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Formatting numeric amounts in Javascript

January 16, 2009
1 comment JavaScript

Dear Lazyweb,

Is there a better method than this to format numeric amounts? Here's a solution I picked up from somewhere and slightly modified. It's heavily string based but passed the tests:


function format_amount(i) {
  if(isNaN(i)) { i = 0.00; }
  var minus = '';
  if(i < 0) { minus = '-'; }
  i = Math.abs(i);
  i = parseInt((i + .005) * 100);
  i = i / 100;
  s = new String(i);
  if(s.indexOf('.') < 0) { s += '.00'; }
  if(s.indexOf('.') == (s.length - 2)) { s += '0'; }
  s = minus + s;
  return s;
}

The "tests" are:


format_amount(100)       == "100.00";
format_amount(100.0)     == "100.00";
format_amount(100.05)    == "100.05";
format_amount(100.051)   == "100.05";
format_amount(-100)      == "-100.00";
format_amount(-100.0)    == "-100.00";
format_amount(-123.45)   == "-123.45";
format_amount(-123.450)  == "-123.45";

So functionally it's OK but I'm not sure it's the best way to do it.

bool is instance of int in Python

December 5, 2008
15 comments Python

I lost about half an hour just moments ago debugging this and pulling out a fair amount of hair. I had some code that looked like this:


result = []
for key, value in data.items():
   if isinstance(value, int):
       result.append(dict(name=key, value=value, type='int'))
   elif isinstance(value, float):
       result.append(dict(name=key, value=value, type='float'))
   elif isinstance(value, bool):
       result.append(dict(name=key, type='bool',
                          value=value and 'true' or 'false'))
...

It looked so simple but further up the tree I never got any entries with type="bool" even though I knew there were boolean values in the dictionary.

The pitfall I fell into was this:


>>> isinstance(True, bool)
True
>>> isinstance(False, bool)
True
>>> isinstance(True, int)
True
>>> isinstance(False, int)
True

Not entirely obvious if you ask me. The solution in my case was just to change the order of the if and the elif so that bool is tested first.

domstripper - A lxml.html test project

November 20, 2008
1 comment Python

I'm just playing with the impressive lxml.html package. It makes it possible to easily work with HTML trees and manipulate them.

I had this crazy idea of a "DOM stripper" that removes all but specified elements from an HTML file. For example you want to keep the contents of the <head> tag intact but you just want to keep the <div id="content">...</div> tag thus omitting <div id="banner">...</div> and <div id="nav">...</div>. domstripper now does that. This can be used for example as a naive proxy that tranforms a bloated HTML page into a more stripped down smaller version suitable for say mobile web browsers. It's more a proof of concept that anything else.

To test you just need a virtual python environment and the right system libs to needed to install lxml. This worked for me:


$ sudo apt-get install cython libxslt1-dev zlib1g-dev libxml2-dev
$ cd /tmp
$ virtualenv --no-site-packages testenv
$ cd testenv
$ source bin/activate
$ easy_install domstripper

Now you can use it like this:


>>> from domstripper import domstripper
>>> help(domstripper)
...
>>> domstripper('bloat.html', ['#content', 'h1.header'])
<!DOCTYPE...
...

Best to just play with it and see if makes sense. I'm not saying this is an amazing package but it goes to show what can be done with lxml.html and the extremely user friendly CSS selectors.

The importance of env (and how it works with virtualenv)

September 18, 2008
8 comments Python

I have for a long time wondered why I'm supposed to use this in the top of my executable python files:


#!/usr/bin/env python

Today I figured out why.

The alternative, which you see a lot around is something like this:


#!/usr/bin/python

Here's why it's better to use env rather than the direct path to the executable: virtualenv. Perhaps there are plenty of other reasons the Linux experts can teach me but this is now my first obvious benefit of doing it the way I'm supposed to do it.

If you create a virtualenv, enter it and activate it so that writing:


$ python

starts the python executable of the virtual environment, then this will be respected if you use the env shebang header. Good to know.

The stupidity of 'id' as a variable name (or stupidity of me)

September 16, 2008
3 comments Python

Both in Zope2 and in Django you need to work with attributes called id. This is a shame since it's such a huge pitfall. Despite having done Python programming for so many years I today fell into this pitfall twice!! The pitfall is that id is a builtin function, not a suitable variable name. The reason is that I was changing a complex app to use something called the UUID as the indentifier instead of the ID which happened to be a name of a primary key in a table.

This meant lots of changes and I tested and tested and kept getting really strange errors. I took the whole thing apart and put it back together when I discovered my error of checking if variable id was set or not. id, if undefined, defaults to the builtin function id() which will always return true on bool(id).

It's been a long day. I'm going home. Two newbie mistakes in one programming session. I'm sure I'm not the only one who's been trapped by this.

Python new-style classes and the super() function

July 12, 2008
5 comments Python

I've never really understood the impact of new-style Python classes and what it means to your syntax until now. With new-style classes you can use the super() builtin, otherwise you can't. This works for new-style classes:


class Farm(object):
   def __init__(self): pass

class Barn(Farm):
   def __init__(self):
       super(Barn, self).__init__()

If you want to do the same for old-style classes you simply can't use super() so you'll have to do this:


class Farm:
   def __init__(self): pass

class Barn(Farm):
   def __init__(self):
       Farm.__init__(self)

Strange that I've never realised this before. The reason I did now was that I had to back-port some code into Zope 2.7 which doesn't support setting security on new-style classes.

Now I need to do some reading on new-style classes because clearly I haven't understood it all.

split_search() - A Python functional for advanced search applications

May 15, 2008
0 comments Python

Inspired by Google's way of working I today put together a little script in Python for splitting a search. The idea is that you can search by entering certain keywords followed by a colon like this:


Free Text name:Peter age: 28

And this will be converted into two parts:


'Free Text'
{'name': 'Peter', 'age':'28}

You can configure which keywords should be recognized and to make things simple, you can basically set this to be the columns you have to do advanced search on in your application. For example (from_date,to_date)

Feel free to download and use it as much as you like. You might not agree completely with it's purpose and design so you're allowed to change it as you please.

Here's how to use it:


$ wget https://www.peterbe.com/plog/split_search/split_search.py
$ python
>>> from split_search import split_search
>>> free_text, parameters = split_search('Foo key1:bar', ('key1',))
>>> free_text
'Foo'
>>> parameters
{'key1': 'bar'}

UPDATE

Version 1.3 fixes a bug when all you've entered is one of the keywords.

Lesson learnt with creating DOM element with jQuery

April 4, 2008
6 comments JavaScript

This took me some seriously wasted time to figure out yesterday. What I was trying to do was to create a DOM element of tag type A and insert it into the DOM tree of my page. As I was coding along, everything was working just fine in Firefox but the damn thing wouldn't show up anywhere in IE 6. I debugged and debugged and tried all kinds of different approaches and I just couldn't work it out. Then Karl Rudd gave the right hint on the jQuery mailing list.

Basically, what I was doing was something like this:


var a = $("<a>").attr('href','#').click(somefunction);
$('#toolbar').append(a);

What was then so strange is now less surprising. When I changed the <a> to a <span> it actually worked but just looked wrong with the rest of the site I was working on. Here's the correct way of doing it:


var a = $("<a></a>").attr('href','#').click(somefunction);
$('#toolbar').append(a);

Notice the difference between <a> and <a></a>. The strange thing is that to reproduce this I created this test.html page but here I noticed that in IE 6 it won't let you add any elements that are enclosing ones that are written as singulars. That's really strange since in the same javascript as the above stuff I did a $("<div>") which was working fine. I'll have to get back to figuring out why that one worked nad the A one didn't.

See you at PyCon 2008

March 11, 2008
0 comments Python

I'm going to Chicago on Wednesday for the PyCon 2008 conference. I'm going to stay at the Crowne Plaza (or whatever it was called) like many of the other people at the conference.

This is what I look like:

See you at PyCon 2008

If you see this mug, go up to it and say Hi. It speaks British, Swedish and some American and loves food, beer and tea which might be helpful to know if you would feel like to talk more to it. Its interests for this conference are: Grok, Zope, Django, Plone, buildout, automated testing, agile development and Javascript. Its main claim-to-fame is an Open Source bug/issue tracker program called IssueTrackerProduct which it is more than delighted to talk about.

I've never been to Chicago before and I'm really excited about Tuesday night as I've bought tickets to a Chicago Bulls NBA game (basketball). All other nights I'm hoping to socialise, get drunk, get full and get down and dirty nerdy all week. See you there!

CommandLineApp by Doug Hellmann

February 22, 2008
0 comments Python

I just read the feature article "Command line programs are classes, too!" by Doug Hellmann in the January 2008 issue of Python Magazine about his program CommandLineApp and I've tried it out on one of my old Python programs where I do the opt parsing manually with getopt. The results are beautiful and quick. It's sprinkled with Doug specific magic but I quickly got over that when I saw out easy it was to work with. There are still a few questions of things I didn't manage to work out but that will unfortunately have to wait.

If anything, the worst thing about this library is that it's not part of the standard library so either you have to tell people to sudo easy_install CommandLineApp in the instructions or include it yourself in your packages if you prefer to ship things with a kitchen sink included.

If you want to check it out in action, either subscribe to the magazine (and support the effort) or just download csvcat

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