How to handle sessions in Google app engine

While developing a GAE app engine recently, I came across the need to handle session variables. While session was a thing that I use to take for granted in ASP.NET (Session[“xyz”]) or php ($_SESSION[‘xyz’]), storing and fetching session variables isn’t something that straightforward with webapp2, the web framework widely used for building GAE apps with python.

However, I found a pretty easy way to implement sessions in my existing app (which was almost half-way done!) by creating a simple BaseHandler class that is session-powered, and then deriving all my url handlers with this new class instead of the usual webapp2.RequestHandler. Once you do that, you can straightaway use the python dictionary self.session[‘my_variable’] (that pretty much resembles $_SESSION[‘my_var’] in php). Here is the implementation for BaseHandler:

class BaseHandler(webapp2.RequestHandler):
    def dispatch(self):
        # Get a session store for this request.
        self.session_store = sessions.get_store(request=self.request)
            # Dispatch the request.
            # Save all sessions.
    def session(self):
        # Returns a session using the default cookie key.
        sess = self.session_store.get_session()
        #add some default values:
        if not sess.get(‘theme’):
        return sess

Here, you may initialize any particular session variables if you want and set default values for them (as I’ve done for the cosmo theme above).

Hopefully, this should save you some development time if you want to implement sessions in existing app engine code. As an example, here is how I make use of a session variable to pass on a user’s preferred theme to the template engine, so that the webpage can be displayed accordingly:

class MainPage(BaseHandler):
    def get(self):
        template = JINJA_ENVIRONMENT.get_template(‘stereo.html’)
        if self.request.get(‘theme’):

A simple pythonic script to log internet bandwidth usage on Linux

We often want to log internet bandwidth on our laptops or PCs in order to track our usage over a period of time. This also comes handy to cross-check our usage with that of our ISP’s invoice. Now how to do it on linux? While the Windows folk have all kinds of utilities for this kind of thing, we lunix folk have to invent it ourselves.

Some time ago, I wrote this small python script that writes your bandwidth stats to a log file each time you disconnect your internet connection. This, I’ve tested on ubuntu 12.04 and debian wheezy:

Source file:

# author: Prahlad Yeri
import subprocess, os, datetime, sys
def execute(command):
        p=subprocess.Popen(command, shell=True,stdout=subprocess.PIPE,stderr=subprocess.PIPE)
        #print str(result)
        if len(result[0])>0:
            return result[0].strip()
            return result[1].strip()
    except Exception as e:
        print ‘error occured:’ + errorstring
        return errorstring
mypath = ‘/etc/NetworkManager/dispatcher.d/’
iface = sys.argv[1].strip()
status = sys.argv[2].strip()
log=open(mypath + ‘tata.log’,’a’)
#log.write(iface + ‘:’ + status + ‘\n’)
for part in sys.argv:
        log.write(‘argv:’ + part + ‘\n’)
if (iface == ‘ppp0’ or iface == ‘ttyUSB0’) and status == ‘down’:
        s = execute(‘iptables -L -v’)
        rules = s.splitlines()
        rx,tx = 0,0
        rxdrop, txdrop = 0,0
        inrule = False
        outrule = False
        for rule in rules:
                log.write(‘rule:’ + rule.strip() + ‘\n’)
                words = rule.strip().split()
                if rule.strip().startswith(“Chain OUTPUT”):
                        log.write(‘switch to outrule’)
                        outrule = True
                        inrule = False
                elif rule.strip().startswith(“Chain INPUT”):
                        log.write(‘switch to inrule’)
                        inrule = True
                        outrule = False
                elif rule.strip().startswith(“Chain FORWARD”):
                        log.write(‘switch to no rule’)
                elif len(words) >=3:
                        if words[6].strip() == “ppp0”:
                                #log.write(‘bytes=’ + words[1]
                                if inrule:
                                        if words[2].strip() == ‘DROP’:
                                                rxdrop += int(words[1].replace(‘K’,’000′).replace(‘M’,’000000′))
                                                rx += int(words[1].replace(‘K’,’000′).replace(‘M’,’000000′))
                                elif outrule:
                                        if words[2].strip() == ‘DROP’:
                                                txdrop += int(words[1].replace(‘K’,’000′).replace(‘M’,’000000′))
                                                tx += int(words[1].replace(‘K’,’000′).replace(‘M’,’000000′))
        #print ‘rx=’ + str(rx) + ‘ tx=’ + str(tx)
        #print ‘rxdrop=’ + str(rxdrop) + ‘ txdrop=’ + str(txdrop)
        execute(“iptables –zero”)
        if not os.path.exists(mypath + ‘usage.csv’):
                #create header
                header = ‘date,rx,tx,rx-dropped,tx-dropped,rx-tot,tx-tot\n’
        f = open(mypath + ‘usage.csv’,’a’)
        if len(header) > 0:
        lines.append(str(‘.’)[0] + ‘,’ + str(rx) + ‘,’ + str(tx) + ‘,’ + str(rxdrop) + ‘,’ + str(txdrop) + ‘,’ + str(rx+rxdrop) + ‘,’ + str(tx+txdrop) + ‘\n’)
        #rx = execute(‘cat /sys/class/net/ppp0/statistics/rx_bytes’)
        #tx = execute(‘cat /sys/class/net/ppp0/statistics/tx_bytes’)
        #line = strdate + ‘,’ + rx + ‘,’ + tx + ‘\n’

In the above script, replace ppp0 with the interface-name for the name of interface through which you access internet. For mobile broadbands it is typically ppp0, but you can issue the “ifconfig” command to list them all and make sure.

You have to place this script at /etc/NetworkManager/dispatcher.d on debian/ubuntu systems so that it runs each time you stop using the modem. In case of other systems, or in case you don’t use NetworkManager, refer to your distro documentation to check the folder/script which is executed when an interface is down.

Android app can now save people’s lives!


Don’t believe it? I too didn’t until I read about this PressureNET open-source android app recently. This is a crowd-sourced app and its working is simple: Just gauge these two parameters:

1) Atmospheric pressure using the pressure sensors (available in latest android phones like Galaxy S4/Nexus), and

2) GPS coordinates using the GPS sensor.

and send the data to the PressureNET web-server through Internet. Now, with enough number of people running this app on their devices, weather scientists across the nation have access to substantial data regarding pressure-belts forming in various parts of the country. They can then input this data to their weather modeling systems and come-up with solid predictions! (not just vague guesses like now) regarding for example, when and exactly where the next hurricane is going to hit. According to Cliff Mass, an atmospheric Scientist at the University of Washington, this data can help predict an occurrence of a hurricane up to six hours earlier. There is currently, no technique to do it now.

Whilst the pressure-sensor is being used for this purpose, the intentions of Google was something totally different when putting this feature into the android OS. It was to fetch the altitude information. You see, the GPS tells you only the Geo-coordinates (Lat/Lon) on a plane surface. But the atmospheric-pressure changes as a person travels uphill or downwards, and thus it can let us know exactly what altitude a person is on!





A Python finds out which is the best performing Linux distro!


With more and more linux distributions competing among themselves for the userspace, we are currently living in the best of times as far as choice in open-source software is concerned. However, more the number of choices, more is the head-scratching to decide what to choose amongst them. Like many, I am also one of the victims of “distro-hopping”. No sooner a major distribution declares a new version, I’m itching to try one out in my virtual box or as a LiveCD version. Few days ago, this benchmarking idea occurred to me.

I’m a software developer and my requirements were clear. I wanted a distro that is good at performance. Since my desktop is cluttered with “heavy-duty” programs like Eclipse-ADT, Java, IDLE, etc., performance was my primary concern. I mostly write application software and android apps, so things like disk I/O, memory and CPU utilization matter.

With these things in my mind, I was seeking a simple way to do a quick benchmark of various distros to find out what suits my needs. Thats when this idea came to me – Almost all distros come preloaded with a general-purpose scripting language called python which could be summoned for the job. I took “time taken in milliseconds” to do 5 basic tasks to decide on the performance. These tasks were:

  1. Zip-test: A test to compress a large file to a zip file, and extract in back in python code.
  2. Random-test: Generate 5 million random numbers and round them to zero decimals.
  3. I/O test: Write a unicode string ‘spam and eggs’ 5 million times to a text file, and read them back.
  4. CSV test: Generate a CSV spreadsheet with 5 million rows in it, and read them back.
  5. Bandwidth test: Download the 800K akonadi RPM file from IIT Kanpur’s website.

(The last one was just for the sake of completeness. Internet bandwidth depends more on other parameters such as your ISP limits, time of the day, latency, etc. rather than the disto you are using).

The contestants in the arena were:

  1. openSUSE 12.2
  2. Debian 7 (GNOME version)
  3. Fedora 18
  4. Ubuntu 12.04 LTS
  5. Xubuntu 12.04

My previous experience with Ubuntu made be biased towards it, but the tests showed me how wrong I was. Also, from what I’d read in most blogs on the Internet, KDE was one of the most bloated distros ever. However, in my tests KDE on openSUSE topped the race in most parameters, though the credit should go to openSUSE for optimizing the KDE. The second spot saw Fedora and Debian fighting amongst them, while Ubuntu (that I was presently using) fared the worst! Here are the detailed results:

Test zt1




random io csv bandwidth
debian1 7622 2753 9142 5732 12784 118978
debian2 7724 2752 9161 5287 12112 92386
fedora18_1 8605 4168 6287 5972 12750 0
fedora18_2 7762 4164 6419 6160 12818 0
openSUSE1 9001 2313 5915 5904 12715 115999
openSUSE2 7253 2245 6035 5935 11492 71257
precise1 9649 5012 8807 6846 13552 78560
precise2 10555 4434 8890 7229 13455 48286
xubuntu1 13305 4827 8954 5839 14676 58538
xubuntu2 10826 4760 8934 7908 12802 52768

*Above figures represent time-taken in milliseconds

The bandwidth test could not be performed on fedora as the Live CD did not have the wget package installed, but I did not need that to get a general idea of things.

My Conclusion: openSUSE wins the race, though Debian-7 and Fedora-18 are also fine distros for performance.

Here is the entire python benchmarking script that you may customize as per your requirement (change initializing variables in the script and run by issuing “python” from the shell):


import sys,time,os
import zipfile,random,csv
import subprocess
from zipfile import ZipFile,ZIP_DEFLATED
from random import random

#This should be initially present before running the program:
file_to_extract = “VirtualBox.exe” #”dotnetfx35.exe”

def timer(test):
if test == “ziptest1”:
elif test == “ziptest2”:
elif test == “randomtest”:
elif test==”iotest”:
elif test==”csvtest”:
elif test==”bwtest”:

print test,timetaken,”msecs.”
return timetaken

def ziptest1():
myzip=zipfile.ZipFile(‘’,’w’,ZIP_DEFLATED) #ZIP_STORED

def ziptest2():

def randomtest():
for i in range(1,5000000):
r = round(random()*100,0);
#print r

def iotest():
for i in range(1,5000000):
file.write(u’spam and eggs’)
while (s!=”):
#print s

def csvtest():
writer= csv.writer(file,delimiter=’,’,quotechar=’|’)#,quoting=csv.QUOTE_MINIMAL)
for i in range(1,5000000):
for row in reader:
s= ‘,’.join(row)

def bwtest():[“wget”,””,”-O akonadi.rpm”])[“wget”,””,”-O”,”akonadi.rpm”])

if __name__ == “__main__”:
#print time.strftime(“%d-%m-%y %H:%M”, time.gmtime())
#print time.strftime(“%d-%m-%y %H:%M”, time.localtime())

#Store results to csv file.
writer= csv.writer(file,delimiter=’,’,quotechar=’|’)
print ‘\n\nWritten results.csv’

How to turn your Linux machine into a wifi Access Point

Wifi Access Point

Update on 08-jul-2014: To easily create a wifi AP on ubuntu and other distros, use the hotspotd daemon – Opensource, available on github.

To install:

tar xvf hotspotd-0.1.tar.gz
cd hotspotd-0.1/
sudo python install

Few weeks ago I stumbled upon the challenge of creating a wifi access-point on my ubuntu 12.04 linux machine. Whilst I knew about ubuntu’s built-in wifi hotspot feature that works in adhoc mode, it was pretty much useless to connect to my new Android smartphone since it did not support the ad-hoc wifi mode. Most phones these days only support the infrastructure mode (a.k.a access-point mode), and in fact, they won’t even detect devices running on ad-hoc mode. After doing some research, I gathered this simple (though a bit lengthy) set of steps to turn your linux machine into a wifi access-point:


1. Ability of your wireless card to support infrastructure mode. To find out:

(i) Find your kernel driver module in use by issuing the below command:
lspci -k | grep -A 3 -i “network”
(In my case, the driver in use was ath9k)
(ii) Now, use the below command to find out your wifi capabilities (replace ath9k by your kernel driver):
modinfo ath9k | grep ‘depend’
(iii) If the above output includes “mac80211” then it means your wifi card will support the AP mode.

2. Hostapd software: Hostapd is the standard linux daemon that will be used to create your access-point.

3. Dhcp software: Even after hostapd creates the AP and your device detects it, you will still need a  dhcp server to assign a dynamic ip-address to your AP client. (unless you are assigning static address to each device)

4. Iptables: In order to share internet on your AP clients through wifi (a.k.a reverse-tethering), you will have to setup a NAT (Network Address Translation), so that your linux machine, acting as a middleman transfers the internet packets to and from your AP client and the internet modem card (typically ppp0).

EDIT: As of 06-Jul-2013, I’ve developed a python program with GTK+ front end called ‘pyforward’ which automates the below procedure for you. You can find it here:


1. Install the hostapd package. On ubuntu:
sudo apt-get install hostapd

2. Install Dhcp server. On ubuntu:
sudo apt-get install dhcp3-server

3. Make sure that packet forwarding is turned on. This means that your computer is able to forward request of connected clients to other devices, which in my case happened to be from wlan0 to ppp0. (forwarding is different than sending and receiving packets). To enable packet forwarding, issue the following linux command:

sysctl -w net.ipv4.ip_forward=1

To make the change permanent, make sure that the below line is uncommented in your /etc/sysctl.conf file. If not, change it and restart your machine:


4. The next step is to set up your dhcp. First, decide the subnet and ip-address range in which your virtual AP will sit and your clients will be automatically assigned using dhcp. In my case, I used the subnet My virtual AP is assigned and each connecting wifi device gets an IP in range of In order to set the rule, add this block to your /etc/dhcp/dhcpd.conf:

subnet netmask {
interface “wlan0”;
# — default gateway
option routers;
# — Netmask
option subnet-mask;
# — Broadcast Address
option broadcast-address;
# — Domain name servers, tells the clients which DNS servers to use.
#option domain-name-servers
option time-offset
default-lease-time 1209600;
max-lease-time 1814400;

5. Now that packet forwarding and dhcp are set up, we have to create a NAT (Network Address Translation) table using iptables. Please note that if you are using any other controlling software on top of iptables such as ufw firewall or firestarter, then you have to manage the NAT yourself. In that case, there is no need to follow this step:

Create a file called iptables_start in your home folder using gedit or nano and add the below contents to it:

            #First, delete all existing rules
iptables –flush
iptables –delete-chain
iptables –table nat –delete-chain
iptables –table nat -F
iptables –table nat -X

#Allow incoming – already established connections:
iptables -A INPUT -m state –state ESTABLISHED,RELATED -j ACCEPT

#Allow outgoing on tcp80, tcp443, udp53
iptables -A OUTPUT -p tcp –dport 80 -j ACCEPT
iptables -A OUTPUT -p tcp –dport 443 -j ACCEPT
iptables -A OUTPUT -p udp –dport 53 -j ACCEPT

#NAT Forwarding for wifi access point
iptables -t nat -A POSTROUTING -o ppp0 -j MASQUERADE
iptables -A FORWARD -i ppp0 -o wlan0 -j ACCEPT -m state –state RELATED,ESTABLISHED
iptables -A FORWARD -i wlan0 -o ppp0 -j ACCEPT

#Block all incoming & outgoing traffic after that
iptables -A INPUT -j DROP
iptables -A OUTPUT -j DROP

Please note that some of the above rules are customized according to my particular requirement. For eg: I’m allowing outgoing traffic on only tcp80, tcp443 & udp53 ports. Your requirements may be different. Only the NAT forwarding part is what you should be concerned about. Also note that here ppp0 refers to the interface that connects to the internet using modem, and wlan0 is your wifi AP interface that connects to client devices that need internet access.

Your particular interfaces might have different names. You may query all interfaces by using ifconfig command to be sure of them.

Once you create the above file, make it executable using a command like:

sudo chmod +x /home/user_xyz/iptables_start

Now run it by issuing “sudo ./iptables_start” in your home folder. Each time you have to make any changes to your firewall, you may edit and run this file. To test your new iptables rules, issue the command “sudo iptables –list”. It will list your newly added rules.

Once you run this however, the iptables rules are set, but they are not stored permanently. To start these rules each time your computer starts, do the following:

1. Save existing rules to a file using “sudo iptables-save > /home/user_xyz/iptables.rules”.
2. Copy the iptables.rules file to your /etc folder.
3. Now, in order for your computer to load rules from iptables.rules when your network interfaces turn up, create a file called /etc/network/if-pre-up.d/iptablesload and add below script to it:
iptables-restore < /etc/iptables.rules
exit 0

4. Make the above file executable by running “sudo chmod +x /etc/network/if-pre-up.d/iptablesload”

Test these settings by restarting your computer and issuing “sudo iptables –list”. This will list your current firewall rules.

6. Now that all hard work is done, you are ready to start your virtual AP. First, create a configuration file for the hostapd called hostapd.conf. It can be located in either /etc or your home folder:

#sets the wifi interface to use, is wlan0 in most cases
#driver to use, nl80211 works in most cases
#sets the ssid of the virtual wifi access point
#sets the mode of wifi, depends upon the devices you will be using. It can be a,b,g,n. Setting to g ensures backward compatiblity.
#sets the channel for your wifi
#macaddr_acl sets options for mac address filtering. 0 means “accept unless in deny list”
#setting ignore_broadcast_ssid to 1 will disable the broadcasting of ssid
#Sets authentication algorithm
#1 – only open system authentication
#2 – both open system authentication and shared key authentication
#####Sets WPA and WPA2 authentication#####
#wpa option sets which wpa implementation to use
#1 – wpa only
#2 – wpa2 only
#3 – both
#sets wpa passphrase required by the clients to authenticate themselves on the network
#sets wpa key management
#sets encryption used by WPA
#sets encryption used by WPA2
#####Sets WEP authentication#####
#WEP is not recommended as it can be easily broken into
#wep_key0=qwert    #5,13, or 16 characters
#optionally you may also define wep_key2, wep_key3, and wep_key4
#For No encryption, you don’t need to set any options

In above script, replace YourAPName with ssid of your AP. This will be detected when you run a scan on your device. Similarly, replace your_passphrase with the actual password you wish to set up.

7. The last and final step is to create an AP script and run it. Create a file called AccessPoint in your home folder:

ifconfig wlan0 up netmask
sleep 5
###########Start DHCP, comment out / add relevant section##########
#Doesn’t try to run dhcpd when already running
if [ “$(ps -e | grep dhcpd)” == “” ]; then
dhcpd wlan0 &
#start hostapd
sleep 2
hostapd ~/hostapd.conf 1>/dev/null
killall dhcpd

8. Make above file by issuing “sudo chmod +x ~/AccessPoint”. Now execute this script in the terminal by “./AccessPoint” and keep it running. If all goes well, your devices should now be able to scan and connect to your new virtual AP.


If for some reasons, all doesn’t go well, then here are some links that can help you:

Iptables basics:

NAT and port forwarding:

(If you have some specific issue, you can always get back to me).

How to create a screencast on your linux desktop

Screen casting is the act of recording your desktop while you do something, for example, running an application, replicating a test-case scenario or creating a training video, etc. Optionally you may or may not want your voice being recorded along with the video. Whilst there are many free and popular tools available for Windows to do such a task, in the Linux world the de-facto standard is a nifty little program called recordMyDesktop. Apart from a command line interface, it also has a very simple but elegant GUI that lets you easily create a screencast with audio recording enabled, while also providing you the option to specify some advanced settings such as screen co-ordinates (the area on your desktop you want recorded), audio/video input devices and the number of channels:


The above image shows recordMyDesktop program running on my Ubuntu 12.04 Linux distro. All you have to do is set the video quality and optionally set the sound quality if you also need voice recording. Most of the time, there is no need to open Advanced Settings. A few scenarios I can think of is where you want to specify an explicit FPS (Frames per second) value, apart from the default which is 15. Or else, you want to specify an explicit screen resolution.

There is also a feature that enables you to record a single window instead of the entire desktop. Just use the select “window button” on bottom left corner to do so.

Once the recording is done, the program encodes the video in .ogv (Ogg Vorbis) format as saves in your home folder with the filename, “out-n.ogv”. Ogg Vorbis is a pretty standard video format most modern programs are able to play.

Installing this program on your linux desktop is pretty easy. Depending on your package management system, this is usually done by issuing a single command in your terminal:

sudo apt-get install recordMyDesktop

This is for apt-based distros such as Debian and Ubuntu. On Fedora you may use yum, or zypper on openSUSE.

Fedora vs Ubuntu vs Linux Mint for Performance

Fedora vs Ubuntu vs Linux Mint

The never ending search for that “holy grail” distribution keeps most people downloading and hopping from one distro to another without ever bothering to ask themselves why they want to use an operating system in the first place. I don’t blame them, as nowadays, the ever active “assembly-lines” of various distro factories like ubuntu, fedora and linux-mint are enough to boggle the minds of most newbies by throwing an enormous number of configuration options.

Until some time ago, I myself was one of those “holy grail” seekers endlessly installing one distro after another in the second partition of my hard drive which is always reserved for Linux. Since my full time job involves .NET programming for my organization, I have to keep the first one reserved for Win7 of course.

Anyways, coming back to my Linux endeavors, I decided to pause and look inwards. I asked myself why do I want to use Linux? Thats because I want to: 1) Learn and understand the Linux operating system and programs using a top-down approach, and 2) Learn programming on Linux – this involves a lot of things including C++, GTK+, Qt, Android and the suchlike, and 3) Doing miscellaneous things such as listening to music, some light gaming, etc.

The most obvious thing that occurred to me was one word – performance. I don’t need high-end graphics. I’m never getting into serious gaming stuff such as OpenGL. All I need is a distro that can handle as many applications and programs as I can throw at it. Since I don’t have time for doing things from scratch like LFS or ArchLinux, I shortlisted the below three candidates:

1. Ubuntu 12.04 LTS – Precise Pangolin

2. Linux Mint 14.1 – Nadia

3. Fedora 18 – Spherical Cow

Rather than search for existing benchmarks, I decided to put these gladiators in my own arena. I prepared my 16GB pendrive for a multi-boot with the above three Live versions and started testing them one by one. The stuff I threw at them was things I would normally do, such as extracting a huge (4GB) archive, format a USB pendrive, surf the net, etc. Here is how each one of them fared in the fight:

Distro Benchmarking

The verdict: Unlike the gladiators of Rome where one used to absolutely dominate over the other, the case of these linux distros is entirely different. One distro fared well in one area, while the other fared in another. For instance, Fedora was good at CPU usage, while Linux-mint did well in the RAM area. However, for my purposes, I regard Linux-mint as an ideal choice overall.