Reading json object from serial port

Question

I want to use ijson to read an object from a serial port. I can read from the port fine, and I can use ijson to deserialize an object from a stream, but using ijson from a serial port just isn't enumerating anything.

This is an example of my code:

    self.serial_port = serial.Serial(
        port=self.port_name, \
        baudrate=115200, \
        parity=serial.PARITY_NONE, \
        stopbits=serial.STOPBITS_ONE, \
        bytesize=serial.EIGHTBITS, \
        timeout=None)
    print 'start reading'
    parser = ijson.parse(self.serial_port)

    for prefix, event, value in parser:
        print `value`
    print 'stop reading'

and my output is just

    start reading

Dennis Shtatnov · Accepted Answer · 2016-07-02 23:26:21Z

1

Try doing:

import ijson
import sys
for x in ijson.parse(sys.stdin):
    print(x)

You can type in json but nothing will print out until you press Ctrl-D twice to signal the end of stdin.

I haven't tested it, but I suspect that something similar is happening with your serial port: because your serial port never closes, ijson doesn't know when to start parsing. Depending on your serial libraries, I would try to send an EOF or EOT character to python or separately find a way to make it trigger the end of the stream.

Some options I can think off:

Read a single line separately from ijson and then parse it
Set a timeout or a inter_byte_timeout to only read the first segment of json from the stream

edited Jul 2, 2016 at 23:26

answered Jul 2, 2016 at 23:21

Dennis Shtatnov

1,38311 silver badges17 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Adam Over a year ago

Thanks Dennis. I can't rely of EOS/EOT however as I intend to keep streaming objects. Good suggestion though. The solution was just to drop the buffer size to 1.

Adam · Accepted Answer · 2016-07-03 00:23:04Z

0

Ok, I figured it out. You have to set the buffer size to 1. So, this works...

objects = ijson.common.items(ijson.parse(self.serial_port, buf_size=1), 'item')
    for o in objects:
        print `o`

My goal was to be able to receive an endless stream of objects. I signal the start of the stream with "[", then send a comma separated list of json objects, and terminate the list with "]". Every time an object is completed within the array, the iterator loops.

Works perfectly.

Test by sending it this (intentional newline in the middle of the object):

[{"foo": "bar"}, {"bar":
"foo"}]

answered Jul 3, 2016 at 0:23

Adam

4,2674 gold badges34 silver badges55 bronze badges

Collectives™ on Stack Overflow

Reading json object from serial port

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related