Author: masnun

Categories
Clojure

Web Scraping with Clojure

I have recently started learning Clojure and I must say I am totally hooked. Clojure is a sane Lisp on the JVM. So I can express myself better while being able to take advantage of the huge JVM eco system. This is just wonderful!

After finishing the popular book Clojure for the Brave and True, I wanted to try something out myself. I decided to try web scraping with clojure. In this post, I am going to walk you through a very simple web scraping task.

We’re going to scrape KAT (Kick Ass Torrents) for TV series. To keep things simple, we would scrape the first page of the TV section and print out the titles. The reason I like KAT is they serve the response gzipped – if your http client can’t handle their response, you probably want to switch.

We will use the following libraries for the task:

  • http-kit
  • Enlive

Most tutorials for Clojure would use Java’s built in URL class with Enlive’s html-resource function but in our case it would not work, because it can’t handle compressed responses well. So we will use http-kit instead.

To begin with, we would add these libraries to our project.clj file (assuming we’re using Leiningen).

Now we’re ready to start writing the codes. Let’s first :require our libraries.

First we have to fetch the HTML response. We can use http-kit’s get function to grab the HTML. This function would return a promise. So we would have to dereference it using deref or the shorthand syntax @. When the promise is resolved, we would get a hashmap which would have a :body key along with :status and few other keys related to the request. We can pass this HTML response to Enlive’s html-snippet function to get an iterable DOM like object from which we can select the elements using select function.

We are using the {:insecure? true} part to ignore issues with SSL. So far, we have a function get-dom which would give us a DOM like object on which we can do select. We will now write another function which will extract the titles from this DOM like object.

Each Torrent title (which is a link, aka anchor tag) has the CSS class cellMainLink so we can select a.cellMainLink to get all the title links. Each title link would have their text part in the :content key. Each text part in the :content key is a vector. So we would need to use first on it to grab the actual text. Here’s what I wrote:

I simply could not resist using comp to do some magic here. comp allows us to combine two functions to compose one which allowed us to first grab the content and then get the first element in our case.

Finally, we can run our functions like this:

Here’s the complete file:

The code is under 20 lines! 😀

Categories
Bangla Django Python Screencast

Django Life – Session 4 (Bangla Screencast)

The audio is in Bangla, you can switch to HD video by toggling the youtube settings.

(I do not prepare the scripts beforehand, so please apologize my clumsiness) .

Categories
Python

Auto Update for your Frozen Python Applications using Esky

Let’s assume you have a desktop application built with Python. It could be a traditional GUI app built with PyQT/wxPython/Kivy or any other GUI framework. Or it could be a web server that serves a browser based HTML GUI for the user. Either way, you have “frozen” the app using cx_freeze, py2app/py2exe or pyinstaller and now you want to add “auto update” to the app, so when there’s a new version of the application is available, the app can download and install the update, automatically. For this particular task, I found esky to be a good viable option. In this article, I am going to demonstrate how we can use esky to deliver updates to our apps.

Quick Introduction to Esky

If we want to use Esky to deliver updates, we need to freeze the app first. But this time, we will ask Esky to freeze the app for us, using our freezer of choice. For example, if we used py2app before, we will still use py2app but instead of directly using it, we will pass it to Esky and Esky will use the py2app to freeze the app for us. This step is necessary so that Esky can inject the necessary parts to handle updates/patches and install them gracefully.

For the apps to locate and download the updates, we need to serve the updates from a location on the internet/local network. Esky produces a zip archive. We can directly put it on our webserver. The apps we freeze needs to know the URL of the webserver and must have access to it.

On the other hand, inside our app, we need to write some codes which will scan the URL of the above mentioned webserver, find any newer updates and install them. Esky provides nice APIs to do these.

So now that we know the steps to follow, let’s start.

Creating a setup file

If you have frozen an app before, you probably know what a setup file is and how to write one. Here’s a sample that uses py2app to freeze an app:

Now we can generate the frozen app using:

This should generate a zip archive in the dist directory.

Hosting the app

Collect the zip file from the dist directory and put it somewhere accessible on the internet. For local testing, you can probably use Python’s built in HTTP server to distribute it.

Finding, Downloading and Installing Updates

Now we will see the client side code that we need to write to locate and install the updates.

Here’s some codes taken from a PyQT app. The find_esky_update method is part of a QMainWindow class. It is called inside the onQApplicationStarted method. So it checks the update as soon as the application starts.

We first check if the app is frozen. If it’s not, then there’s no way we can install updates. sys.frozen will contain information about the app if it’s frozen. Otherwise it will not be available. So we first ensure that it is indeed a frozen app.

Then we create an Esky app instance by providing it the URL of our webserver (where the updates are available). We only pass the root URL (without the zip file name). The find_update() method on the Esky app will find newer update and return some information if a new update is available. Otherwise it will be falsy.

If an update is available, we ask our user if s/he wants to update. Here we used QMessageBox for that. If they agree, we call the auto_update method with a callback. We will see the callback soon. The auto_update downloads the update and installs it. The callback we pass – it gets called every time something happens during the process. It can be a good way to display download progress using this callback.

Let’s see our example code here:

As you can see from the code, the callback gets a dictionary which has a key status and if it is “downloading”, we also have the amount of data we have received so far and the total size. We can use this to calculate the progress and print it. We can also display a nice progress bar if we wish.

So basically, this is all we need to find and install updates.

Rolling a new update

We have learned to use Esky, we have seen how to add auto update to our app. Now it’s time to build a new update. That is easy, we go back to the setup.py file we defined earlier. We had version="0.1", inside the setup() function. We need to bump it. So let’s make it 0.2 and build it. We will get a new zip file (the file contains the version if you notice carefully). Drop it on the webserver (the URL where we put our app). Run an older copy of the app (which includes the update checking codes described above). It should ask you for an update 🙂

Please note, you need to call the find_esky_update() method for the prompt to trigger. As I mentioned above, I run it in onQApplicationStarted method for PyQt. You need to find the appropriate place to call it from in your application.

Further Reading

You can find a nice tutorial with step by step instructions and code samples here: https://github.com/cloudmatrix/esky/tree/master/tutorial