I’ve recently got myself a couple of Arduino Duemilanove boards from Little Bird Electronics. Now the Arduino is a pretty cool bit of kit, and it comes with a great integrated development environment, which makes writing simple little programs an snap. Of course, I’m too much of an old curmudgeon to want to use and IDE and some fancy-schmancy new language, I want make, I want emacs, I want gcc. So I embarked on the task of building a cross-compiler for the AVR board, that will work on OS X.

Now, my ideal solution for this would have been to create a package for Homebrew, my current favourite package manager for OS X. Unfortunately, that just isn’t going to happen right now. GCC toolchains pretty much insist in putting things in $prefix/$target. So, in a standard homebrew install, that would mean I need a directory /usr/local/avr, unfortunately, Homebrew insists your package only dump things in etc, bin, sbin, include, share or lib. Now, after wasting a bunch of time first fighting GCC’s autogoat build system to try and convince it to conform to Homebrew’s idea of a filesystem hierarchy, and then a whole other bunch of time trying to learn Ruby and convince Homebrew that other directories should be allowed, I took the corwads way out and decided /opt/toolchains is a perfectly respectable place for my cross-compilers to live.

And so, the cross compiler dance begins. We start with binutils, and as the code dictates, we start by trying with the latest version, which at time of writing is 2.20.1.

Of course, building a toolchain for a non-x86 based architecture wouldn’t be the same if you didn’t need to patch the source. The patches for the avr are mostly minimal; adding some extra device definitions, for devices you probably don’t have anyway. If you miss this step, expect some pain when you try to compile avr-libc. I got my patches from the FreeBSD AVR binutils patchset. I try to avoid patches I don’t need so have only applied the patch-newdevices patch. If things break for you, you might want to try the other patches as well.

1. $ mkdir avr-toolchain-build
2. $ cd avr-toolchain-build
3. $ wget http://ftp.gnu.org/gnu/binutils/binutils-2.20.1.tar.bz2
4. $ wget "http://www.freebsd.org/cgi/cvsweb.cgi/~checkout~/ports/devel/avr-binutils/files/patch-newdevices?rev=1.16;content-type=text%2Fplain" -O patch-newdevices
5. $ tar jxf binutils-2.20.1.tar.bz2
6. $ cd binutils-2.20.1
7. $ patch -p0 < ../patch-newdevices
8. $ cd ..
9. $ mkdir binutils-build
10. $ cd binutils-build
11. $ ../binutils-2.20.1/configure --target=avr --prefix=/opt/toolchains/ --disable-werror
12. $ make -j2
13. $ make install
14. $ cd ../

Fairly simple, the only gotcha is the --disable-werror, seems that binutils added the -Werror flag to the build, good move!, unfortunately, seems that the code isn’t warning free, so things barf (bad move!), but it comes with a free flag to disable the error... that’s good.

So, now we move on to GCC. Now, since I’m really only interested in compiling C code, (remember the bit about being a curmudgeon, none of this fancy C++ stuff for more, no siree!), we can just grab GCC’s core package.

Now GCC has some dependencies these days, so you need to get gmp, mpc and mpfr installed somehow. I suggest using Homwbrew. In fact it was this step with my old Mac Ports setup that forced me to switch to Homebrew; too many weird library conflicts with iconv between the OS X version and the ports version; but hey, you mileage may vary! (Note: make sure you have the latest version of Homebrew that includes my fix for building mpfr).

And of course, just like binutils, you need some patches as well. Unfortunately things here aren’t as easy. There doesn’t appear to be any semi-official patch for new devices for gcc 4.5.0! So, based on this patch, I managed to hack something together. The formats have changed a little, so I’m not 100% confident that it works, if you are actually trying to use one of the new devices in this patch, be a little skeptical, and double-check my patch. If you are using an existing supported AVR like the atmega328p, then the patch should work fine (well, it works for me).

1. $ brew install gmp libmpc mpfr
2. $ wget http://ftp.gnu.org/gnu/gcc/gcc-4.5.0/gcc-core-4.5.0.tar.bz2
3. $ wget http://benno.id.au/drop/patch-gcc-4.5.0-avr-new-devices
4. $ tar jxf gcc-core-4.5.0.tar.bz2
5. $ cd gcc-core-4.5.0
6. $ patch -p1 < ../patch-gcc-4.5.0-avr-new-devices
7. $ mkdir gcc-build
8. $ cd gcc-build
9. $ ../gcc-4.5.0/configure --target=avr --prefix=/opt/toolchains
10. $ make -j2
11. $ make install
12. $ cd ..

So, the final piece to complete the toolchain is getting a C library. You almost certainly want AVR Libc, or you can be really hard-core and go without a C library at all, your call.

1. $ wget http://mirror.veriportal.com/savannah/avr-libc/avr-libc-1.7.0.tar.bz2
2. $ tar jxf avr-libc-1.7.0.tar.bz2
3. $ mkdir avr-libc-build
4. $ cd avr-libc-build
5. $ ../avr-libc-1.7.0/configure --prefix=/opt/toolchains --host=avr --build=
6. $ make
7. $ make install

This should work with out any problems, however, if you messed up the new devices patches when building GCC and binutils, you might get errors at this point.

So, now we have a working GCC toolchain for the AVR we can start programming. Of course, you really want to have a good reason to do things this way, otherwise I really recommend just using the Arduino development environment..

Next time I’ll be looking at getting some example C programs running on the Arduino.

References

• AVR Libc User Manual

Wow, I can’t believe it has been over two years since I last wrote about Android’s for of the QEMU emulator. Turns out there have been some changes since I last looked at it.

The most important is that the Android emulator no longer has a fixed layout of devices in the physical memory address space. So, while it may have previously been the case that the event device was always at 0xff007000, now it might be at 0xff008000, or 0xff009000, depending on what other devices have been configured for a particular device configuration.

Now, if a device may exist at some random physical address, how does the OS know how to setup the devices drivers? Well, as I’m sure you’ve guessed, the addresses and really random, they are located at page-offset addresses through a restricted range of memory. OK, so how does the OS know what the range is? Well, there is the goldfish_device_bus device.

Basically, this device provides a mechanism to enumerate the devices on the bus. The driver writes PDEV_BUS_OP_INIT to the PDEV_BUS_OP register, the goldfish_device_bus then raises an interrupt. The driver the reads the PDEV_BUS_OP register. Each time the value is PDEV_BUS_OP_ADD_DEV, the driver can read the other registers such as PDEV_BUS_IO_BASE, PDEV_BUS_IO_SIZE, PDEV_BUS_IRQ, to determine the properties of the new device. It continues doing this until it reads a PDEV_BUS_OP_DONE, which indicates the bus scan has finished.

The driver can determine what type of device it has found by writing a pointer to the PDEV_BUS_GET_NAME register. When this happens the device writes an the device’s name (as an ASCII string) to the pointer.

Linux uses these strings to perform device to driver matching as described in the Platform Devices and Drivers document.

JavaScript keeps throwing up interesting new tidbits for me. One that kind of freaked me out the other day is when functions and variables are created in a given block of code.

My expectation was that in a block of code, a sequence of statements, the statements would be executed in order, one after another, just like in most imperative languages. So for example in Python I can do something like:

 def foo(): pass
 print foo

and expect output such as:

 <function foo at 0x100407b18>

However, I would not expect the same output if I wrote the code something like this:

 print foo
 def foo(): pass

and in fact, I don’t. I get something close to:

  NameError: name 'foo' is not defined

Now in JavaScript, by contrast, I can do something like:

 function foo() { }
 console.log(foo);

and the console will dutifully have foo () printed in the log. However, and here comes the fun part, I can also do this:

 console.log(foo);
 function foo() { }

which blows my mind. I guess this really comes in handy when... nope, can’t think of any good reason why this is a useful feature. (I’m sure there must be a good reason, but damned if I can work it out. But this is only where the fun begins. Because the same thing works for variables!

Now usually in Javascript, if you have a function, and write code like:

 function foo() { 
     x = 37;
 }
 foo();
 console.log("x:", x);

You find out you’ve stuffed up, and accidently written to the global object because for some brain-dead reason when you assign to a variable that doesn’t exist in JavaScript it will merrily walk up the scope chain to the global object and plonk it in there. If you do something like:

 function foo() { 
     var x;
     x = 37;
 }
 foo();
 console.log("x:", x);

x will be undefined, and the x = 37 line will update the function’s locally scoped variable, and not mess with the global object. But now comes the part that screws with your head. You can just as easily write this as:

 function foo() { 
     x = 37;
     var x;
 }
 foo();
 console.log("x:", x);

and it will have exactly the same effect. Now it is fairly clear what is happening here; as a function is parsed any variables and functions are created at that time. It turns out though that although variables are created, they are not initialised, so code such as:

 var x = 12;
 function foo() { 
     console.log("x:", x);
     var x = 37;
 }
 foo();

will output x as undefined, (not 37, or 12). Now this behaviour isn’t wrong, or necessarily bad, but it was certainly counter to my expectation and experience in other languages.

Javascript is a very permissive language; you can go messing around of the innards of other classes to your heart’s content. Of course the question is should you?.

Currently I’m playing around with the channel messaging feature in HTML5. In a nutshell, this API lets you create communication MessageChannel, which have two MessagePort object associated with it. When you send a message on port1 an event is triggered on port2 (and vice-versa). You send a message by calling the postMessage method on a port object. E.g:

  port.postMessage("Hello world");

This opens up a range of interesting possibilities for web developers, but this blog post is about software design, not cool HTML5 features.

Unfortunately the postMessage is very new, and the implementation has not yet caught up with the specification. Although you are meant to be able to transfer objects using postMessage currently only strings are supported, and any other objects are coerced into strings. This has an interesting side-effect. If we have code such as:

  port.postMessage({"op" : "test"});

the receiver of the message ends up with the string [Object object], which is mostly useless; actually it is completely useless. So, we want to transfer structured data over a pipe that just supports standard strings, sounds like a job for JSON. So, now my code ends up looking like:

  port.postMessage(JSON.stringify({"op" : "test"});

Now this is all good, but it gets a bit tedious having to type that out every time I want to post a message, so a naïve approach we can simple create a new function, postObject:

 function postObject(port, object) {
   return port.postMessage(JSON.stringify(object));
 }

 postObject(port, {"op" : "test"});

OK, so this works, and it is pretty simple to use, but there aesthetic here that makes this grate just a bit, why do I have to do postObject(port, object), why can’t I do port.postObject(object), that is more “object-oriented”, so, thankfully JavaScript lets us monkey patch objects at run time. So, if we do this:

 port.postObject = function (object) {
   return this.postMessage(JSON.stringify(object));
 }
 
 port.postObject({"op" : "test"});

OK, so far so good. What problems does this have? Well, firstly we are creating a new function for each port object, which isn’t great for either execution time, or memory usage if we have a large number of ports. So instead we could do:

  function postObject(object) {
    return this.postMessage(JSON.stringify(object));
  }
  port.postObject = postObject;

  port.postObject({"op" : "test"});

This works well, except we have two problems. The postObject function ends up as a global function, and if you called it as a global function, the this parameter would be the global, window object, rather than a port object, which would be an easy mistake to make, and difficult to debug. Additionally, we end up with additional per-object data for storing the pointer. Thankfully javascript has a powerful mechanism for solving both the problems: the prototype object (not to be confused with the javascript framework of the same name).

So, if we update the prototype object, instead of object directly, we don’t need to add the function to the global namespace, we avoid per-object memory usage, and we avoid extra code having to remember to set it for every port object:

 MessagePort.prototype.postObject = function postObject(object) {
   return this.postMessage(JSON.stringify(object));
 } 

 port.postObject({"op" : "test"});

Now, this ends up working pretty well. The real question is should be do this? Or rather which of these options should we choose? There is an argument to be made that monkey-patching anything is just plain wrong, and it should be avoided. For example rather than extending the base MessagePort class, we could create a sub-class (Exactly how you create sub-classes in JavaScript is another matter!).

Unfortunately sub-classing doesn’t get us to far, as we are not the ones directly creating the MessagePort instance, the MessageChannel construct does this for us. (I guess we could monkey-patch MessageChannel, but that defeats the purpose of avoiding monkey-patching!).

Of course, another option would be to create a class that encapsulates a port, taking one as a constructor. E.g:

 function MyPort(port) {
   this.port = port;
 }

 MyPort.prototype.postObject = function postObject(object) {
   this.port.postMessage(JSON.stringify(object));
 }

 port = MyPort(port);
 port.postObject({"op" : "test"});

Of course, this means we have to remember to wrap all our port objects in this MyPort class. This is kind of messy (in my opinion). Also we can no longer call the standard port methods. Of course, we could create wrappers for all these methods too, but then things are getting quite verbose, and we are stuffed if it comes to inspecting properties.

Unlike some other object-oriented languages, Javascript provides another alternative, we could change the class (i.e: prototype) of the object at runtime. E.g:

 function MyPort(port) { }
 MyPort.deriveFrom(MessagePort); /* Assume this is how we create sub-classes */

 MyPort.prototype.postObject = function (object) {
    this.portMessage(JSON.stringify(object));
 }

 port.__proto__ = MyPort.prototype; /* Change class at runtime */
 port.postObject({"op" : "test"});

This solves most of the problems of the encapsulted approach, but we still have to remember to adapt the object, and aditionally, __proto__ is a non-standard extension.

OK, so after quickly looking at the sub-classing approaches I think it is fair to discount them. We are still left with trying to determine if any of the monkey-patching approaches is better than a simple function call.

So, there is mostly a consesus out there that monkey patching the base object is verboten, but what about other objects?

Well, if you are in you own code, I think it is a case of anything goes, but what if we are providing reusable code modules for other people? (Of course, even in your own code, there might be libraries that you include that are affected by your overloading). When base objects start working in weird and wonderful ways just because you import a module debugging becomes quite painful. So I think changing the underlying implementation (like the example does when monkey-patching the postMessage method) should probably be avoided.

OK, so now the choices are down to function vs. add a method to the built-in class’s prototype. So if we just add a new global function we could be conflicting with any libraries that also name a global function in the same way. If we add a method to the prototype, at least we are limiting of messing with the namespace to just the MessagePort object; but really, both the options aren’t ideal.

The accepted way to get around this problem is to create a module specific namespace. This reduces the number of potential conflicts. E.g:

 var Benno = {};

 Benno.postObject = function postObject(port, object) {
   return port.postMessage(JSON.stringify(object));
 }

 Benno.postObject(port, {"op" : "test"});

Now, this avoids polluting the global namespace (except for the single Benno object). So, it would have to come out above the prototype extension approach. Now, we should consider if it is possible to play any namespace tricks with the prototype approach. It might be nice to think we could do something like:

  MessagePort.prototype.Benno = {}
  MessagePort.prototype.Benno.postObject = function postObject(object) {
    return this.postMessage(JSON.stringify(object));
  }

  port.Benno.postObject(object);

but this doesn’t work because of the way in which methods and the this object work. this in the function ends up referring to the Benno module object, rather than the MessagePort object.

Even assuming this did work, the function approach has some additional benefits. If the user wants to reduce the typing they can do something like:

   var $B = Benno;
   $B.postObject(port, object);

or even,

  var $P = Benno.postObject;
  $P(port, object);

The other advantage of this scheme is that for someone debugging the code it should be much more obvious where to look for the code and to understand what is happening. If you were reading code and saw Benno.postObject(port, object), it would be much more obvious where the code came from, and where to start looking to debug things.

So, in conclusion, the best approach is also the simplest: just write a function. (But put it in a decent namespace first). Sure instance.operation(args) looks nicer than operation(instance, args), but in the end the ability to namespace the function, along with the advantage of making a clear distinction between built-in and added functionality means that the latter solution wins to day in my eyes.

If you have some other ideas on this I’d love to hear them, so please drop me an e-mail. Thanks to Nicholas for his insights here.

I’m really impressed with the way the HTML5 spec is going, and the fact that it is quickly going to become the default choice for portable application development.

One of the lastest additions to help support application development is the File API. This API enables a developer to gain access to the contents of files locally. The main new data structure that a developer if provided with is a FileList objects which represents an array of File objects. FileList objects can be obtained from two places; input form elements and from drag & drop DataTransfer objects.

Based on this latest API, I’ve created a simple library, JsJpegMeta for parsing Jpeg meta data.

I’ve hacked together a example that demonstrates the library. Just select a JPEG file from the form, or drag a JPEG file onto the window. For large JPEG files you might need to be a little bit patient, as it can be a little slow. This slowness, suprisingly, doesn’t appear to be the Javascript part, but rather Firefox’s handling of large data: URLs and JPEG display in general.

The rest of this post goes into some of the details. Unfortunately only Firefox 3.6 supports these new APIs right now.

Using the File API

Here is an example of how to get access to a FileList. When the user chooses a file, it calls the Javascript function loadFiles. (Assuming you have already defined that function).

  <form id="form" action="javascript:void(0)">
    <p>Choose file: <input type="file" onchange="loadFiles(this.files)" /></p>
  </form>

A File object just provides a reference to a file; to actually get some data out of the file you need to use a FileReader object. The FileReader object provides an asynchronous API for reading the file data into memory. Three different methods are provided by the FileReader object; readAsBinaryString, readAsText and readAsDataURL. A callback, onloadend, is executed when the file has been read into memory, the data is then available via the result field.

Here example of what the loadFiles function might look like:

function loadFiles(files) {
    var binary_reader = new FileReader();

    binary_reader.file = files[0];
    
    binary_reader.onloadend = function() {
        alert("Loaded file: " + this.file.name + " length: " + this.result.length);
    }

    binary_reader.readAsBinaryString(files[0]);
    
    $("form").reset();
}

Note the $("form").reset(); clears the input form.

Drag & Drop

Forms are not the only way to get a FileList, you can also get files from drag and drop event. You need to handle three events; dragenter, dragover and drop.

<body ondragenter="dragEnterHandler(event)" ondragover="dragOverHandler(event)" ondrop="dropHandler(event)">

The default handling of these are fairly striaght forward:

function dragEnterHandler(e) { e.preventDefault(); }
function dragOverHandler(e) { e.preventDefault(); }
function dropHandler(e) {
    e.preventDefault();
    loadFiles(e.dataTransfer.files);
}

Parsing files

The interesting thing here is the readAsBinaryString, when this method is used result ends up being a binary string. This is pretty new because, as far as I know, there hasn’t really been a good way to access binary data in Javascript before. Each character in the binary string represents a byte, and has a character code in the range [0..255].

This is great, because it means that we can parse binary strings locally, without having to upload files to a server for processing. Unfortunately there isn’t a great deal of support for handling binary data in Javacript; there isn’t anything like Python’s struct module.

Luckily it isn’t too hard to write something close to this. Mostly we wanted to parse unsigned and signed integers of arbitrary length. To be useful, we need to handle both little and big endianess. A very simple implementation of parsing an unsigned integer is:

    function parseNum(endian, data, offset, size) {
	var i;
	var ret;
	var big_endian = (endian === ">");
	if (offset === undefined) offset = 0;
	if (size === undefined) size = data.length - offset;
	for (big_endian ? i = offset : i = offset + size - 1; 
	     big_endian ? i < offset + size : i >= offset; 
	     big_endian ? i++ : i--) {
	    ret <<= 8;
	    ret += data.charCodeAt(i);
	}
	return ret;
    }

endian specifies the endianess; the string literal ">" for big-endian and "<" for little-endian. (Copying the Python struct module). data is the binary data to parse. An offset can be specified to enable parsing from the middle of a binary structure; this defaults to zero. The size of the integer to parse can also be specified; it defaults to the remainder of the string.

Signed integers require a little bit more work. Although there are multiple ways of representing signed numbers, by far the most common is the two’s complement method. A function that has the same inputs as parseNum is:

    function parseSnum(endian, data, offset, size) {
	var i;
	var ret;
	var neg;
	var big_endian = (endian === ">");
	if (offset === undefined) offset = 0;
	if (size === undefined) size = data.length - offset;
	for (big_endian ? i = offset : i = offset + size - 1; 
	     big_endian ? i < offset + size : i >= offset; 
	     big_endian ? i++ : i--) {
	    if (neg === undefined) {
		/* Negative if top bit is set */
		neg = (data.charCodeAt(i) & 0x80) === 0x80;
	    }
	    ret <<= 8;
	    /* If it is negative we invert the bits */
	    ret += neg ? ~data.charCodeAt(i) & 0xff: data.charCodeAt(i);
	}
	if (neg) {
	    /* If it is negative we do two's complement */
	    ret += 1;
	    ret *= -1;
	}
	return ret;
    }

JpegMeta API

JpegMeta is a simple, pure Javascript library for parsing Jpeg meta-data. To use it include the jpegmeta.js file. This creates a single, global, module object JpegMeta. The JpegMeta module object has one public interface of use, the JpegFile class. You can use this to construct new JpegFile class instances. The input is a binary string (for example as returned from a FileReader object. An example is:

	var jpeg = new JpegMeta.JpegFile(this.result, this.file.name);

After creation you can then access various meta-data properties, categorised by meta-data groups. The main groups of meta-data are:

general

Information extracted from the JPEG SOF segment. In particular the hieght, width and colour depth.

jfif

Meta-data from the JFIF APP0 segment. This usually includes resolution, aspect ratio and colour space meta-data.

tiff

Generic TIFF meta-data extracted from the Exif meta-data APP1 segment. Includes things such as camera make and model, orientation and date-time.

exif

Exif specific meta-data extracted from the Exif meta-data APP1 segment. Includes camera specific things such as white balance, flash, metering mode, etc.

gps

GPS related information extracted from the Exif meta-data APP1 segment. Includes atitude, longitude etc.

Meta-data groups can be access directly, for example:

 var group = jpeg.gps;

A lookup table is also provided: jpeg.metaGroups. This associative array can be used to determine which meta-groups a particular jpeg file instance actually has.

The MetaGroup object has a name field, a description field and an associative array of properties.

Properties in a given group can be accessed directly. E.g:

 var lat = jpeg.gps.latitude;

Alternatively, the metaProps associative array provides can be used to determine which properties are available.

The metaProp object has a name field, description field, and also a value field.

Conclusion

The File API adds a poweful new capability to native HTML5 applications.

pexif is the python library for editing an image’s EXIF data. Somewhat embarrassingly, the last release I made (0.12) had a really stupid bug in it. This has now been rectified, and a new version (0.13) is now available.

Cool! Someone in Japan seems to be porting OKL4 to the SuperH architecture.

A couple of months ago I posted about my OLPC XO laptop, and how I was looking to give it away to someone who can make good use of it.

As it happens, I’m not the only one in this position, and John came up with the idea of creating an OLPC library that would enable developers to loan out a laptop. So I will be donating my laptop to the new OLPC library.

I’m currently sitting in Melbourne airport waiting for my plan to Hobart. I’m heading off for the best conference in the world, where I’ll be running the Open Mobile Miniconf. There is an awesome set of talks (which I’ll be blogging about shortly).

For a little personal project I’ve been working on recenly, I needed to create some unit tests for a CGI script, sorry, that should be, a cloud computing application. Of course, I turned to my trusty hammer, Python, and was a little disappointed to discover it didn’t quite have the batteries I needed included.

I kind of thought that urllib2 would more or less do what I wanted out of the box, but unfortunately it didn’t and (shock, horror!) I actually needed to write some code myself! The first problem I ran into is that urllib2 only supports GET and POST out of the box. HTTP is constrained enough in the verbs it provides, so I really do want access to things like PUT, DELETE and HEAD.

The other problem I ran into, is that I did’t want things automatically redirecting (although clearly this would be the normal use-case), because I wanted to check I got a redirect in certain cases.

The final problem that I had is that only status code 200 is treated as success, and other 2xx codes raise exceptions. This is generally not what you want, since 201, is a perfectly valid return code, indicating that new resource was created.

So, urllib2 is billed as An extensible library for opening URLs using a variety of protocols, surely I can just extend it to do what I want? Well, it turns out that I can, but it seemed to be harder than I was expecting. Not because the final code I needed to write was difficult or involved, but because it was quite difficult to work out what the right code to write is. I want to explore for a little bit why (I think) this might be the case.

urllib2 is quite nice, because simple things (fetch a URL, follow redirects, POST data), are very easy to do:

ret = urllib2.urlopen("http://some_url/") data = ret.read()

And, it is definitely possible to do more complex things, but (at least for me), there is a sharp discontinuity in the API which means that learning the easy API doesn’t help you learn the more complex API, and also the documentation (at least as far as I read it), doesn’t make it apparent that there are kind of two modes of operation.

The completely frustrating thing is that the documentation in the source file is much better than the online documentation! Since it talks about some of the things that happen in the module, which are otherwise “magic”.

For example, the build_opener function is pretty magical, since it does a bunch of introspection, and ends up either adding a handler or replacing a handler depending on the class. This is explained in the code as: if one of the argument is a subclass of the default handler, the argument will be installed instead of the default. , which to me makes a lot of sense, where-as the online documentation describes it as: Instances of the following classes will be in front of the handlers, unless the handlers contain them, instances of them or subclasses of them: ....<list of default handlers>. For me the former is much clearer than the latter!

Anyway, here is the code I ended up with:

opener = urllib2.OpenerDirector()
opener.add_handler(urllib2.HTTPHandler())

def restopen(url, method=None, headers=None, data=None):
    if headers is None:
        headers = {}
    if method is None:
        if data is None:
            method = "GET"
        else:
            method = "POST"
    return opener.open(HttpRestRequest(url, method=method, 
                                       headers=headers, data=data))

So, conclusions, if the dos don’t make sense, read the code. If you are writing an API, try to make it easier to get from the easy case to the more complex case. (This is quite difficult to do, and I have definitely been guilty in the past of falling into the same trap in APIs I have provided!). If you can’t get the API to easily transition from easy to hard, make sure you document it well. Finally, Python is great language for accessing services over HTTP, even if it does require a little bit of work to get the framework in place.