Python Serial: How to use the read or readline function to read more than 1 character at a time

Question

I'm having trouble to read more than one character using my program, I can't seem to figure out what went wrong with my program.

import serial

ser = serial.Serial(
    port='COM5',\
    baudrate=9600,\
    parity=serial.PARITY_NONE,\
    stopbits=serial.STOPBITS_ONE,\
    bytesize=serial.EIGHTBITS,\
        timeout=0)

print("connected to: " + ser.portstr)
count=1

while True:
    for line in ser.read():

        print(str(count) + str(': ') + chr(line) )
        count = count+1

ser.close()

here are the results I get

connected to: COM5
1: 1
2: 2
3: 4
4: 3
5: 1

actually I was expecting this

connected to: COM5
1:12431
2:12431

something like the above mentioned which is able read multiple characters at the same time not one by one.

https://stackoverflow.com/questions/59564715/serial-port-getting-engaged-after-reading-single-command-in-python Please can anyone guide me on this — nishant, Jan 04 '20 at 08:16
Of all the documentation I found, nobody specified that I should be writing "port = 'COM5'" instead of just dev/usb0 or similar. Thank you! — IcanCwhatUsay, Jun 14 '20 at 17:36

score 36 · Answer 1 · answered Apr 18 '13 at 18:34

36

I see a couple of issues.

First:

ser.read() is only going to return 1 byte at a time.

If you specify a count

ser.read(5)

it will read 5 bytes (less if timeout occurrs before 5 bytes arrive.)

If you know that your input is always properly terminated with EOL characters, better way is to use

ser.readline()

That will continue to read characters until an EOL is received.

Second:

Even if you get ser.read() or ser.readline() to return multiple bytes, since you are iterating over the return value, you will still be handling it one byte at a time.

Get rid of the

for line in ser.read():

and just say:

line = ser.readline()

answered Apr 18 '13 at 18:34

jwygralak67

924
7
13

Hi, when I'm using line=ser.readline() , it always ask me for an integer print(str(count) + str(': ') + chr(line) ) TypeError: an integer is required – user2294001 Apr 19 '13 at 02:18
1

chr(line) is expecting an integer argument. You're giving it a string. – jwygralak67 Apr 19 '13 at 14:28

score 20 · Answer 2 · edited Sep 09 '18 at 02:19

20

I use this small method to read Arduino serial monitor with Python

import serial
ser = serial.Serial("COM11", 9600)
while True:
     cc=str(ser.readline())
     print(cc[2:][:-5])

edited Sep 09 '18 at 02:19

Stephen Rauch

47,830
31
106
135

answered Sep 09 '18 at 01:57

Thushara Madushan

231
2
4

Nice, I needed to clean that response, good one! I think for future readers it's worth mentioning in your answer what exactly your code performs. – brasofilo May 06 '19 at 04:47
5

I assume you use `cc[2:][:-5]` to strip the `b'` and `\r\n'`. Wouldn't it be better to use `.decode("utf-8")` for this? – Wouter Sep 29 '20 at 09:14

score 12 · Answer 3 · edited Jun 22 '18 at 09:00

12

Serial sends data 8 bits at a time, that translates to 1 byte and 1 byte means 1 character.

You need to implement your own method that can read characters into a buffer until some sentinel is reached. The convention is to send a message like 12431\n indicating one line.

So what you need to do is to implement a buffer that will store X number of characters and as soon as you reach that \n, perform your operation on the line and proceed to read the next line into the buffer.

Note you will have to take care of buffer overflow cases i.e. when a line is received that is longer than your buffer etc...

EDIT

import serial

ser = serial.Serial(
    port='COM5',\
    baudrate=9600,\
    parity=serial.PARITY_NONE,\
    stopbits=serial.STOPBITS_ONE,\
    bytesize=serial.EIGHTBITS,\
        timeout=0)

print("connected to: " + ser.portstr)

#this will store the line
line = []

while True:
    for c in ser.read():
        line.append(c)
        if c == '\n':
            print("Line: " + ''.join(line))
            line = []
            break

ser.close()

edited Jun 22 '18 at 09:00

Joe

12,057
5
39
55

answered Apr 18 '13 at 08:27

1337holiday

1,924
1
24
42

Hi, could you give me some guide on the buffering? I'm really confused right now. – user2294001 Apr 19 '13 at 02:30
I edited the code, take a look. Just remember one thing, the code above will KEEP reading into the `line` array until it sees a `\n` character so make sure in your serial device sends a `\n`. – 1337holiday Apr 19 '13 at 06:50
Hi, the codes aboves doesn't seems to work. it says char is not defined. – user2294001 Apr 26 '13 at 00:36
Try now, i had char when i should have had 'c', just make sure ur serial program does indeed send '\n' characters after each line – 1337holiday Apr 26 '13 at 02:18
4

"You need to implement your own method that can read characters into a buffer until some sentinel is reached" - isn't this *exactly* what `serial.readline` does? – paxdiablo Sep 13 '16 at 02:43

Desprit · Answer 4 · 2014-04-21T11:39:58.973

6

I was reciving some date from my arduino uno (0-1023 numbers). Using code from 1337holiday, jwygralak67 and some tips from other sources:

import serial
import time

ser = serial.Serial(
    port='COM4',\
    baudrate=9600,\
    parity=serial.PARITY_NONE,\
    stopbits=serial.STOPBITS_ONE,\
    bytesize=serial.EIGHTBITS,\
        timeout=0)

print("connected to: " + ser.portstr)

#this will store the line
seq = []
count = 1

while True:
    for c in ser.read():
        seq.append(chr(c)) #convert from ANSII
        joined_seq = ''.join(str(v) for v in seq) #Make a string from array

        if chr(c) == '\n':
            print("Line " + str(count) + ': ' + joined_seq)
            seq = []
            count += 1
            break


ser.close()

edited Apr 21 '14 at 11:39

answered Mar 25 '14 at 11:42

Desprit

707
2
11
24

4

I think your comment #convert from ansii should be #convert from ascii – Nato Saichek Oct 03 '15 at 17:44
This chr(c) made it for me after few days of turning around the problem. I have an ATTiny85 sending an int to an ATTiny45 sending this data to a Raspberry Pi 0. all using serial communication. I was just checking why it was displaying 48 when I was sending 0. ASCII was published in 1963. Always, back to the source. Thank you. – Jul 25 '21 at 18:28

Python Serial: How to use the read or readline function to read more than 1 character at a time

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