Python – pySerial program doesn’t read serial correctly

arduinopyserialpythonserial-portwindows

I have a problem using pySerial, and I don´t know from where to start looking for.
I have a 64 bits Windows Seven OS, with Python 2.7.5 (32 bits), and pySerial and Arduino (Arduino working correctly) already installed.

My Arduino code is the following:

// the setup routine runs once when you press reset:
void setup() {                
  // initialize the serial in 19200 baud rate
  Serial.begin(19200);     
}

// the loop routine runs over and over again forever:
void loop() {
  delay(1000);               // wait for a second
  Serial.print("hello");
}

(Arduino conected in COM8, when using Serial Monitor I can see it saluting)

And my PySerial code looks like this:

import serial
import time

arduino = serial.Serial("COM8", 19200)
time.sleep(2)  

while True:
    print arduino.readline()

When I start this script, the program runs, but I can´t see the serial output (I think the configuration in the Python script is OK because if something – e.g. the port – is wrong, it crashes).

I don´t know what to do to find a solution.
Can you help me?

Best Answer

You might try using println instead of print on the Arduino/C side, and/or set a timeout for the serial read on the Python side.

Since serial.readline() waits for a \n, and you never send one using print, the serial read will just wait for a timeout. (But it's a bit more complicated than this, and it's worth reading the docs on readline and EOL.)

If this doesn't work, at least switch readline to just read and print out each character you may (or may not) be reading, but don't make it more complicated by waiting for the \n that readline requires.

From the demo docs:
Be carefully when using readline(). Do specify a timeout when opening the serial port otherwise it could block forever if no newline character is received. Also note that readlines() only works with a timeout. readlines() depends on having a timeout and interprets that as EOF (end of file). It raises an exception if the port is not opened correctly.

Related Solutions

Python – How to execute a program or call a system command

Use the subprocess module in the standard library:

import subprocess
subprocess.run(["ls", "-l"])

The advantage of subprocess.run over os.system is that it is more flexible (you can get the stdout, stderr, the "real" status code, better error handling, etc...).

Even the documentation for os.system recommends using subprocess instead:

The subprocess module provides more powerful facilities for spawning new processes and retrieving their results; using that module is preferable to using this function. See the Replacing Older Functions with the subprocess Module section in the subprocess documentation for some helpful recipes.

On Python 3.4 and earlier, use subprocess.call instead of .run:

subprocess.call(["ls", "-l"])

Python – pyserial – How to read the last line sent from a serial device

Perhaps I'm misunderstanding your question, but as it's a serial line, you'll have to read everything sent from the Arduino sequentially - it'll be buffered up in the Arduino until you read it.

If you want to have a status display which shows the latest thing sent - use a thread which incorporates the code in your question (minus the sleep), and keep the last complete line read as the latest line from the Arduino.

Update: mtasic's example code is quite good, but if the Arduino has sent a partial line when inWaiting() is called, you'll get a truncated line. Instead, what you want to do is to put the last complete line into last_received, and keep the partial line in buffer so that it can be appended to the next time round the loop. Something like this:

def receiving(ser):
    global last_received

    buffer_string = ''
    while True:
        buffer_string = buffer_string + ser.read(ser.inWaiting())
        if '\n' in buffer_string:
            lines = buffer_string.split('\n') # Guaranteed to have at least 2 entries
            last_received = lines[-2]
            #If the Arduino sends lots of empty lines, you'll lose the
            #last filled line, so you could make the above statement conditional
            #like so: if lines[-2]: last_received = lines[-2]
            buffer_string = lines[-1]

Regarding use of readline(): Here's what the Pyserial documentation has to say (slightly edited for clarity and with a mention to readlines()):

Be careful when using "readline". Do specify a timeout when opening the serial port, otherwise it could block forever if no newline character is received. Also note that "readlines()" only works with a timeout. It depends on having a timeout and interprets that as EOF (end of file).

which seems quite reasonable to me!

Best Answer

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