How to send response headers and status from CGI scripts

It is possible to implement support for a Status: code message header that overrides the HTTP status line (first line of HTTP response, e.g. HTTP/1.0 200 OK). This requires:

1. sub-classing CGIHTTPRequestHandler in order to trick it into writing the CGI script's output into a StringIO object instead of directly to a socket.
2. Then, once the CGI script is complete, update the HTTP status line with the value provided in the Status: header.

It's a hack, but it's not too bad and no standard library code needs to be patched.

import BaseHTTPServer
import SimpleHTTPServer

from CGIHTTPServer import CGIHTTPRequestHandler
from cStringIO import StringIO

class BufferedCGIHTTPRequestHandler(CGIHTTPRequestHandler):
    def setup(self):
        """
        Arrange for CGI response to be buffered in a StringIO rather than
        sent directly to the client.
        """
        CGIHTTPRequestHandler.setup(self)
        self.original_wfile = self.wfile
        self.wfile = StringIO()
        # prevent use of os.dup(self.wfile...) forces use of subprocess instead
        self.have_fork = False

    def run_cgi(self):
        """
        Post-process CGI script response before sending to client.
        Override HTTP status line with value of "Status:" header, if set.
        """
        CGIHTTPRequestHandler.run_cgi(self)
        self.wfile.seek(0)
        headers = []
        for line in self.wfile:
            headers.append(line)    # includes new line character
            if line.strip() == '':  # blank line signals end of headers
                body = self.wfile.read()
                break
            elif line.startswith('Status:'):
                # Use status header to override premature HTTP status line.
                # Header format is: "Status: code message"
                status = line.split(':')[1].strip()
                headers[0] = '%s %s' % (self.protocol_version, status)

        self.original_wfile.write(''.join(headers))
        self.original_wfile.write(body)


def test(HandlerClass = BufferedCGIHTTPRequestHandler,
         ServerClass = BaseHTTPServer.HTTPServer):
    SimpleHTTPServer.test(HandlerClass, ServerClass)

if __name__ == '__main__':
    test()

Needless to say, this is probably not the best way to go and you should look at a non-CGIHTTPServer solution, e.g. a micro-framework such as bottle, a proper web-server (from memory, CGIHTTPServer is not recommended for production use), fastcgi, or WSGI - just to name a few options.