`time.sleep` in python script reading serial output causes erratic behavior

Question

I am attempting to control the position of my computer's cursor using a potentiometer hooked up to my arduino nano. The arduino is running the following code:

int potPin = 3; // select the input pin for the potentiometer
int val = 0; // variable to store the value coming from the sensor
void setup() {
 Serial.begin(9600);
}
void loop() {
 val = analogRead(potPin); // read the value from the sensor
 Serial.println(val);
}

When I ran the following python script, it worked properly and the sensor values were printing out in my terminal.

import serial
import pyautogui
ser = serial.Serial("/dev/cu.usbserial-141220", 9600, timeout=0.1)
while True:
 data = ser.readline().decode("UTF-8").strip()
 if data:
 print(data)

However, when I added a small time delay in the while loop, the output would either get stuck on a single value or take multiple seconds to reflect a change to the potentiometer.

import serial
import pyautogui
import time
ser = serial.Serial("/dev/cu.usbserial-141220", 9600, timeout=0.1)
while True:
 data = ser.readline().decode("UTF-8").strip()
 if data:
 print(data)
 time.sleep(0.01)

I'd appreciate any help fixing this issue.

• chrisl: What is a small delay for you? 100ms? You are sending the values as fast as the Arduino can.
• Manav Bokinala: @chrisl the issue happens even with a delay in the python while loop of 10ms. I added the code with the loop to my question.
• timemage: @ManavBokinala, Like chrisl said, you're sending as fast as you can. What keeps approximately 9.6 characters per iteration from piling up in the host's inbound buffer?
• timemage: You may find printing ser.in_waiting inside your loop enlightening.
• Manav Bokinala: @timemage Thank you so much, this solved my problem! I'm new to all of this, so I did not understand the concept of the inbound buffer. Your comment made things clear for me. I really appreciate it.

Answer 1

Thanks to chrisl and timemage's comments, I figured out the issue: the Arduino was sending the data too fast and the bytes were piling up in the computer's inbound buffer. I solved this by adding a delay to the loop() function of the Arduino code.