Python byte array for Serial communication

Question

i got a program which needs to send a byte array via a serial communication. And I got no clue how one can make such a thing in python. I found a c/c++/java function which creates the needed byte array:

byte[] floatArrayToByteArray(float[] input)
{
 int len = 4*input.length;
 int index=0;
 byte[] b = new byte[4];
 byte[] out = new byte[len];
 ByteBuffer buf = ByteBuffer.wrap(b);
 for(int i=0;i<input.length;i++) 
 {
 buf.position(0);
 buf.putFloat(input[i]);
 for(int j=0;j<4;j++) out[j+i*4]=b[3-j];
 }
 return out;
}

but how can I translate that to python code. edit: the serial data is send to a device. where I can not change the firmware. thanks

• samgak: That looks like Java code, not C++
• zehnpaard: Must it be bytearrays, or can it be bytes? If bytes is ok you probably want the struct module.
• zehnpaard: Also it would be good to have sample input/output.
• user2206668: it might be java sry. well since the code has to be send to a device which expects exactly that format it has to be that byte array

Answer 1

Put your data to array (here are [0,1,2] ), and send with: serial.write(). I assume you've properly opened serial port.

>> import array
>> tmp = array.array('B', [0x00, 0x01, 0x02]).tostring()
>> ser.write(tmp.encode())

Ansvered using: Binary data with pyserial(python serial port) and this:pySerial write() won't take my string

• Basya: Thank you. This helped me. However, I had to remove the call to "encode"; it was not recognized. I just passed the array. I don't know -- maybe it has to do with different versions of Python.

Answer 2

It depends on if you are sending a signed or unsigned and other parameters. There is a bunch of documentation on this. This is an example I have used in the past.

x1= 0x04
x2 = 0x03
x3 = 0x02
x4 = x1+ x2+x3
input_array = [x1, x2, x3, x4]
write_bytes = struct.pack('<' + 'B' * len(input_array), *input_array) 
ser.write(write_bytes)

To understand why I used 'B' and '<' you have to refer to pyserial documentation.

https://docs.python.org/2/library/struct.html