How to properly install isapi_wsgi on IIS for Python2.7?

Question

I've worked through installing Python as a CGI application on IIS on Windows 7. This is pretty straightforward, but I'd like to use the WSGI stuff, for better flexibility.

I downloaded the archive for isapi_wsgi, unzipped it, and then ran the install as per the instructions, like this:

\python27\python.exe setup.py install

This succeeded:

Then I coded a .py module that had the wsgi glue in it, and tried installing it. This failed like so:

It's a COM Moniker error, and I know that the IIS6-compatible management stuff is based on COM Monikers, which reminded me that there is a pre-req for isapi_wsgi of the IIS6-compatible management stuff. I ran \windows\system32\OptionalFeatures.exe and installed that, then re-ran the .py module and it installed correctly.

C:\dev\wsgi>\Python27\python.exe app1_wsgi.py
Configured Virtual Directory: /wsgi
Installation complete.

Ok, wonderful. Now when I look in the current directory, I see a new DLL named _app1_wsgi.dll, and when I look in IIS Manager I can see a new IIS vdir, and a scriptmap within that vdir for '*', which is mapped to the _app1_wsgi.DLL. All good. But! making a request to http://localhost/wsgi gives me a 500 error.

Through some trial-and-error I see that the .py module that defines my handlers must be in the site-packages directory. I am very surprised by this.

Can I avoid this? Can I simply put the .py module in the same directory as the generated .dll file? Or do I need to deploy all of my python logic to site-packages in order to run it from the WSGI mechanism?

Answer 1

The answer is:

• the installation of isapi_wsgi as described in the question, is correct.

• with the basic boilerplate of app.py as shown in the example code accompanying isapi_wsgi, the python classes for the web app need to be in the site-packages directory.

• it is possible to allow the python source modules to reside in the same directory as with the generated *.dll file, but it requires some special handling in the *wsgi.py file.

• a better way to run python on Windows for development purposes is to simply download the Google App Engine and use the builtin dedicated http server. The framework that comes with the GAE SDK handles reloading and allows the .py modules to be placed in particular directories.

---

If you don't want to download and install the GAE SDK, then you might try the following. Using this code, when a request arrives on isapi_wsgi, the handler looks in the home directory for a py module, and loads it. If the module is already loaded, it checks the file "last modified time" and reloads the module if the last mod time is later than the time from the prior load. It works for simplistic cases but I suppose it will be brittle when there are nested module dependencies.

import sys
import os
import win32file
from win32con import *

# dictionary of [mtime, module] tuple;  uses file path as key
loadedPages = {}

def request_handler(env, start_response):
    '''Demo app from wsgiref'''
    cr = lambda s='': s + '\n'
    if hasattr(sys, "isapidllhandle"):
        h = None
        # get the path of the ISAPI Extension DLL
        hDll = getattr(sys, "isapidllhandle", None)
        import win32api
        dllName = win32api.GetModuleFileName(hDll)
        p1 = repr(dllName).split('?\\\\')
        p2 = p1[1].split('\\\\')
        sep = '\\'
        homedir = sep.join(p2[:-1])

        # the name of the Python module is in the PATH_INFO
        moduleToImport = env['PATH_INFO'].split('/')[1]

        pyFile = homedir + sep + moduleToImport + '.py'

        fd = None
        try:
            fd = win32file.CreateFile(pyFile, GENERIC_READ, FILE_SHARE_DELETE, None, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0)
        except Exception as exc1:
            fd = None

        if fd is not None:
            # file exists, get mtime
            fd.close()
            mt = os.path.getmtime(pyFile)
        else:
            mt = None


        if mt is not None:
            h = None
            if not pyFile in loadedPages:
                # need a new import
                if homedir not in sys.path:
                    sys.path.insert(0, homedir)

                h = __import__(moduleToImport, globals(), locals(), [])
                # remember
                loadedPages[pyFile] = [mt, h]
            else:
                # retrieve handle to module
                h = loadedPages[pyFile][1]
                if mt != loadedPages[pyFile][0]:
                    # need to reload the page
                    reload(h)
                    loadedPages[pyFile][0] = mt

            if h is not None:
                if 'handler' in h.__dict__:
                    for x in h.handler(env, start_response):
                        yield x
                else:
                    start_response("400 Bad Request", [('Content-Type', 'text/html')])
            else:
                start_response("404 Not Found", [('Content-Type', 'text/html')])
                yield cr()
                yield cr("<html><head><title>Module not found</title>" \
                             "</head><body>")
                yield cr("<h3>404 Not Found</h3>")
                yield cr("<h3>No handle</h3></body></html>")

        else:
            start_response("404 Not Found", [('Content-Type', 'text/html')])
            yield cr()
            yield cr("<html><head><title>Module not found</title>" \
                 "</head><body>")
            yield cr("<h3>404 Not Found</h3>")
            yield cr("<h3>That module (" + moduleToImport + ") was not found.</h3></body></html>")


    else:
        start_response("500 Internal Server Error", [('Content-Type', 'text/html')])
        yield cr()
        yield cr("<html><head><title>Server Error</title>" \
                 "</head><body><h1>Server Error - No ISAPI Found</h1></body></html>")


# def test(environ, start_response):
#     '''Simple app as per PEP 333'''
#     status = '200 OK'
#     start_response(status, [('Content-type', 'text/plain')])
#     return ['Hello world from isapi!']


import isapi_wsgi
# The entry point(s) for the ISAPI extension.
def __ExtensionFactory__():
    return isapi_wsgi.ISAPISimpleHandler(request_handler)


def PostInstall(params, options):
    print "The Extension has been installed"


# Handler for our custom 'status' argument.
def status_handler(options, log, arg):
    "Query the status of the ISAPI?"
    print "Everything seems to be fine..."


if __name__=='__main__':
    # This logic gets invoked when the script is run from the command-line.
    # In that case, it installs this module as an ISAPI.

    #
    # The API provided by isapi_wsgi for this is a bit confusing.  There
    # is an ISAPIParameters object. Within that object there is a
    # VirtualDirs property, which itself is a list of
    # VirtualDirParameters objects, one per vdir.  Each vdir has a set
    # of scriptmaps, usually this set of script maps will be a wildcard
    # (*) so that all URLs in the vdir will be served through the ISAPI.
    #
    # To configure a single vdir to serve Python scripts through an
    # ISAPI, create a scriptmap, and stuff it into the
    # VirtualDirParameters object. Specify the vdir path and other
    # things in the VirtualDirParameters object.  Stuff that vdp object
    # into a sequence and set it into the ISAPIParameters thing, then
    # call the vaguely named "HandleCommandLine" function, passing that
    # ISAPIParameters thing.
    #
    # Clear as mud?
    #
    # Seriously, this thing could be so much simpler, if it had
    # reasonable defaults and a reasonable model, but I guess it will
    # work as is.

    from isapi.install import *

    # Setup the virtual directories -
    # To serve from root, set Name="/"
    sm = [ ScriptMapParams(Extension="*", Flags=0) ]
    vdp = VirtualDirParameters(Name="wsgi", # name of vdir/IIS app
                              Description = "ISAPI-WSGI Demo",
                              ScriptMaps = sm,
                              ScriptMapUpdate = "replace"
                              )

    params = ISAPIParameters(PostInstall = PostInstall)
    params.VirtualDirs = [vdp]
    cah = {"status": status_handler}

    # from isapi.install, part of pywin32
    HandleCommandLine(params, custom_arg_handlers = cah)

Using this model, requesting http://foo/wsgi/bar will try loading bar.py from the home directory with the WSGI .dll file. If bar.py cannot be found, you get a 404. If bar.py has been updated since the last run, it reloads. If bar cannot be loaded, you get a 500.

bar.py must export a method called handler, publicly. That method must be a generator. like so:

import time

def handler(env, start_response):
    start_response("200 OK", [('Content-Type', 'text/html')])
    cr = lambda s='': s + '\n'
    yield cr("<html><head><title>Hello world!</title></head><body>")
    yield cr("<h1>Bargle Bargle Bargle</h1>")
    yield cr("<p>From the handler...</p>")
    yield cr("<p>(bargle)</p>")
    yield cr("<p>The time is now: " + time.asctime() + " </p>")
    yield cr("</body></html>")

__all__ = ['handler']

But as I said, I think GAE is probably a better way to develop Python webapps using Windows.