add this settings to your $HOME/.vimrc file
set smartindent
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set shiftwidth=4
set expandtab
Author Archives: Jinesh
2012 in review
The WordPress.com stats helper monkeys prepared a 2012 annual report for this blog.
Here’s an excerpt:
600 people reached the top of Mt. Everest in 2012. This blog got about 2,000 views in 2012. If every person who reached the top of Mt. Everest viewed this blog, it would have taken 3 years to get that many views.
Operator overloading in Python
Very recently my friend Hari wrote an article about operator overloading in ruby, and he asked me to implement the same program which he used in his article in python. His aim was to compare the two hottest languages of technology lovers.
Though our example is very simple and straight forward, we consider this as a stepping stone for our comparative study. Below we are giving our example which compares the operator overloading for ‘+’ (addition).
I expect you know about operator overloading. If you dont know, in simple words, it is just like giving special meaning to a language’s operators(‘+’, ‘-‘,’/’,’*’ etc) via new definition though class methods.Read more about operator overloading from here.
In python, operator overloading is achieved via overriding python’s special methods from a python class. If you want to do operator overloading in python you should know about it’s special methods ( which is also called magic methods). Python has a number of special methods, which should be of the form __xxx__. The list is very big, but most commonly used ones are, __add__, __sub__, __repr__, __str__, __len__, __gt__, __lt__ etc. you will get the complete list from here.
lets see how we can give special meaning to __add___. I hope you should be knowing that we cant add two objects, we can add only two numbers. In python you can use ‘+’ to append two strings too. But look how we can add special meaning to ‘+’ to add two objects.
class Add(object):
def __init__(self, val):
self.val = val
def __add__(self, obj):
return self.val + obj.val
a = Add(10)
b = Add(20)
print a + b
as you expect this will print 30. In order to understand how this works, you can read the print statement like this. print a.__add__(b)
a = Add(‘Hello’)
b = Add(‘ World’)
print a + b #This will print ‘Hello World’.
The above examples are straight forward use of ‘+’ operator. We can further extend our __add__ method to intelligently add different data types. Normally python wont allow to add different datatypes. ( try some examples on your python shell)
Here is our extended __add__ method. And see the examples.
class Add(object):
def __init__(self, val):
self.val = val
def __add__(self, obj):
if isinstance(self.val, str) or isinstance(obj.val, str):
return str(self.val) + str(obj.val)
return self.val + obj.val
a = Add(10)
b = Add(20)
print a + b #30
c = Add(‘Hello’)
d = Add(‘ World’)
print c + d # Hello world
e = Add(5)
f = Add(‘ is my number.’)
print e + f # 5 is my number
In the above definition of __add__ we are checking whether any of the value is a string, if then we will append the two values, otherwise add the values.
Basic authentication mechanism using OAuth.
The current third party apps revolution led to the scenario where apps need to access user’s private data from different websites. In order to access user’s data from other sites (especially personal data) , we have to go through a basic authentication mechanism. Many of them follows OAuth. You can read more about OAuth from here.
Here am just sharing a diagrammatic representation of how we are establishing a secure connection with a distant website using OAuth. My intention is to give an high level overview of OAuth mechanism.
1) Initially we will send a get request to the url from where you intend to get data and you will provide some information such as our application id, scope of data access ( r/w permission for different data sets, eg read_products, write_tweet), and redirect_uri(url to redirect after authentication) for authentication purpose.
2)If user grant the access to the specified scope, we will receive a code(long id) and some other details as response from the site.
3) We are interested only in the code, which we will send back to the website/url(normally this url will be given in developer Api) for getting an access_token. We use this access token for establishing a secure connection. We can save this access_token for future use also. That is once a connection is established and we got access token we can receive user data at anytime until access_token expires or user revoke their permission.
A little about HTACCESS
Introduction.
htaccess is simply a server side configuration file, used for webserver configuration management. Which contain some instructions in a text file. And those instructions can be used to improve your server settings and do many other helpful things such as customize your own 404 page, rewrite long urls with more readable ones, disable directory listing, put security restrictions for a directory etc.
How to add htaccess file to Server:
What we will do is, just create a file with the name .htaccess and put it in our webserver directory. This file would contain some instructions (we call them rules ) in a pre-specified format, that webserver can understand. We would discuss about the rules and its structure in more detail.
Things we can acheive with htaccess rules:
- Redirect / Rewrite urls
- Password protection for directories
- Deny visitors by IP address
- Adding MIME types ( instruct the server how to treat different varying file types)
- Disable directory listings
- Preventing access to your PHP includes files
- Prevent access to php.ini
- Forcing scripts to display as source code
- Ensuring media files are downloaded instead of played
- Setting up Associations for Encoded Files
- Preventing requests with invalid characters
Lets write a simple .htaccess file:
We will write a simple .htaccess file to show how we can redirect one url to another.
Some times our url will be long and complex to the user. In those times you can simply replace such long urls with more readable and search engine optimised urls .
This is how we acheive url redirection through htaccess rule.
RewriteEngine On RewriteRule ^myurl.*$ http://example.com/ [NC,L]
This rule in .htaccess file will redirect request http://www.example.com/myurl to http://www.example.com/
Explanation:
a general format of the rule for url redirect will be
Redirect /old_url/ http://www.yourdomain.com/new_url [flags]
Now lets do cross section in the real example:
RewriteEngine On : will turn on the rewriting engine. We will write this line only once in a
.htaccess file.
RewriteRule ^myurl.*$ http://example.com/ [NC,L]
The above line specify the rule we want to apply.
^myurl.*$ : The server will compare the url of all the requests to this pattern. If it matches
then it will replace the pattern with the following argument in rewrite rule. ( here it is
http://www.example.com/ ).
[NC,L] : These are “Flags”, that tell Apache how to apply the rule. “NC”, tells Apache that this rule should be case-insensitive, and “L” tells Apache not to process any more rules if this one is used.
Condition
Now lets try another example, To redirect an old domain to a new domain. That is if you have chnaged your website domain name to a new name with out changing its directory structure, you can put this .htaccess file in your webserver , which will redirect all requests to old domain to new domain.
RewriteEngine On
RewriteCond %{HTTP_HOST} old_domain\.com [NC]
RewriteRule ^(.*)$ http://www.new_domain.com/$1 [L,R=301]
Here we have a RewriteCond (rewrite condition) preceded by RewriteRule. Which is the most common case in htaccess files. By using rewriteCond before rewrite rule we can limit the RewriteRule only if that condition is met. We can also write multiple condition using and, or.
How RewriteCond works?
General format: RewriteCond server variable string
The RewriteCond operates in a similar way with RewriteRule. The string to test can be a variety of things. Such as which browser used, the IP address etc, it depends on the server variable we have specified. In the above example we have used the HTTP_HOST variable. It verifies whether the given string matches the host domain, and if it does do RewriteRule.
List of server variable that we can use:
HTTP Headers
HTTP_USER_AGENT
HTTP_REFERER
HTTP_COOKIE
HTTP_FORWARDED
HTTP_HOST
HTTP_PROXY_CONNECTION
HTTP_ACCEPT
Connection Variables
REMOTE_ADDR
REMOTE_HOST
REMOTE_USER
REMOTE_IDENT
REQUEST_METHOD
SCRIPT_FILENAME
PATH_INFO
QUERY_STRING
AUTH_TYPE
Server Variables
DOCUMENT_ROOT
SERVER_ADMIN
SERVER_NAME
SERVER_ADDR
SERVER_PORT
SERVER_PROTOCOL
SERVER_SOFTWARE
Dates and Times
TIME_YEAR
TIME_MON
TIME_DAY
TIME_HOUR
TIME_MIN
TIME_SEC
TIME_WDAY
TIME
Special Items
API_VERSION
THE_REQUEST
REQUEST_URI
REQUEST_FILENAME
IS_SUBREQ
List of flags that we can use:
C (chained with next rule)
CO=cookie (set specified cookie)
E=var:value (set environment variable var to value)
F (forbidden – sends a 403 header to the user)
G (gone – no longer exists)
H=handler (set handler)
L (last – stop processing rules)
N (next – continue processing rules)
NC (case insensitive)
NE (do not escape special URL characters in output)
NS (ignore this rule if the request is a subrequest)
P (proxy – i.e., apache should grab the remote content specified in the substitution section and return it)
PT (pass through – use when processing URLs with additional handlers, e.g., mod_alias)
R (temporary redirect to new URL)
R=301 (permanent redirect to new URL)
QSA (append query string from request to substituted URL)
S=x (skip next x rules)
T=mime-type (force specified mime type)
References:
http://www.addedbytes.com/for-beginners/url-rewriting-for-beginners/
http://perishablepress.com/stupid-htaccess-tricks/
http://www.htaccess-guide.com/
http://www.freewebmasterhelp.com/tutorials/htaccess
Linux commands for daily usage
File/Basic commands
These are some useful basic linux commands. For every command I followed this convention.
1)The name of the command with a short description is given.
2)Some of the useful options that can be used with the command is given after the command.
ls – list the current directory contents.
-l long listing
-a list including hidden files
-t list by last modified
cat – show the content of a file.
format: cat [options] [file name(s)]
-n show the line numbers also.
head – output the first 10 lines of each FILE to standard output
format: head [options] [file name(s)]
–bytes=5k or -c 5k
-n specify the number of lines.
tail – output last part of file
same as head
sort – sort lines of text files
sort [OPTION]… [FILE]…
-n numerical sort
-f ingnore case
-r reverse the results
-R random
-d in dictinary order
free – Display amount of free and used memory in the system
free [-b|-k|-m|-g]
-b bytes
-k kilo bytes
-m mega bytes
-g giga bytes
Search commands:
find – search for files in a directory hierarchy
find [directory to search] [option] [filename]
eg:- find . -name abc.py
find [directory] -group gname (File belongs to group gname)
grep – print lines matching a pattern
grep [options] [pattern] [filename]
eg:
grep ‘head’ words.txt
grep ‘^head’ words.txt
grep ‘head$’ words.txt
grep ‘^head$’ words.txt
grep ‘^….$’ words.txt
grep ‘^h.*d$’ words.txt
sed – stream editor for filtering and transforming text
sed [OPTION]… {script-only-if-no-other-script} [input-file]…
two examples:
sed ‘s/cat/dog/g’ data ( substitute all cat with dog globally )
sed 2,4d data ( delete lines from 2 to 4 in file data)
Getting details from Youtube URL
We can easily get many details from a youtube url using some of Python libraries. Here am going to show you how we can get
1)Video id
2)Video title
3)Author of video
4)Video thumbnails
We use Python’s urlparse, urllib and simplejson libraries to do this.
import urlparse
import urllib
import simplejson
url_data = urlparse.urlparse("http://www.youtube.com/watch?v=z_AbfPXTKms&NR=1")
video_id = urlparse.parse_qs(url_data.query)["v"][0] # this is the video id
# from this url you will get all data you needed about video, by just replacing the video id
url = 'http://gdata.youtube.com/feeds/api/videos/%s?alt=json&v=2' % video_id
json = simplejson.load(urllib.urlopen(url))
# this is the title of the video
title = json['entry']['title']['$t']
# author of video
author = json['entry']['author'][0]['name']['$t']
print "id:%s\nauthor:%s\ntitle:%s" % (id, author, title)
You will get thumbnails from this url: http://img.youtube.com/vi/video_id/image_id.jpg
image_id can be 0,1,2 or 3.
Install Javascript shell
If you are familiar with Python, you will be liking its interactive command prompt. Because we can experiment its features on the fly in terminal. Similarly if you wish to play with javascript in terminal we have some standalone javascript shells. You can find a list of useful links from here.
Recently I have installed javascript shell from spider monkey source distribution. I feel it will be a handy tool for you.
Pre-requisites:
1) Mercurial version control. (Hg)
2) autoconf2.13 ( I installed it from synaptic package manager. It is the easier way)
Here is the step for the installation:
1) Get the spidermonkey source code.
Just copy this in your terminal:
hg clone http://hg.mozilla.org/mozilla-central/
the source code will be downloaded to the folder ‘mozilla-central’.
2)
cd mozilla-central/
3)
cd js/src
4)
autoconf2.13
5)
./configure
6)
make
A simple decorator in Python
Decorators are very useful functionality in Python. Here am going in a straight forward way to get a quick basic idea about what a decorator means. You can find what is a decorator and more details from here.
Excerpt from python wiki about decorator: A decorator is the name used for a software design pattern. Decorators dynamically alter the functionality of a function, method, or class without having to directly use subclasses or change the source code of the function being decorated.
Lets go through an example.
@login_required
def account_settings(request):
#some code
Here login_required is a decorator. Decorator is nothing but another function. But it’s functionality is different. While running this code, when the python interpreter see some decorator ( here it is login_required) it calls the decorator with the below function as argument (that means ‘account_settings’ as argument). Then we can do some preprocessing with the function inside decorator. Such as, using the details from function arguments we can check whether user is logined or not, if not logined redirect to login page etc.
In short using a decorator we can do some action before a particular function is called.( can do more than this!).
So how a decorator look like. Am giving a simple outline of a decorator:
def login_required(func_name):
def decorator(request, *args, **kwargs):
#if not logined:
# redirect to log in page
#else:
#allow to access settings
return decorator
Some points:
1) The inner function name ‘decorator’ is optional. You can choose a different name also. But you should return that name in the end. That means if you use the name ‘myfunc’ as function name, then you should return ‘myfunc’.
2) If you are sure about the number of parameter and its type you can use it directly as inner function (here ‘decorator’) parameter. If you know there should be two parameters(say ‘a’ and ‘b’ ) for account_settings function, then you can define inner function as ‘decorator(a,b)’ .
if you want to know more about decorators, this is a good tutorial.
how to add a script tag inside a jquery template.
You cannot directly add a script tag inside a jquery template.
suppose you want to add a script tag inside a jquery template in the following way:
<script id="filaVideoTemplate" type="text/x-jQuery-tmpl"> <!-- Some HTML here --> <script type="text/javascript"> <!-- Some javascript here --> </script> </script>
This will not work.Because browsers don’t understand nesting of tags, so whenever it parses a , it closes the outermost script tag.
You should write ” </scr{{= “”}}ipt> “instead of closing the inside script directly as ” </script> “.
so that the code look like:
<script id="filaVideoTemplate" type="text/x-jQuery-tmpl">
<!-- Some HTML here -->
<script type="text/javascript">
<!-- Some javascript here -->
{{html "</sc"+"ript>"}}
</script>
so this is the solution.
Further for the django developers, this is not the solution.
because ‘{‘ and ‘}’ are reserved characters in django templates. so instead of writing
{{html “</sc”+”ript>”}}
we have to write:
{% templatetag openbrace %}{% templatetag openbrace %}html “</sc”+”ript>”}{% templatetag closebrace %}{% templatetag closebrace %}
This will work for django templates.
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