Switch Case Statements in Python

Switch case statements are very popular conditional control statements in almost all programming languages. But surprisingly this is not available in python.

Question: Is there any switch case statements in python ?

Answer: The direct answer is NO

Alternative options for switch case statements in python

Option 1: Using If – elif – else statements. An example is given below.

if case == "case1":
    execute_func_case1()
elif case == "case2":
    execute_func_case2()
elif case == "case3":
    execute_func_case3()
else:
    execute_default_func()

Wow. Excellent.  The above code looks good right ?. It works exactly like switch-case statements, then why need switch-case statements in Python ?

Have you noticed a problem ?. The above if-elif-else conditions are fine as long as we have less number of cases. Imagine the situation with 10 or more elif conditions. Now you got the problem right ?.

Lets try the second option

Option 2: Using List in Python as an alternative to switch case statements

An example is given below.

def add(a, b):
    return a + b

def sub(a, b):
    return a-b

case_funcs = [add, sub]

case_funcs[0](1, 2)
case_funcs[1](1, 2)

 

In the above program, we don’t have to use if-elif-else blocks, instead, we can call using the position or index of the list and call the function. This looks better than the previous option right ?. But what about the default case ?. Also what if someone types an option greater than the size of the list ?. It will throw exception and there is no way to handle default case.

Option 3: Using Dictionary as alternative to switch case statements in python

An example is given below.

def add(a, b):
    return a + b

def sub(a, b):
    return a-b

case_funcs = {'sum':add, 'subtract':sub}

case_funcs['sum'](1,2)

 

Here the implementation is much similar to the switch case statement. We use a key to identify or route to the required case or function. The keys can be anything and are not limited by the indices or positions.

Now lets talk about the drawbacks of the above implementation. The above method will throw KeyError if we pass an unknown key. Also there is no default case statement. How will we handle these problems?

Check the below program

def add(a, b):
    return a + b

def sub(a, b):
    return a-b

def default(a, b):
    return "Default Return"

case_funcs = {'sum':add, 'subtract':sub}

# sum is the key for the add(). 
# default is the default function that gets called for non existent keys
# (1, 2) are the arguments for the function
print(case_funcs.get('sum',default)(1,2))

 

Python dictionary has a get() method that returns the value based on the key. This has one more feature. We can configure a default value for non-existent keys. Wow now we got the solution.

So by using this feature, we can implement the switch-case like feature in python.

Can’t open config file: /usr/local/ssl/openssl.cnf on Windows

I faced the above issue while trying to use openssl in window. I have downloaded openssl and extracted in my windows server. After that I tried executing the following command.

openssl.exe genrsa -out subdomain.mydomain.com.key 2048

Solution:

Open Powershell

set OPENSSL_CONF=c:\[PATH TO YOUR OPENSSL DIRECTORY]\bin\openssl.cnf

Now execute the openssl command to generate the certificate. It will work without any issues. I hope this is helpful.

Cassandra not getting started in CentOS 7 and RHEL 7

After a recent update in CentOS 7 and RHEL 7, the cassandra daemon stopped working. I was getting the following error while trying to start the cassandra using systemd. Similar installations were working fine in the recent past and suddenly it stopped working.

Mar 20 13:22:34 localhost systemd[1]: New main PID 72596 does not belong to service, and PID file is not owned by root. Refusing.
Mar 20 13:22:34 localhost systemd[1]: New main PID 72596 does not belong to service, and PID file is not owned by root. Refusing.
Mar 20 13:22:34 localhost systemd[1]: Failed to start LSB: distributed storage system for structured data.

Root Cause

The cassandra starts, but the systemd cannot control it. The cause is that when the cassandra starts, the old initialization SysV script is used, in which it is obviously impossible to specify the user and group to start the service.

It’s about user/group options for systemd:
—————–
[Service]
User=cassandra
Group=cassandra
—————–

But since the process pid is created with the permissions of the cassandra user, and the user and group are not specified in the initialization script, the systemd consider that it uses the root to start the service (by default) and does not allow creating the pid with cassandra user permissions.
——————
systemd[1]: New main PID 2545 does not belong to service, and PID file is not owned by root. Refusing.
——————

More details in CVE-2018-16888 (https://access.redhat.com/security/cve/cve-2018-16888)
——————
It was discovered systemd does not correctly check the content of PIDFile files before using it to kill processes. When a service is run from an unprivileged user (e.g. User field set in the service file), a local attacker who is able to write to the PIDFile of the mentioned service may use this flaw to trick systemd into killing other services and/or privileged processes.
——————

Solution

Update the /etc/rc.d/init.d/cassandra file. Either make the following patch manually or replace the entire file with the file that I provided below.

Option: 1 – Manual Patch

Open /etc/rc.d/init.d/cassandra file and make the modifications as per comments in the below script. The below snippet is not the complete script, it is only the portion which needs update. Do not copy paste and replace the file completely with this

case "$1" in
start)
# Cassandra startup
echo -n "Starting Cassandra: "
[ -d `dirname "$pid_file"` ] || \
install -m 755 -o $CASSANDRA_OWNR -g $CASSANDRA_OWNR -d `dirname $pid_file`
# -Commented for fix
#su $CASSANDRA_OWNR -c "$CASSANDRA_PROG -p $pid_file" > $log_file 2>&1
# +Added for fix
runuser -u $CASSANDRA_OWNR -- $CASSANDRA_PROG -p $pid_file > $log_file 2>&1
retval=$?
# +Added new
chown root.root $pid_file
[ $retval -eq 0 ] && touch $lock_file
echo "OK"
;;
stop)
# Cassandra shutdown
echo -n "Shutdown Cassandra: "
# -Commented as per the fix
#su $CASSANDRA_OWNR -c "kill `cat $pid_file`"
# +Added for fixing the issue
runuser -u $CASSANDRA_OWNR -- kill `cat $pid_file`
retval=$?
[ $retval -eq 0 ] && rm -f $lock_file
for t in `seq 40`; do
status -p $pid_file cassandra > /dev/null 2>&1
retval=$?
if [ $retval -eq 3 ]; then
echo "OK"
exit 0
else
sleep 0.5
fi;
done

Option:2 – Replace the file

Replace the /etc/rc.d/init.d/cassandra file with the file present in the following link. This patch was made as per the JIRA issue CASSANDRA-15273.

Steps to replace the file are given below.

mv /etc/rc.d/init.d/cassandra /etc/rc.d/init.d/cassandra.old
curl -o /etc/rc.d/init.d/cassandra https://gist.githubusercontent.com/amalgjose/74cf98e0110c27b6124b0adbb698d372/raw/c08ce3481e9cb0601e79e127c78a65bf82080e5f/cassandra
systemctl daemon-reload
systemctl restart cassandra

 
The Gist code is pasted below.

 

This solution helped me. I hope this will help someone else also.

How to connect a CentOS computer to Internet using USB Wifi Adapter ?

I have an old desktop computer with CentOS 7 operating system installed without GUI. I wanted to connect to internet using a USB wifi adapter. My internet router was located in a different room and LAN cable was not available with me. So I used netgear USB wifi adapter for establishing the internet connection. This post is about troubleshooting and fixing the connectivity issue.

The model that I have used is Netgear WNA3100M Wireless-N300 USB Mini Adapter. The picture is shown below.

wifi_network_adapter

Netgear WNA3100M

I checked the network interfaces and ip address using ifconfig command. It listed an interface named wlp18s0b1. But no ip address was assigned.

I tried ifup command. But it gave me an error as follows.

ifup wlp18s0b1

/sbin/ifup: configuration for wlp18s0b1 not found.
Usage: ifup 

Then I tried listing the USB interfaces using lsusb command and it listed the network adapter usb device. This means that the device is getting detected.

The next steps that I tried are using the nmcli command.

The following command will list all the available Wifi connection profiles.

nmcli connection show

To connect to a wifi network, use the below command. You have to pass your wifi ssid and password as shown below as arguments.

nmcli dev wifi connect your-wifi-ssid password wifi-password

My desktop got connected to the internet after triggering the above command. After this, every time I start my computer, if the desktop is not automatically connected to the internet, I issue the following commands.

nmcli connection show

nmcli connection up your-wifi-connection

This solution helped me. Hope this will help someone else also :).

Merge two dictionaries in Python

This is the simplest way to merge or combine two dictionaries in python. This operation in supported in python version above 3.5.

 

Sample Output

{'p': 2, 'q': 4, 'r': 6, 's':8}

Convert csv to json using pandas

The following sample program explains you on how to read a csv file and convert it into json data. Two programs are explained in this blog post. The first program expects the column names in the csv file and second program does not need column names in the file.

The first program expects the headers in the first line of the csv. In case of missing headers, we have to pass it explicitly in the program.

Sample Input

EMPID,FirstName,LastName,Salary
1001,Amal,Jose,100000
1002,Edward,Joe,100001
1003,Sabitha,Sunny,210000
1004,John,P,50000
1005,Mohammad,S,75000

Here the first line of the csv data is the header

Sample Output

[{"EMPID":1001,"FirstName":"Amal","LastName":"Jose","Salary":100000},{"EMPID":1002,"FirstName":"Edward","LastName":"Joe","Salary":100001},{"EMPID":1003,"FirstName":"Sabitha","LastName":"Sunny","Salary":210000},{"EMPID":1004,"FirstName":"John","LastName":"P","Salary":50000},{"EMPID":1005,"FirstName":"Mohammad","LastName":"S","Salary":75000}]

 

If the csv file contains a header row, then you should explicitly pass header=0 to override the column names. If headers are not present in the csv file, we have to explicitly pass the field names in a list to the argument names. Duplicates in this list are not allowed. A sample implementation is given below.

 

How to convert a csv file to json file ?

Sometimes we may get dataset in csv format and need to be converted to json format.  We can achieve this conversion by multiple approaches. One of the approaches is detailed below. The following program helps you to convert csv file into multiline json file.  Based on your requirement, you can modify the field names and reuse this program.

The sample input is give below.

1001,Amal,Jose,100000
1002,Edward,Joe,100001
1003,Sabitha,Sunny,210000
1004,John,P,50000
1005,Mohammad,S,75000

 

Output multiline json is given below.

{"EmpID": "1001", "FirstName": "Amal", "LastName": "Jose", "Salary": "100000"}
{"EmpID": "1002", "FirstName": "Edward", "LastName": "Joe", "Salary": "100001"}
{"EmpID": "1003", "FirstName": "Sabitha", "LastName": "Sunny", "Salary": "210000"}
{"EmpID": "1004", "FirstName": "John", "LastName": "P", "Salary": "50000"}
{"EmpID": "1005", "FirstName": "Mohammad", "LastName": "S", "Salary": "75000"}

 

 

Functions as Objects in Python

Python is very powerful. It is easy to learn. Applications can be developed very quickly using python because of the simplicity.

Everything in python is an object. This includes functions also. Are you aware of the following features of functions in python. I was not aware during my initial few years.

  • Functions can be the elements inside various data structures like lists, dictionaries etc.

Few examples

Function as argument to another function

A Sample program in python to explain the implementation of using function as an argument of another function is given below.

Functions as elements within data structures like list or dict()

A simple implementation of passing list of functions as argument to another function is shared below.

I hope this will help someone. 🙂