Catching an Exception in Python

Catching a exception without name: 
try:
      x = 1/0
except:
     print "it will be catched"

To get information about a exception,  use sys.exc_info() in except clause.:
excep_info = sys.exc_info()   // it returns a tuple
excep_info[0]   == type of exception
excep_info[1]  == exception message or arguments
excep_info[2]  == exception object value.


Catching a exception by name:
 try:
      x = 1/0
except ValueError:
     print "it will be catched"

or  (to get exception information)

 try:
      x = 1/0
except ValueError as exp:
     print "it will be catched"
     print type(exp)    // type of exception
     print exp.args     //exception message or argumenst


Catching exceptions by names , incorrectly :
 try:
      x = 1/0
except ZeroDivisionError, TypeError:
     print "it will be catched"

There are two issues with above code:
1) It will not catch the  ZeroDivisionError exception in a wrong way.
    Reason: except statement is interpreted  in a wrong way.
    except ZeroDivisionError as  TypeError:
    It is because in old python except ZeroDivisionError, e, anything after comma is interpreted as        alias to the exception before comma.

 TypeError is used as a reference to the ZeroDivisionError as following code shows.
      print "it will be catched", TypeError
output
     it will be catched integer division or modulo by zero

2) It will not catch the TypeError.
    Reason: since TypeError will be used as a reference.


Catching exceptions by names , correctly :
In this case either of the exception will be catched.
  try:
      x = 1/0
except ( ZeroDivisionError, TypeError ):
     print "it will be catched"

Boolean Value of a Data Types in Python

Boolean values in python are True, False and None. While going through the pages of DiveIntoPython , i found a sentence which says that :

You can use virtually any expression in a boolean context.

then i thought ANY expression ?  so i tried following code :

if "poi":
     print "yes"    
else:
    print "no"

My first guess was that it should give error, since "poi" is not evaluates to be a boolean value. It didn't give any error and prints the "yes". but .....Why ?

Every datatype in python has a boolean associated value with it. So string, list , tuple and dictionary are all can be used in the boolean context.

Then what evaluates true or False for these data types ?

Well, whenever these data types are empty, their boolean value will be False.
>>var=""
>>bool(var)
False
Same is true for [], (), {} i.e. non-empty list, tuple and dictionary.

And when these data types are not empty having atleast one element , their boolean value will be True.
>>var="python"
>>bool(var)
True
Same is true for [3], (4,5), {"key": 3} i.e. non-empty list, tuple and dictionary.

For numbers ,
None evaluates to be False.
non-zero number evaluates to be True.
0 (or 0.0) evaluates to be False.

Zen of Python

The guideline principles behind the python language design by Tim Peters.

>> import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!


while coding the python above points should be considered. Then on can say that it is a pythonic code.

Python Collection: Counter Class

Conceptual idea of Counter is class is to represents the multiset of mathematics. Mutiset is set which allows the a member(element) to appear more than once like [a,a,a,b,b,b,b,c,c].
Number of times a element appear is said to be multiplicity of element.
Where this multiset can be useful ?
Prime factor of 144 is [2,2,2,2,3,3],  which is a multiset !!

In python terms
Counter is a subclass of dict which stores the element as a key and count as a value. It is unordered collection type.
This class supports the mathematical operations like addition, subtraction, union and intersection between two objects. To use this class one needs to import Counter class from collections package.
In this post i'm going to work around this class for basic operations.

Use case 1:
Let say we have a list  ['red', 'blue', 'red', 'green', 'blue', 'blue']. from which we want to find out the number of occurrence of a element.
>>> color_list = ['red', 'blue', 'red', 'green', 'blue', 'blue']
>>> cnt = Counter()

>>> for word in color_list:

...     cnt[word] += 1
>>> cnt
Counter({'blue': 3, 'red': 2, 'green': 1})

creating the counter object :
>>> from collections import Counter             //importing Counter
>>> c = collections.Counter(a=3, b=2, c=5)

printing counter object :
>>> c
Counter({'c': 5, 'a': 3, 'b': 2})
>>> c.items()
[('a', 3), ('c', 5), ('b', 2)]

list all elements in a counter object :
>>> list(c.elements())
['a', 'a', 'a', 'c', 'c', 'c', 'c', 'c', 'b', 'b']

finding the count for an element :
>> c['a']
3
one interesting thing here is if we tries to find the element which is not in c , say c['k']
>> c['k']
0         // returns zero not KeyError,  which is a case when using the dictionary.

get the all keys and values in counter object :

>>> c.keys()

['a', 'c', 'b']

>>> c.values()

[3, 5, 2]

finding the most common elements and their count using most_common(n) method, where n is number of elements :

>>> c.most_common(1)   // n is 1

[('c', 5)]

and 

>>> c.most_common(2)  // n is 2

[('c', 5), ('a', 3)]

updating the counter object : 

>> c2 = {'a': 5}

>> c.update(c2)

>> c

Counter({'a': 8, 'b': 5, 'b': 2})    // value of a element is updated to (3+5=8).

Use case 2 : Find out the most common words in a file 

>>> import re
>>> words = re.findall(r'\w+', open('hamlet.txt').read().lower())
>>> Counter(words).most_common(10)
[('the', 1143), ('and', 966), ('to', 762), ('of', 669), ('i', 631),
 ('you', 554),  ('a', 546), ('my', 514), ('hamlet', 471), ('in', 451)]

Django Import Cheat-sheet

In large full fledged Django web project consist the several applications and each applications having the view which are using various django's methods or classes. I  have noticed that some imports for methods or classes are used in almost every view. 

To keep them in sight i'm listing it out here : 

from django.http import HttpResponse, HttpResponseRedirect, Http404
from django.shortcuts import render_to_response
from django.template import RequestContext
from django.contrib.auth.decorators import login_required, permission_required
from django.contrib.admin.views.decorators import staff_member_required
from django.contrib.auth.decorators import login_required
from django.core.exceptions import ObjectDoesNotExist, ViewDoesNotExist

from django.utils import simplejson
from django.template.defaultfilters import slugify
from django.template.loader import render_to_string
from django.core.urlresolvers import reverse
from django.db.models import Q


from django.core.exceptions import PermissionDenied
from django.utils.translation import ugettext as _

from django.forms.models import modelformset_factory

Stacktrace of Django's Request and Response

In this post I'll going to explore what happens behind the scenes when we make a request for a resource (normally a template) and how Django sends the response.
All of the action happens in the django/core/handlers/base.py file.

1. On request server make server instance which is django.core.handlers.BaseHandler object. The server can be mod_python or wsgi type.

2. Import project's setting.py file and Django custom exception classes.

3. loads middleware classes by calling the load_middleware() of BaseHandler. 
    (ImproperlyConfigured exception is thrown from  here)

4. server instance dispatches the request_signal and create the instance of HttpRequest. this request object may be of mod_python [ django.core.handlers.wsgi.ModPythonRequest ]  or wsgi [ django.core.handlers.wsgi.WSGIRequest] depending upon the server type.

5. get_reponse(request) of BaseHandler is called. Its main purpose is to return HttpResponse object. This is most important method to for a request to get successful response or otherwise. 
I have divided the working of get_reposne(request) in following two phases:

phase 1: request
a). Execute the process_request(reqest) of middleware by passing the request object.
b). If process_request() returns the None then resolves the url by calling resolve() on    
     django.core.urlresolvers. 
c). If url resolution is successful then  calls process_view( request, view_fuc, url_arg )
     If url is not successful then returns the django.http.Http404.

process_view( request, view_fuc, url_arg ) either returns None or HttpResponse object.
 If it returns None then Django will continue processing the request and will call appropriate view. 
If it returns the HttpResponse object then django will not call view. 


phase 2: response
When view() is called and it should either response or exception.
If response is  response_object :
a).process_template_response(request, reponse ) is called by view() if response instance has render() associated with it, response parameter is of type TemplateResponse.
It is where you get the required response (html). 
 b).process_template_response(request, reponse ) is called by view() if response instance has render() associated with it, response parameter is of type HttpResponse or StramingHttpResponse.
If response is  None: 
c). process_excpetion(request, exception) is called which throws ValueError like:

raise ValueError("The view %s.%s didn't return an HttpResponse object." % (callback.__module__, view_name))

So for every request(url) either you will get response (e.g. html) or exception. 

No JSON object could be decoded: line 1 column 0 (char 0)

I have written a python code which makes the HTTP requests to fetch the json data.
The json data seems like :
{    
  "items": [

          {
            "question_id": 18384375,
            "answer_id": 18388044,
            "creation_date": 1377195687,
            "last_activity_date": 1377195687,
            "score": 0,
            "is_accepted": false,
            "owner": {
                "user_id": 1745001,
                "display_name": "Ed Morton",
                "reputation": 10453,
                "user_type": "registered",
                "link": "http://stackoverflow.com/users/1745001/ed-morton"
                 }
          },

      ]
}

Code : 

>>url="https://api.stackexchange.com/2.1/answers?order=desc&sort=activity&site=stackoverflow"

>>obj = urllib.urlopen(url)
>>response_str = obj.read()
>>response_json = simplejson.loads(response_str)

Error : 

The last line above shoots following  error.
JSONDecodeError at /
No JSON object could be decoded: line 1 column 0 (char 0)

when i tried to change the simplejson to json object for loads() then following  error occurred
ValueError at /
No JSON object could be decoded

I tried to check the json format using the Jsonlint which gave me error :

The error suggested that problem is of encoding type of the response . Then i inspects the response information like
    >>obj.info()

         Server: nginx
         Date: Fri, 23 Aug 2013 04:51:01 GMT
         Content-Type: application/json; charset=utf-8
         Connection: close
         Cache-Control: private
         Content-Encoding: gzip
         Access-Control-Allow-Origin: *
         Access-Control-Allow-Methods: GET, POST
         Access-Control-Allow-Credentials: false
         Content-Length: 2334

Solution:

The problems seems to be of content Content-Encoding of the response which is gzip format and simplejson or json  is failed to correctly parse that. After many searches on stackoverflow i found the following the solution(s) of problem.
Solution: better to use requests object.

1) using only requests

   import requests

   response_json = requests.get(url).json()

2)  using only requests with simplejson or json.

     import requests

     response = requests.get(url)

     response_json = simplejson.loads(response.text)

     print response_json

Above both the codes  returning the correct Content-Encoding for json.
>>reposnse_json.encoding
     utf-8
>>response_json.headers['content-type']
    "application/json; charset=utf8'

Selecting checkboxes in jquery

In my recent side project (clone of imdb.com), number of checkbox options were provided from which user can select the genre(s) of the movie and on the basis of that movies are filtered and shown.
Basically this is divided into two parts, first in which i'll get the all checked genre(s) and second part is to show movie(s) based on that genre(s).

In this post i'm wrting about how i accomplished first part which is divided into number of tasks.

In my template , following is code for showing genre(s) each having  checkbox option.

<div class="refine_box">
        {% for k, v in genre_dic.items  %}
            <input type="checkbox" class="check_id" name="genre" value="{{ k }}">{{ k }} ({{ v }})<br>
        {% endfor %}
</div>

genre_dic is a dictionary having key as a genre and value is number of movies for that genre.

task1: get the value of checkbox which is checked. 
 $(".refine_box input[type=checkbox]").click(function() { 
        
            var isChecked = $(this).is(':checked');
            if(isChecked){
                    alert(#(this).val())
            }
});


task2: get the values of all checkboxes which are checked.
 $(".refine_box input[type=checkbox]").click(function() { 
        
        var arr_refine = [];    //this will store all clicked checkboxes

        $(".check_id:checked").each(function() {
                arr_refine.push(this.value);

        });
 
  }); 

task3: passing array from ajax request to Django view.
In previous task i have created a array holding all genre, so in this task that array we will send to view.
      $.ajax({
                    url: "{% url ajax_refine_movieslist %}",
                    data: {arr_refine: arr_refine},
                   
            });    


Follwoing is whole code of jquery :
 $(".refine_box input[type=checkbox]").click(function() { 
        
        var arr_refine = [];

        $(".check_id:checked").each(function() {
                arr_refine.push(this.value);
               
        });

        $.ajax({
                    url: "{% url ajax_refine_movieslist %}",
                    data: {arr_refine: arr_refine},
                    dataType:'html',
                    success: function(newData){
                         $('.movie_listing').html(newData);
                    }
                   
            });       
   
    }); 
   
task4 : printing the list holding the selected genre(s)
def ajax_refine_movies(request):
    genre_list = request.GET.getlist('arr_refine[]')
    print genre_list


This is how i'm getting the all checkbox selected options.
Soon i'll post second part of this.