# Keeping an eye on a file where data lines are added.
# Uses Python file read.
# The file can be a log file, stock markets, electrity spot prices etc.
# The generator watch_file keeps reading the last line from file as soon as it appears.
# 
# The rest is for testing.
#
import numpy as np
import time
import threading 

def watch_file(filename,*args,delay=0.1):
    print('watch file',filename)
    while True:
        try:
            with open(filename,'r') as f:
                # starting from beginning (0), move file pointer to end of file (2)
                f.seek(0, 2) 
                while True:
                    line = f.readline()
                    if not line:
                        time.sleep(delay)
                        continue
                    yield line
        except:
            pass  # no file yet, retry

        
def analyze_data(filename):
    ave = 0.0
    N = 0
    for line in watch_file(filename):
        d = float(line) # or float(line.split()[0])
        ave += d
        N +=1
        print(f'got {d:10.5f} average {ave/N:10.5f} number of data {N}')

def add_new_data(filename):
        for _ in range(10):
            # append data to file
            with open(filename,'a') as f:
                f.write(f'{np.random.random():<10.5f}\n')
            time.sleep(1)
        
if __name__=='__main__':

    filename = 'random.dat'
    # with 2 threads:
    t1 = threading.Thread(target=add_new_data,args=(filename,)).start()
    t2 = threading.Thread(target=analyze_data,args=(filename,)).start()
    # feature: t1 writes one line *before* watch_file wakes up in t2 and moves past that line
    # feature: never stops, has to kill with ctrl-C
    #
    # with one thread:
    #t1 = threading.Thread(target=add_new_data,args=(filename,)).start()
    #analyze_data(filename) # executes forever
    # feature: t1 writes one line *before* watch_file wakes up in t2 and moves past that line
    # feature: never stops, won't die with ctrl-C, has to ctrl-z and kill %