How to get the size (in bytes) of output in Python

Question

firstly, I would like to thanks to whomever would help me.

- Environment

I am using Python v2.7 in Windows 8 OS. I am using COM4 to talk to robot by sending some commands in Python code.

I send a command getversion to robot and suppose to get a bunch of data which is in the following format (I omit some, it is too long):

Component,Major,Minor,Build,Aux

APPassword,956FC721

BaseID,1.2,1.0,18000,2000,

BatteryType,4,LIION_4CELL_SMART,

Beehive URL, beehive.cloud.com

BlowerType,1,BLOWER_ORIG,

Bootloader Version,27828,,

BrushMotorType,1,BRUSH_MOTOR_ORIG,

BrushSpeed,1400,,

BrushSpeedEco,800,,

ChassisRev,1,,

Cloud Selector, 2

DropSensorType,1,DROP_SENSOR_ORIG,

LCD Panel,137,240,124,

LDS CPU,F2802x/c001,,

LDS Serial,KSH13315AA-0000153,,

• My Question Is: Based on this format, how to get the size (in byte) of the above result? The reason for this question is, I can get the full/complete result as long as I know how large it is.

To be specific, my code is:

ser.write('getver \n')   # send 'getversion' cmd to robot

ser.read(1305)   

The response size of getver is 1305 byte, yes, I count it manually, that is why I would like to ask Python to tell me how large it is automatically.

Answer 1

In order to be able to communicate with a device, you have to know what the protocol is for that communication. Whoever designed the protocol had to define a way for you to know how many bytes to read. If you have a specification, it probably covers that question.

So, there is either a way to determine number of bytes beforehand or to detect the end of transmission, e.g. by the existance of a special end character.

Without some sort of specification, we can only guess what the protocol is.

• The response message size is apparently not fixed. Maybe there is a way to ask the device "what would be the length of the answer to getversion"?
• Some protocols would prefix each message with the length information. Here there is none. Perhaps you can put the device in a different mode where it deos something like that by sending it some special command?
• Your message does not look like it has as the end marked, but perhaps it is just not visible, e.g. might there be a null character ('\0') at the end? If there is one, you could read character-by-character until it appears.
• Failing to find any other solution, you can try setting a reasonable timeout on reads (ser = serial.Serial(..., timeout=2, ...)). Then try to read everything. When there is nothing more to read, the read function will freeze indefinitely, unless there is a timeout. If you set a reasonably long timeout and no date is received in that time, you can assume the transmission is over.