Q:

Python: How to unit test a custom HTTP request Handler?

I have a custom HTTP request handler that can be simplified to something like this:
# Python 3:
from http import server

class MyHandler(server.BaseHTTPRequestHandler):
    def do_GET(self):
        self.send_response(200)
        self.send_header("Content-type", "text/html")
        self.end_headers()

        # Here's where all the complicated logic is done to generate HTML.
        # For clarity here, replace with a simple stand-in:
        html = "<html><p>hello world</p></html>"

        self.wfile.write(html.encode())

I'd like to unit-test this handler (i.e. make sure that my do_GET executes without an exception) without actually starting a web server. Is there any lightweight way to mock the SimpleHTTPServer so that I can test this code?

A:

Here's one approach I came up with to mock the server. Note that this should be compatible with both Python 2 and python 3. The only issue is that I can't find a way to access the result of the GET request, but at least the test will catch any exceptions it comes across!
try:
    # Python 2.x
    import BaseHTTPServer as server
    from StringIO import StringIO as IO
except ImportError:
    # Python 3.x
    from http import server
    from io import BytesIO as IO

class MyHandler(server.BaseHTTPRequestHandler):
    """Custom handler to be tested"""
    def do_GET(self):
        # print just to confirm that this method is being called
        print("executing do_GET") # just to confirm...

        self.send_response(200)
        self.send_header("Content-type", "text/html")
        self.end_headers()

        # Here's where all the complicated logic is done to generate HTML.
        # For clarity here, replace with a simple stand-in:
        html = "<html><p>hello world</p></html>"

        self.wfile.write(html.encode())

def test_handler():
    """Test the custom HTTP request handler by mocking a server"""
    class MockRequest(object):
        def makefile(self, *args, **kwargs):
            return IO(b"GET /")

    class MockServer(object):
        def __init__(self, ip_port, Handler):
            handler = Handler(MockRequest(), ip_port, self)

    # The GET request will be sent here
    # and any exceptions will be propagated through.
    server = MockServer(('0.0.0.0', 8888), MyHandler)

test_handler()

A:

Expanding on the answer from jakevdp, I managed to be able to check the output, too:
try:
    import unittest2 as unittest
except ImportError:
    import unittest
try:
    from io import BytesIO as IO
except ImportError:
    from StringIO import StringIO as IO
from server import MyHandlerSSL  # My BaseHTTPRequestHandler child

class TestableHandler(MyHandlerSSL):
    # On Python3, in socketserver.StreamRequestHandler, if this is
    # set it will use makefile() to produce the output stream. Otherwise,
    # it will use socketserver._SocketWriter, and we won't be able to get
    # to the data
    wbufsize = 1

    def finish(self):
        # Do not close self.wfile, so we can read its value
        self.wfile.flush()
        self.rfile.close()

    def date_time_string(self, timestamp=None):
        """ Mocked date time string """
        return 'DATETIME'

    def version_string(self):
        """ mock the server id """
        return 'BaseHTTP/x.x Python/x.x.x'

class MockSocket(object):
    def getsockname(self):
        return ('sockname',)

class MockRequest(object):
    _sock = MockSocket()

    def __init__(self, path):
        self._path = path

    def makefile(self, *args, **kwargs):
        if args[0] == 'rb':
            return IO(b"GET %s HTTP/1.0" % self._path)
        elif args[0] == 'wb':
            return IO(b'')
        else:
            raise ValueError("Unknown file type to make", args, kwargs)

class HTTPRequestHandlerTestCase(unittest.TestCase):
    maxDiff = None

    def _test(self, request):
        handler = TestableHandler(request, (0, 0), None)
        return handler.wfile.getvalue()

    def test_unauthenticated(self):
        self.assertEqual(
                self._test(MockRequest(b'/')),
                b"""HTTP/1.0 401 Unauthorized\r
Server: BaseHTTP/x.x Python/x.x.x\r
Date: DATETIME\r
WWW-Authenticate: Basic realm="MyRealm", charset="UTF-8"\r
Content-type: text/html\r
\r
<html><head><title>Authentication Failed</title></html><body><h1>Authentication Failed</h1><p>Authentication Failed. Authorised Personnel Only.</p></body></html>"""
                )

def main():
    unittest.main()

if __name__ == "__main__":
    main()

The code I am testing returns a 401 Unauthorised for "/". Change the response as appopriate for your test case.

