Category / My Work

by Nirupam Biswas

Django-Select2 version 3.1.1 released

Just released Django-Select2‘s version 3.1.1.

The key changes are:-

  • Updated Select2 Javascript library to versino 3.2. This new version fixes a lot of bugs, along with providing a high resolution icon image, to be used in Retina displays.
  • In my last release (version 3.0.2) I introduced some new template tags but unfortunately I did not update the setup.py and Manifest.in files to include that directory. That directory was missing in the released package. Now it is included and installed.

If you see the Changelog then you will notice that I jumped from version 3.0.2 to 3.1.1. In fact there was a version 3.1.0 in-between, but that was again a faulty package. That package had the templatetags directory but that does not install that. That was again due to some missing code in setup.py.

by Nirupam Biswas

Hack sbicapsec.com to run on Firefox

`AG_BACKQUOTE_TURN_ON`

www.sbicapsec.com is State Bank of India’s online share trading portal. This is a good portal but their site’s code quality is amazingly of low quality, dare I say, crappy. SBI being a public sector government institute, we can rest assured that the code quality or bugs in it would not get fixed soon, if ever.

Anyway as of now if you try using this portal in Firefox or Chrome, then maybe you will endup with a screen like this.

sbicapsec.com after login in Firefox. Notice no applet running, instead we have weird “:10”.

Notice the weird “:10” character in the screenshot, instead of the Java Applet.

Fixing sbicapsec.com to run in Firefox

Don’t worry the “hack” here is not illegal. This is used merely as an expression for making things work the way you want it which requires a lot of technical trickery to get it right.

The following has been tested in Firefox 13 and 14 in Mac OSX and Windows XP. You are free to try this as you may want it though. Also this should work in Chrome too.

Now let’s get into fixing this in few simple steps:-

  1. For your sake, open this blog page in your Firefox browser.
  2. Install GreaseMonkey browser extension. You may need to restart your browser.
  3. Install my GreaseMonkey user script by clicking on the link here – AppVersion Patch for SBICAPSEC.
  4. Click on Install button in the dialog box that you get.

That is it! You should now get the applet after login.

Fixed sbicapsec.com, after login.

How the fix works (for the technically inclined)

This section is for geeks, who would like to know how the script works. Also it is better to understand this, so that you understand, there is no malicious code in my script.

What is wrong in sbicapsec.com’s site? (Root cause)

The portal’s JS code assumes that `navigator.appVersion` will always return a string which will have a semi-colon (;). In fact that is true for IE and Chrome (in Mac OSX version only), but not for others. After this failure everything goes down like dominoes.

The popup we get after login has a `frameset` with three frames. The second one is the one which is supposed to present the Java applet. The layout of that page is roughly as below:-

[code lang=”html”]
<body>
…
<applet>
<param>
<param>
<param>
…
</applet>
</body>
[/code]

It seems the devs there had a requirement to set the `width` and `height` of the `applet` based on user screen’s dimension. For this they modified the code to use JS to dynamically generate the upper `applet` tag.

[code lang=”html”]
<body>
…
<script>
// This script will generate the upper applet tag with appropriate width and height.
</script>
<param>
<param>
:10
<param>
…
</applet>
</body>
[/code]

It is in the above `script` tag where it reads `navigator.appVersion` and tries to split it by `;` and then read the other part. When that errors out, so does the code following it. That code is supposed to write the upper `applet` tag. So, at the end we are left with many `param` tags and a dangling `</applet>` tag. The weird `:10` characters are written between two `param` tags.

From the way they have written the JS code, it seems the devs did not visualize the html page as a tree of blocks, instead for them it was a file stream; like the kind when you use your Java or C++ file output stream. The `script` block above uses `document.write()` to write the opening `applet` tag, instead of using JS to directly manipulate the DOM objects. I wonder how these devs can layout a page, who visualize it as a stream of characters? And, this is just a fraction of the real code, who knows what else is inside. It seems SBI needs to seriously train its devs. All this makes me loose faith over the security and reliability of their site.

Anyway, enough of the rant, back to the topic. The patch script I wrote will, simply try to do what there code was meant to do, add the `applet` tag. Because of the dangling `applet` end tag, I was unable to wrap my `applet` around the existing `param` tags. So, instead I detached all the `param` tags, emptied the parent (this reference was stored before detaching `param`), then added `param` tags inside the newly create `applet` tag and finally added the `applet` inside the previous parent of `param` tags.

I also tried using other techniques but they did not work. The first one was directly modifying `navigator.appVersion` to return a string with `;`, but it seems you cannot modify them. The second options was to replace existing `String.split()` function with my own version. In my version I would always return an array of at least length two, so that the code does not error out. Anyway this too did not work out since GreaseMonkey scripts are ran after the page is executed. GreaseMonkey does provide an option to run our scripts at the beginning too, but according to docs that is not supported inside frames.

Anyway, all’s well that ends well. 🙂

by Nirupam Biswas

SplitBill: IOU manager for and by Geeks

There are hundreds of IOU software on the net. What makes SplitBill different is that it has no UI. Yes, you read that right – No UI. Now the more surprising part. It is a breeze to use and setup. In fact all you need is a sever with Python and MySql installed to host this, and then simply run the code after setting up the DB and it is ready. You might be wondering now, how do we access the system? Easy; just send a mail to a pre-configured mail box. Suppose John owes you 100 bucks then simply send a mail to SplitBill with John in CC or To fields. In subject write Loan 100 For Pizza. Now just send the mail. You are done. Moments later SplitBill will reply to your mail, confirming that your transaction has been recorded. So, SplitBill is like a command-line program, where your mail box is the Terminal window. We already have mail clients for pretty much every device hooked to the net. So, SplitBill is accessible to you from practically from everywhere. When you want to checkout who owes you or you owe whom, simply mail to SplitBill with Get in the mail’s subject. The response mail will have all the summary along with list of all last hundred transactions.

SplitBill Architecture

SplitBill Architecture

Because of the above design, SplitBill is inherently scalable. Many users can simultaneously send mails to SplitBill’s mail box without affecting it at all. It polls the mail server every five seconds (configurable) via IMAP connection and processes the mails in the order they were received. This way spikes in requests are eased out at mail server end. Mail server acts like a very large buffer for SplitBill system. Also if for some reason SplitBill crashes, the users can still file transaction, which will get processed when it is back online.

SplitBill System Design Assumptions

SplitBill is a simple product. To keep it simple to use, and light, it has only basic abuse protection facility. For example, suppose you filed a transaction that John owes you 100 bucks. Now John can command SplitBill to simply delete that transaction. SplitBill will honour that and will purge that transaction without the possibility for a rollback. However, when John is notified that his request has been fulfilled, you too will be CCed in that response mail. From the response mail you will come to know what exact transaction has been deleted. You can then use that info to refile the transaction, and give a nice little trashing to John. SplitBill has no mechanism for explicitly registering users. Whenever you send a mail to SplitBill, it will quietly register all new mail ids in it.

SplitBill recognizes a person by there mail id, so if you happen to own multiple mail ids then each one would be treated as separate user. Currently there is no way to combine them together. This is something which can be addressed in future.

SplitBill Error Handling

In spite of its simplicity it does not trade off reliability. Most of the stuffs you can checkout in its code, but there is one part you need to be aware of. When it errors out five consecutive times (configurable), then it auto shuts down. This is because if on consecutive tries if the command fails then it is assumed that something bad, like a DB crash, has happened which might take quite sometime or forever to be fixed.

It does not mark a mail as read unless and until the transaction has been successfully recorded in the database. However, if it is unable to mark the mail as read (even after retrying), then it shuts itself down to prevent recording the same transaction again. Unfortunately currently the mail UID is not recorded in database, preventing it from identifying if the current transaction has already been processed.

Whenever it shuts itself down, it sends mail to all registered owners, notifying the problem. The owners also receive mails with stack traces when they happen. Me and my friends have been using this for couple of months over two years now, and we have hardly faced any issues till now.

SplitBill Command Reference

There is already a separate page on this. Please see it here.

Get SplitBill

Get or fork SplitBill on GitHub – https://github.com/applegrew/splitbill.

Setting up SplitBill

  1. Make sure you have Python 2.6 or up installed on your server.
  2. Download and extract the splitbill sub-folder into your server.
  3. Install pytz package.
  4. Install MySql and MySql connector for Python.
  5. Run setup_db.sql script to setup your database. If you  want your schema to be named something other than splitbill, then modify this sql file.
  6. In splitbill folder you will find settings.ini. Set the DB and mail credentials and other settings as required.
  7. Finally! Run SplitBill. You can run it from the parent folder of splitbill using command nohup python -m splitbill.main &.

A Little History

SplitBill was envisioned not by me, but my friend Rohit. He designed the original system and coded that in C#. Later when we wanted to move the system to my (Linux) server, I decided to recode it in Python. This version of SplitBill is only couple of months a year old and was coded by both of us. We had used the original version for almost a year, during which time the SplitBill architecture evolved to its current state.

by Nirupam Biswas

Django-Select2: Select2 for Django

Select2

Select2 is an excellent Javascript framework which transforms mundane <select> fields to cool looking and searchable. This is a very handy when there are quite a number of options to select from.

Basic Select2 options field.

Select2 also allows dynamic fetching of options from server via Ajax. Select2’s webpage has a neat demo of this.

Select2 fetching data via Ajax. In the above screenshot it is using RottenTomatoes’ API to get them.

Django-Select2

Django includes basic select widget, which just generates <select><option>...</option>...</select> tags.  Although their ‘looks’ can be improved using basic CSS, but we hit a usability problem when there are too many options to select from. This is where Django-Select2 comes into picture.

Light Components

Django-Select2 includes many widgets suited to various use-cases. Select2Widget and Select2MultipleWidget widgets are suited for scenarios where we have a static list of choices which may not may not be large. They are not meant to be used when the options are too many, say, in thousands. This is because all those options would have to be pre-rendered onto the page and Javascript would be used to search through them. Said that, they are also one the most easiest to use. They are almost drop-in-replacement for Django’s default select widgets, and they look much much better.

Heavy Components

HeavySelect2Widget and HeavySelect2MultipleWidget widgets are suited for scenarios when the number of options are large and need complex queries (from maybe different data sources) to get the options. This dynamic fetching of options undoubtably requires Ajax communication with the server. Django-Select2 includes a helper JS file which is included automatically, so you need not worry about writing any Ajax related JS code. Although on the server side you do need to create a view specifically to respond to the queries. The format of the response is decided by the JS code being used on the client side. The included abstract view – Select2View, will make sure to format the response into the format expected by the helper JS code. Below is a example on how to use it.

#!python
from django.db.models import Q
from django_select2 import Select2View, NO_ERR_RESP
from .models import Employee

class EmployeeSelect2View(Select2View):
    def check_all_permissions(self, request, *args, **kwargs):
        user = request.user if not (user.is_authenticated() and user.has_perms('emp.view_employees')):
            raise PermissionDenied

    def get_results(self, request, term, page, context):
        emps = Employee.objects.filter( Q(first_name__icontains=term) | Q(last_name__icontains=term) | Q(emp_no__icontains=term))
        res = [ (emp.id, "%s %s" % (emp.first_name, emp.last_name),) for emp in emps ]
        return (NO_ERR_RESP, False, res) # Any error response, Has more results, options list

How many such views you will need depends totally on your use-case. From Django-Select2 there is no restriction on their reuse. If you feel that writing these views are too much of a hassle then you have an alternate option – sub-class AutoSelect2Field field. In your sub-classed field you need to override security_check(self, request, *args, **kwargs) and get_results(self, request, term, page, context) methods. When your field will be instantiated for the first time, it will register its own instance with AutoResponseView. When the related field is used in the browser, the queries would be directed to AutoResponseView which will direct it to your ‘auto’ field instance. For ‘auto’ fields to work you must use the following code in your urls.py to register the url for AutoResponseView.

#!python
urlpatterns += patterns("", url(r"^select2/", include("django_select2.urls")), )

Django-Select2 Fields

The following fields are available in Django-Select2.

  • Select2ChoiceField – Uses Select2Widget.
  • Select2MultipleChoiceField – Uses Select2MultipleWidget.
  • HeavySelect2ChoiceField – Uses HeavySelect2Widget.
  • HeavySelect2MultipleChoiceField – Uses HeavySelect2MultipleWidget.
  • ModelSelect2Field – Uses Select2ChoiceField. It additionally requires queryset argument. It similar to Django’s ModelChoiceField.
  • AutoSelect2Field – Uses HeavySelect2ChoiceField. Auto register’s itself with AutoResponseView.
  • AutoModelSelect2Field – Similar to AutoSelect2Field, but like ModelSelect2Field, normalizes values to Django model objects.

Download Django-Select2

You can download it or fork it from https://github.com/applegrew/django-select2. You can also add this to you pip requirement files as:-

-e git+https://github.com/applegrew/django-select2.git#egg=django-select2

Update: Now can simply add django_select2 to your pip requirement. If you want to install it manually then you can simply run:-

pip install django_select2

Update: Now you can install beta version of django_select2 compatible with Python3.

pip install Django-Select2-Py3

Closing Statement

It is recommended that you go through the codes to familiarize yourself with how to efficiently use Django-Select2. The code is not very complex so you should not face much problem in understanding it.