rfile.read(content_length))

After you have the form_data stored in the data_input variable, you can then use a URL parser to access the values you need from the form.

cornelius · Accepted Answer · 2019-11-10 18:17:16Z

2

The FieldStorage class has a constructor defined on this page. It's important parameters are fp, headers, and environ. OP is right in that cgi.FieldStorage() is usually called without parameters in a CGI script, in which case the parameters are filled in for you. However, we can create a FieldStorage object with variables that CGIHTTPRequestHandler provides for us:

form = cgi.FieldStorage(
        fp=self.rfile,
        headers=self.headers,
        environ={
            'REQUEST_METHOD': 'POST',
            'CONTENT_TYPE': self.headers['Content-Type'],
        }

answered Nov 10, 2019 at 18:17

cornelius

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1 Comment

Hkrie Over a year ago

this works like a charm <3 I completely overlooked the possibility of providing the parameters manually.

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