Mysterious 404 responses with foursquare OAuth2 + google-app-engine

I’ve done a lot of work on my aforementioned foursquare / last.fm mashup project, which I’m still not exactly ready to start crowing about, but whose source is, as ever, available.

I’m currently hitting a bunch of problems in getting my Python code to work while it’s deployed out to Google’s appspot.com production environments.  I’ve been able to authenticate successfully against foursquare from my local environment, following their straightforward instructions, but every time I try to run the same code out in Google’s cloud, once I hit the point where I retrieve an access code from foursquare and make a request for a legit session token, this is what I wind up with:

    HTTP blarg, feh
    Traceback (most recent call last):
    File "/base/python27_runtime/python27_lib/versions/third_party/webapp2-2.3/webapp2.py", line 545, in dispatch
      return method(*args, **kwargs)
    File "/base/data/home/apps/s~how-you-been/0-2-4.357616581640073459/whence.py", line 97, in get
      accessCode = self.getFoursquareAccessToken(code)
    File "/base/data/home/apps/s~how-you-been/0-2-4.357616581640073459/howyoubeen/Foursquare.py", line 49, in getFoursquareAccessToken
      httpResponse = urllib2.urlopen(url)
    File "/base/python27_runtime/python27_dist/lib/python2.7/urllib2.py", line 126, in urlopen
      return _opener.open(url, data, timeout)
    File "/base/python27_runtime/python27_dist/lib/python2.7/urllib2.py", line 400, in open
      response = meth(req, response)
    File "/base/python27_runtime/python27_dist/lib/python2.7/urllib2.py", line 513, in http_response
      'http', request, response, code, msg, hdrs)
    File "/base/python27_runtime/python27_dist/lib/python2.7/urllib2.py", line 438, in error
      return self._call_chain(*args)
    File "/base/python27_runtime/python27_dist/lib/python2.7/urllib2.py", line 372, in _call_chain
      result = func(*args)
    File "/base/python27_runtime/python27_dist/lib/python2.7/urllib2.py", line 521, in http_error_default
      raise HTTPError(req.get_full_url(), code, msg, hdrs, fp)

Dang it!  My initial instinct is that I’m hitting a timeout in Google’s cloud environment, but are they honestly passing a 404 back to my stack at that point?  (I suspect, as an alternate explanation, something about my error-handling handlers are deficient.)

While looking into this I’ve stumbled upon a few Python OAuth2 libraries, about which OAuth2 isn’t exactly rocket science, but if the libraries have a decent interface and can overcome this issue, maybe I’ll give them a shake.  (On the js side of the world I’ve been having a few similar issues, where I’d like to move to an open-source library but the ones I’ve seen make odd assumptions about the execution environment.)

Another possibility I’ve considered is that there’s some sort of SSL wildcard issue at play, as suggested by this stack overflow answer which foursquare has bravely elected as their replacement for tech support.  But for now, it is a mystery.

Generating weighted-probability lists in Javascript

js-weighted-list test page screenshot

I’ve made a good deal of progress on my Google App Engine application, about which more later.  During the course of it, I started to keenly feel the need for a weighted random list.  That is, I have a list and want to select a random sample from it, but it has an unequal (weighted) probability distribution; I want some of the items to be selected more frequently than others.  I also need to modify the weights depending on what else is going on in the program (eg, in response to user input or web service results.  Also, this is all happening on the front-end, so it needs to work in javascript.

Poking around for a while I couldn’t find anything obviously relevant to this, but I did find a pretty good breakdown of the problem and the algorithm to follow in this Stack Overflow answer.  This implementation makes use of a heap in order to enable a binary search on the probability space.  While digging through to understand it, I wound up reimplementing the thing in Javascript, and then made a bunch of improvements to the API for it.  It’s pretty basic and doesn’t have any particular dependencies, so I’m releasing it as its own library, js-weighted-list.

It is available on github under the MIT license.  There are still several things I need to improve with it, in particular it’s severely under-tested, but it’s decently documented and immediately usable for what I need it to do.

Using pyjade with webapp2 on Google App Engine

And now for something completely different. I got a little distracted again by an idea for a simple social networking mashup I’ve had and decided that the best place to deploy it would probably on Google App Engine.  App Engine supports Scala and I’ve seen one or two positive reports about using Scalatra on it, so I thought it wouldn’t be too hard to spin something up quickly using my existing code.

It turned out not to be as easy as that, unfortunately; I may try to elaborate on the reasons in a later post, but it seems to come down to Scalate always checking the filesystem for its template routines.  In any event, I found that I was spending a lot of time trying to glue weird bits of infrastructure to one another rather than working on my idea.

With that in mind, I decided to start over in Python, and of course as my first task I spent a long time glueing random bits of infrastructure together.  In this case I’ve been successful, though, so here’s a brief writeup on getting Jade templates to work with Google App Engine and Google’s webapp2 in python.

1. Add dependencies to include webapp2’s jinja2 module in your project.

This is as simple as adding this bit of code to app.yaml:

libraries:
- name: jinja2
  version: latest

2. Install pyjade in your environment.

I’ve been using a layout similar to the one mentioned in this Stack Overflow answer.  It’s puzzling that Google doesn’t really mention anything about a standard way to get libraries and stuff from the wider Python ecosystem installed in a development environment, but essentially I set up a virtualenv, ran pip install pyjade, and then symlinked the pyjade directory from the virtualenv lib directory into the project’s src directory.

3. Add a custom Jinja2 factory.

This is where the magic happens.  We write a factory function, jinja2_factory, whose purpose is to add pyjade’s included Jinja2 extension into the Jinja2 instance’s Environment.

  def jade_factory(app):
    j = jinja2.Jinja2(app)
    j.environment.add_extension('pyjade.ext.jinja.PyJadeExtension')
    return j

Then, in our handler’s jinja2 method, we pass along the factory method to it:

  @webapp2.cached_property
  def jinja2(self):
    return jinja2.get_jinja2(app=self.app, factory=jade_factory)

(This is based on moraes’s Stack Overflow answer here.)

4. Use the factory and get a template.

Here’s the full source code for a simple “hello world” app. I’m following the examples on Google’s jinja2 page and creating a JadeHandler subclass of RequestHandler.

import os
import webapp2
from webapp2_extras import jinja2

class JadeHandler(webapp2.RequestHandler):
  # Per http://stackoverflow.com/a/7081653/87990
  @staticmethod
  def jade_factory(app):
    j = jinja2.Jinja2(app)
    j.environment.add_extension('pyjade.ext.jinja.PyJadeExtension')
    return j
  
  @webapp2.cached_property
  def jinja2(self):
    return jinja2.get_jinja2(app=self.app, factory=JadeHandler.jade_factory)

  def render_response(self, _template, **context):
    # Renders a template and writes the result to the response.
    rv = self.jinja2.render_template(_template, **context)
    self.response.write(rv)

class MainPage(JadeHandler):
  def get(self):
    context = {'message': 'Hello, world!'}
    self.render_response('index.jade', **context)

app = webapp2.WSGIApplication([('/', MainPage)], debug=True)

Caveat

I’m by no means an expert in Google App Engine, and I’m a little worried that there’s something badly inefficient or similarly wrong about this approach. In particular, I’m hoping the returned templates are cached, but I’m not certain, and I’ve seen reports that Jinja2 Environment creation is an expensive operation on GAE. More to the point, pyjade seems to work as a preprocessor for Jinja2, so it would obviously be better not to need to run it for every request. In any event, this does at least find jade templates and render them properly as HTML.

Edit: as usual, the current code is available on GitHub.

The cake falls

(I’m running out of snappy cake-related puns here, but I’m saving “the cake is a lie” for a special occasion.)

So after thinking about my interface / implementation dilemma for a while (recap: I’m aiming to separate out the implementations of hexmap from their interface), I decided to approach it from a Java vantage point and see how I’d implement the same thing in Java.  The immediately obvious thing that jumped out at me was that the specific problem I was dealing with was sort of a classic application of plain old vanilla inheritance from an abstract class.  Having drunk deeply from the well of Spring over my last several years of Java experience, plain old inheritance had begun to seem in some ways like a quaint relic of a bygone era (having been replaced in many cases by service composition).

At any rate, a quick bit of refactoring to get rid of traits altogether got me back to a fairly sane codebase, where I’ve got this:

abstract class HexMap (h: Int, w: Int) {
  val height = h
  val width = w
  // This is where the magic happens
  def data(addr: Address): HexData
}
abstract class HexData(a: Address) {
  def toJson(): JObject = JObject(List()) // default: empty JSON object
}

case class ColorData(a: Address, color: String) extends HexData (a) {
  override def toJson(): JObject = {
    ("color" -> color)  // json {"color": "foo"} via JsonDSL
  }
}
class RandomColorMap(h: Int, w: Int) extends HexMap(h, w) {
  private val store: Map[Address, ColorData] = createRandomMap()
  def data(addr: Address): ColorData = {
    store(addr)
  }
  // Note, we're able to access width and height here
  private def createRandomMap(): Map[Address, ColorData] = {
    val pairs = for (x <- 0 until width; y <- 0 until height) yield {
      val addr = Address(x, y)
      addr -> ColorData(addr, randomColor())
    }
    pairs toMap
  }
}

I will come back and revisit this later; I’m certain there’s a way traits could be useful in this code, but for now I’m leaving them aside and pressing on with more actual program functionality.

As far as that is concerned, my next worry is persisting the hexmap the user sees in their Scalatra session, so that when the user asks for a random path from the server, he or she gets one within the actual bounds of the actual map he or she is seeing. The Scalatra session docs make this look fairly straightforward; we’ll see.

More delicious cake

I’ve made some progress on both the client and server sides of the hexmap project.  On the client side, there’s now some code to highlight tiles (though it’s still imperfect).  I’ve also updated the map representation some, so it looks like this:

[ {"x":1, "y":1, "data":{"color": "yellow"}},
  {"x":1, "y":2, "data":{"color":"white"}},
  {"x":2, "y":1, "data":{"color":"white"}},
  {"x":2, "y":2, "data":{"color":"lightgreen"}} ]

The idea is that the data object will expand to have more game or application-specific data.  I’m not 100% sure that moving to a sparse representation (versus a 2d matrix) is a good idea, but I don’t think it will hurt me all that much, and it shouldn’t be too difficult to change back if I need to.

On the Scala side of the fence, I’m using the lift-json library to perform serialization to JSON.  I’m still not sure I’ve structured this in a way that really makes sense in the long-term.  One thing I’m hoping to achieve with this project is to wind up with a fairly abstract, generic hexmap library which can be augmented by application-specific code to implement a particular game or the like.  So I’m trying to partition off some of the specific stuff, such as in this case tile colors, into its own conceptual space, but I’m not sure I’m doing it idiomatically.

In particular, I think I’m hitting a slight impedance mismatch between Java and Scala revolving around traits, self-type declarations, and the Cake Pattern versus interfaces and Spring-style dependency injection.  I’d like to separate out the interface of a hexmap from the implementation used to actually implement the hexmap, so what I have is this:

class HexMap (h: Int, w: Int) {
  // Every instance of a HexMap will be mixed in with a HexStore
  self: HexStore =>

  val height = h
  val width = w

  def hex(x: Int, y:Int): Hex // Return data for a specific cell
}

And then HexMapStore implementations derive from this:

abstract class HexData(a: Address) {
  def toJson(): JValue // return a json representation of this data
}
abstract trait HexStore {
  def data(addr: Address): HexData  // Get the specific data from addr
}

My idea for this was that there might be one implementation which uses a MongoDB store, one which uses an in-memory map, etc. The fundamental operation, though, is to pass in a grid address and retrieve the data for that cell. The actual implementation I’ve got now is this:

case class ColorData(a: Address, color: String) extends HexData (a) {
  def toJson(): JValue = {
    ("color" -> color)  // json {"color": "foo"} via JsonDSL
  }
}
trait RandomHexMapStore extends HexStore {
  private lazy val store: Map[Address, ColorData] = createRandomMap()
  def data(addr: Address): ColorData = {
    store(addr)
  }
}

I think where I’m hitting problems most is trying to figure out how exactly the actual instantiation of “a HexMap with a RandomHexMapStore mixed in” should take place, or in other words where the definition of the specific component assembly I’m creating should appear.  In Spring this would be an object graph defined either in a (probably big, crazy, and unwieldy) XML file or via annotations on various class declarations.

What I’ve been trying for starters is just to define a factory method which returns a new random map:

object RandomHexMapStore {
   def create(height: Int, width: Int): HexMap = {
     new HexMap(height, width) with RandomHexMapStore
  }
}

This is working OK for now, and it makes sense to me that different implementations of HexMapStore might have different constructor parameters, so I’m not too bothered by factory methods in general. I’m a little worried by how this approach might or might not scale to much larger class / object graphs, though.

Moreover, so far I’ve been glossing over the most serious immediate problem I have with this: the implementation of the RandomHexMapStore.createRandomMap() method. This method is meant to construct a map composed of randomly selected color elements. The problem is that, being a trait which is mixed in to HexMap, is has no access to HexMap’s height and width class parameters. What I have at the moment is this:

  private def createRandomMap(): Map[Address, ColorData] = {
    // Well, this isn't ideal
    val pairs = for (i <- 1 to 6; j <- 1 to 6) 
                yield Address(i,j) -> 
                      new ColorData(Address(i,j), randomColor())
    pairs toMap
  }

I’m a little baffled by how to get access to class parameters from a trait which is mixed in to that class. When I think about it, I can’t really see a good reason for Scala to allow it, since, after all, RandomHexMapStore could be mixed into any class, not just a descendant of HexMap. This suggests to me that I am probably not using the proper language mechanism for what I’m trying to do. I will have to do some thinking on this, and possibly call on Stack Overflow for help.