So..rosserial_arduino was always setting the baud rate of arduino as 57600. So, for rosserial_python, only the baud rate of 57600 was working.I made rosserial_arduino run at the baud rate of 9600 (or 19200, etc.) by editing ArduinoHardware.h ArduinoHardware.h in <sketchbook>/libraries/ros_lib <sketchbook>/libraries/ros_lib and then specified the same baud rate for rosserial_python and it is working now.