Multi-LED Toggle Switch
Adding comments to your code would help us understand your intentions.

It is assumed you are sending the ASCII values for 'a', 'b', 'c', ... for different buttons and their actions.  And that you want to receive these on an Arduino over the USB using the USB serial port protocol. It is also assumed that your Python script works and the Linux OS is creating a /dev/ttyACM0 or some such serial device when you plug in your Arduino.

Looking at your Arduino code we see you are only checking for the number of bytes in the Arduino's serial port receive buffer by using the Serial.available() method call.  If your intentions were to read a byte of data you would instead call the serial.read() method.

It is suggested that you check if there is any data available and if there is you would read it once and save it to a variable. Then you can check this variable many times and decide what action to take based on its value.  If you read the buffer many times you would be pulling a new byte of data from the buffer with each read.  If you run out of data to read you will be supplied with the value of -1 as an indication there is no more data.