S Anand

After watching Bret Victor’s Inventing on Principle, I just had to figure out a way of getting live reloading to work. I know about LiveReload, of course, and everything I’ve heard about it is good. But their Windows version is in alpha, and I’m not about to experiment just yet.

This little script does it for me instead:

(function(interval, location) {
  var lastdate = "";
  function updateIfChanged() {
    var req = new XMLHttpRequest();
    req.open('HEAD', location.href, false);
    req.send(null);
    var date = req.getResponseHeader('Last-Modified');
    if (!lastdate) {
      lastdate = date;
    }
    else if (lastdate != date) {
      location.reload();;
    }
  };
  setInterval(updateIfChanged, interval);
})(300, window.location)

It checks the current page every 300 milliseconds and reloads it if the Last-Modified header is changed. I usually include it as a minified script:

<script>(function(d,c){var b="";setInterval(function(){var a=new
XMLHttpRequest;a.open("HEAD",c.href,false);a.send(null);
a=a.getResponseHeader("Last-Modified");if(b)b!=a&&
c.reload();else b=a},d)})(300,window.location)</script>

There are no dependencies on any library, like jQuery. However, it requires that the file be on a web server. (It’s easy to fix that, but since I always run a local webserver, I’ll let you solve that problem yourself.)

Here’s the easiest way to set up a Windows XP virtual machine that I could find.

(This is useful if you want to try out programs without installing it on your main machine; test your code on a new machine; or test your website on IE6 / IE7 / IE8.)

1. Go to the Virtual PC download site. (I tried VirtualBox and VMWare Player. Virtual PC is better if you’re running Windows on Windows.)
   
   If you have Windows 7 Starter or Home, select “Don’t need XP Mode and want VPC only? Download Windows Virtual PC without Windows XP Mode.”
   If you have Windows Vista or Windows 7, select “Looking for Virtual PC 2007?”
2. Download it. (You may have to jump through a few hoops like activation.)
3. Download Windows XP and run it to extract the files. (It’s a 400MB download.)
4. Open the “Windows XP.vmc” file – just double-clicking ought to work. At this point, you have a working Windows XP version. (The Administrator password is “Password1”.)
5. Under Tools – Settings – Networking – Adapter 1, select “Shared Networking (NAT)”
   

That’s pretty much it. You’ve got a Windows XP machine running inside your other Windows machine.

Update (18 Sep 2012): I noticed something weird. The memory usage of VMWindow and vpc.exe is tiny!

Between the two processes, they take up less than 30MB of memory. This is despite the Windows XP Task Manager inside the virtual machine showing me 170MB of usage. I’ve no clue what’s happening, but am beginning to enjoy virtualisation. I’ll start up a few more machines, and perhaps install a database cluster across them.

Lisp users would laugh, since they have macros, but Python supports some basic code inspection and modification.

Consider the following pieces of code:

margin = lambda v: 1 - v['cost'] / v['sales']

What if you wanted another function that lists all the dictionary indices used in the function? That is, you wanted to extract cost and sales?

This is a real-life problem I encountered this morning. I have 100 functions, each defining a metric. For example,

1. lambda v: v['X'] + v['Y']
2. lambda v: v['X'] - v['Z']
3. lambda v: (v['X'] + v['Y']) / v['Z']
4. …

I had to plot the functions, as well as each of the corresponding elements (‘X’, ‘Y’ and ‘Z’) in the formula.

Two options. One: along with each formula, maintain a list of the elements used. Two: figure it out automatically.

Each function has a func_code attribute. So, when you take

margin = lambda v: 1 - v['cost'] / v['sales']

margin.func_code is a “code object”. This has a bunch of interesting attributes, one of which is co_consts

>>> margin.func_code.co_consts
(None, 1, 'cost', 'sales')

There — I just pick the strings out of that list and we’re done (for simple functions at least.)

Check out http://docs.python.org/reference/datamodel.html and search for func_ — you’ll find a number of interesting things you can do with functions, such as

1. Finding and changing the default parameters
2. Accessing the global variables of the namespace where the function was defined (!)
3. Replacing the function code with new code

Also search for co_ — you’ll find some even more interesting things you can do with the code:

1. Find all local variable names
2. Find all constants used in the code
3. Find the filename and line number where the code was compiled from

Python also comes with a disassembly module dis. A look at its source is instructive.

Google’s Protocol Buffers is a “language-neutral, platform-neutral, extensible mechanism for serializing structured data – think XML, but smaller, faster, and simpler”

XML is slow and large. There’s no doubting that. JSON’s my default alternative, though it’s a bit large. CSV’s ideal for tabular data, but ragged hierarchies are a bit difficult.

I was trying to see if Protocol Buffers would be smaller and faster, at least when using Python. I took JSON as the base, and checked the write speed, read speed and file sizes. Here’s the comparison:

Protocol Buffers are 17 times slower to write and almost 6 times slower to read than JSON files. File sizes are smaller, but then, all it takes is a simple gzip operation to compress the JSON files even smaller. Reading json.gz files is just 2% slower than JSON files, and writing them is only 4 times slower.

The code base is at https://bitbucket.org/sanand0/protobuftest

On the whole, it appears that GZipped JSON files are smaller, faster, and just as simple as Protocol Buffers. What am I missing?

Update: When you add GZipped CSV to the mix, it’s twice as fast as GZipped JSON to read: clearly a huge win. It’s only slightly slower to write, and but compresses a tiny bit more than JSON.