Sunday, November 24, 2013

No more pdb.set_trace() committed: git pre-commit hooks

After 1, 2, ... 5 times it happens, you must find a way to solve the problem.
I'm talking of committing to your git repository a pdb.set_trace() you forgot to remove.

What is really nice of git (something missing in SVN for what I know) is the support to two type of hooks: client side and server side.
While server side hooks are complex and triggered when you push to the repository, client side hooks are simpler and under your control.

So, I can use client side git hook to solve the pdb problem?

Git hooks are executable file (of any kind) inside the .git/hooks directory of your repository. Those files must have a precise name, that match the action they capture (you can find a complete list in the Git Hooks reference).
In our case: we need to use an executable named pre-commit.

As you probably noticed, hooks are inside the repository. But can I have a centralized database of hooks that will be replied in every repository?
Yes, you must use Git templates.
To quick set you global templates, see this nice article: "Create a global git commit hook".
For impatiens, in two lines of code:

$ git config --global init.templatedir '~/.git-templates'
$ mkdir -p ~/.git-templates/hooks

Now put the executable file named pre-commit inside this new directory. After that, when you create a new repository, hooks inside the template directory will be replicated inside the repository.

The pdb commit hook is a well know problem, already solved. The best reference I found is in the article "Tips for using a git pre-commit hook", that use a simple bash script.

I simple changed the author idea a little, because I don't want to block commit when a commented pdb is found in the code (bugged! See next section!):

git diff --cached --name-only | \
    grep -E $FILES_PATTERN | \
    GREP_COLOR='4;5;37;41' xargs grep --color --with-filename -n \
    -e $FORBIDDEN_PATTERN && echo 'COMMIT REJECTED Found "pdb.set_trace()" references. Please remove them before commiting' && exit 1

There are some other info you'd like to know about client side hooks:
  • You can ignore the hook for a single commit (git commit --no-verify)
  • Only a single hook can exits in a repository. This is a limit, however you can find workarounds.

EDIT (21 December)

Normally I don't modify old articles, but I found a bug in the solution above. The problem: grep command return non-zero status code when it found no match.

Here another working solution (I'm not a Linux bash ninja... any other cleaner suggestion will be accepted!):

git diff --cached --name-only | \
    grep -E $FILES_PATTERN | \
    GREP_COLOR='4;5;37;41' xargs grep --color --with-filename -n \
    -e $FORBIDDEN_PATTERN && echo 'COMMIT REJECTED Found "pdb.set_trace()" references. Please remove them before commiting'
[ $RETVAL -eq 1 ] && exit 0

Saturday, November 16, 2013

Dive into HTML5 Canvas

In last weeks I read a book about the new canvas element of HTML 5: HTML 5 Canvas, by Steve and Jeff Fulton
I don't want to review the book itself (just two word: is ok) but reading it lead me think about "how Canvas can change the Web in future".

First of all: I found all the HTML 5 Canvas feature interesting, but while reading the book I felt a sort of deja-vu. Every time I run one of the given example about drawing and painting, it was like I had already see it on a Web browser...
No, I'm not talking of Flash (I never had any experience with that) but about Java Applet! Yes, I really said "Java Applet"!
When I started Web development 10 years ago applet was "cool" because you were able to do a lot of stuff impossible to to with pure HTML + JavaScript. But you already know that applet failed.
Now using canvas you can do (again) a lot of drawing work, but still: how can this be useful to Web users?

HTML 5 Canvas: The Good Part

I was not able to find every answers myself, so I asked to Twitter and I get back some useful feedback..
  • Videogames development. A lot of this. In my opinion this is great! I don't have time for developing videogames anymore, but is still a discipline I like to follow. A lot of new JavaScript frameworks for game development are coming out, thanks to canvas.
  • Graphs and Histograms. We have a lot of powerful graph generator for server side languages, that can create images on the fly, but we can now stop query a remote server for that (remember that HTML 5 means also "bein offline").
  • File handling and preview. As HTML 5 is able to manage files, and canvas can also manage some type of media like images, audio and video (see below), plugins like jQuery File Upload are now possible.
I get some more good examples, but in the meantime I continued reading the book, and I found new super-cool answers myself! So: keep reading.

HTML 5 Canvas: The Super Fancy Cool Part

First of all: video!
I don't want to simply talk about HTML 5 video support or how you can control video with JavaScript, but what you can do with video and canvas.
This can be sum in a simple sentence: canvas can handle a video like it were an image. What does it means? You can draw the current video frame (taken from a standard HTML video element) inside a canvas; write the current video frame every 20 milliseconds and you are really playing the video inside it.

Why is this cool? I can already see a video element inside my HTML... so?
The great part is that you can draw a video frame inside the canvas then draw other stuff on the top of it! You can add comments and images on the running video! Wow!

And now the best part: HTML Media Capture API.
A lot of browser already support this new technology that make possible from JavaScript to access native webcam (and microphone) of the user's device. And how JavaScript can use this privileged access is not putting it into canvas?
And after that? Can I upload my work to a server? Yes!

Playing with this new toy I spent some times developing a new Plone add-on: collective.takeaportrait. This sum all the stuff I learned about media capture and video manipulation:
  • If the browser support getUserMedia call, a new button appear
  • The button will open an overlay where the webcam output is displayed
  • A viewfinder with the standard Plone ratio for user's portrait and a countdown are drawn over the streaming video
  • Use can save a photo and send it to the server for replacing the current portrait

This is something that you already see somewhere, some social networks (for what I remember, Facebook for sure) already give that chance to users, but it's really amazing to see how few lines of JavaScript code can raise the usability of your site!

Now the bad news (also reported in the Fulton & Fulton book): there's no support for those features inside mobile devices right now. Really sad.