Refresh JSON on Flask RESTful

Question

My script prints sensor information (temperature, humidity, barometric pressure) to an LCD screen and also sends it as JSON via Flask RESTful api for other devices to use. The python script below works fine, but it only runs once and the sensor info never updates.

How can I refresh the JSON on the RESTful api every, say, 30 seconds? (The most important elements of the script are at the bottom). I have tried making the TempHum class into a while loop, but the RESTful api doesn't like that at all. My question is also how to loop the script so that just the JSON updates. Where should I add the while loop or equivalent.

from flask import Flask
from flask_restful import Resource, Api
from Adafruit_BME280 import *
import RPi.GPIO as GPIO
import math, decimal
from datetime import datetime
import time

# Define GPIO to LCD mapping
LCD_RS = 7
LCD_E  = 8
LCD_D4 = 25
LCD_D5 = 24
LCD_D6 = 23
LCD_D7 = 18
LED_ON = 15

# Define some device constants
LCD_WIDTH = 20    # Maximum characters per line
LCD_CHR = True
LCD_CMD = False

LCD_LINE_1 = 0x80 # LCD RAM address for the 1st line
LCD_LINE_2 = 0xC0 # LCD RAM address for the 2nd line
LCD_LINE_3 = 0x94 # LCD RAM address for the 3rd line
LCD_LINE_4 = 0xD4 # LCD RAM address for the 4th line

# Timing constants
E_PULSE = 0.0005
E_DELAY = 0.0005

# Main program block

GPIO.setwarnings(False)
GPIO.setmode(GPIO.BCM)       # Use BCM GPIO numbers
GPIO.setup(17, GPIO.IN)      # Pin 17 for reset
GPIO.setup(LCD_E, GPIO.OUT)  # E
GPIO.setup(LCD_RS, GPIO.OUT) # RS
GPIO.setup(LCD_D4, GPIO.OUT) # DB4
GPIO.setup(LCD_D5, GPIO.OUT) # DB5
GPIO.setup(LCD_D6, GPIO.OUT) # DB6
GPIO.setup(LCD_D7, GPIO.OUT) # DB7
GPIO.setup(LED_ON, GPIO.OUT) # Backlight enable

sensor = BME280(mode=BME280_OSAMPLE_8)
in_degrees = sensor.read_temperature()
in_pascals = sensor.read_pressure()
in_hectopascals = in_pascals / 100
in_kilopascals = in_hectopascals / 10
in_humidity = sensor.read_humidity()
in_degrees_short = int(round(sensor.read_temperature(), 1))
in_degrees_short_f = (int(round(sensor.read_temperature(), 1)) * 1.8) + 32
in_humidity_short = int(round(sensor.read_humidity(), 1))

app = Flask(__name__)
api = Api(app)

def sleep():
        time.sleep(5)

def lcd_init():
  # Initialise display
  lcd_byte(0x33,LCD_CMD) # 110011 Initialise
  lcd_byte(0x32,LCD_CMD) # 110010 Initialise
  lcd_byte(0x06,LCD_CMD) # 000110 Cursor move direction
  lcd_byte(0x0C,LCD_CMD) # 001100 Display On,Cursor Off, Blink Off
  lcd_byte(0x28,LCD_CMD) # 101000 Data length, number of lines, font size
  lcd_byte(0x01,LCD_CMD) # 000001 Clear display
  time.sleep(E_DELAY)

def lcd_byte(bits, mode):
  # Send byte to data pins
  # bits = data
  # mode = True  for character
  #        False for command

  GPIO.output(LCD_RS, mode) # RS

  # High bits
  GPIO.output(LCD_D4, False)
  GPIO.output(LCD_D5, False)
  GPIO.output(LCD_D6, False)
  GPIO.output(LCD_D7, False)
  if bits&0x10==0x10:
    GPIO.output(LCD_D4, True)
  if bits&0x20==0x20:
    GPIO.output(LCD_D5, True)
  if bits&0x40==0x40:
    GPIO.output(LCD_D6, True)
  if bits&0x80==0x80:
    GPIO.output(LCD_D7, True)

  # Toggle 'Enable' pin
  lcd_toggle_enable()

  # Low bits
  GPIO.output(LCD_D4, False)
  GPIO.output(LCD_D5, False)
  GPIO.output(LCD_D6, False)
  GPIO.output(LCD_D7, False)
  if bits&0x01==0x01:
    GPIO.output(LCD_D4, True)
  if bits&0x02==0x02:
    GPIO.output(LCD_D5, True)
  if bits&0x04==0x04:
    GPIO.output(LCD_D6, True)
  if bits&0x08==0x08:
    GPIO.output(LCD_D7, True)

  # Toggle 'Enable' pin
  lcd_toggle_enable()

def lcd_toggle_enable():
  # Toggle enable
  time.sleep(E_DELAY)
  GPIO.output(LCD_E, True)
  time.sleep(E_PULSE)
  GPIO.output(LCD_E, False)
  time.sleep(E_DELAY)

def lcd_string(message,line,style):
  # Send string to display
  # Style=1 Left justified
  # Style=2 Centred
  # Style=3 Right justified

  if style==1:
          message = message.ljust(LCD_WIDTH, " ")
  elif style==2:
          message = message.center(LCD_WIDTH, " ")
  elif style==3:
          message = message.rjust(LCD_WIDTH, " ")

  message = message.ljust(LCD_WIDTH," ")

  lcd_byte(line, LCD_CMD)

  for i in range(LCD_WIDTH):
    lcd_byte(ord(message[i]),LCD_CHR)

def lcd_backlight(flag):
        #Toggle backlight on-off-on
        GPIO.output(LED_ON, flag)

class TempHum(Resource):
    lcd_init()
    lcd_string("Temperature: " + str(round(sensor.read_temperature(), 1)) + "C",LCD_LINE_1,2) 
    lcd_string("Humidity: " + str(round(sensor.read_humidity(), 1)) + "% ",LCD_LINE_2,2)
    lcd_string("Pressure: " + str(round(in_kilopascals, 1)) + "kPa",LCD_LINE_3,2)
    def get(self):
         temp = round(in_degrees, 1)
         humi = round(in_humidity, 1)
         pres = round(in_kilopascals, 1)
         return {'Temperature' : temp,
                 'Humidity' : humi,
                 'Pressure' : pres 
                 }

api.add_resource(TempHum, '/') 

if __name__ == "__main__":
    app.run(host='0.0.0.0', port=5000, debug=True)

Answer 1

If you want data to update for every GET request to the API, you need to move all sensor.read_* method calls into the get function. Thereby, reading the sensor for every time a request is made.

To refresh a chart or web page, you put the loop there, on the UI, not in the server code. Regarding the LCD screen, that could be a loop, but you should be using a separate thread than Flask's for it