Python - Live Graph Update From A Changing Text File

i have a thread that writes continuously into a text file for every 2 seconds. the same file is referenced by a Matplotlib graph (live updating graph). so when i start the script,

Solution 1:

The problem is unrelated to matplotlib, but rather it has to do with how you're reading and writing your data to the text file.

Python file objects are usually line-buffered by default, so when you call f.write(str(i)+','+str(j)+'\n') from inside your FileWriter thread, your text file is not immediately updated on disk. Because of this, open("sampleText.txt","r").read() is returning an empty string, and therefore you have no data to plot.

To force the text file to be updated "instantly", you could either call f.flush() immediately after writing to it, or you could set the buffer size to zero when you open the file, e.g. f = open('sampleText.txt', 'w', 0) (see this previous SO question as well).

Solution 2:

Its been a few years since this was posted, but I was working with this and I ran into an issue where I needed the graph to update after every cycle.

I tried to use ali_m's suggestion: f=open('./dynamicgraph.txt','a', 0), but there was an error with buffering "cannot be set to 0".

If you put a flush() function into the FileWriter while loop, it will update the graph after each cycle. Here is the complete code for the program which will draw a graph as it is running:

#!usr/bin/python3import matplotlib.pyplot as plt
import matplotlib.animation as animation
import time
from random import randrange
from threading import Thread

fig = plt.figure()
ax1 = fig.add_subplot(1,1,1)


defFileWriter():
    f=open('./dynamicgraph.txt','a')
    i=0whileTrue:
        i+=1
        j=randrange(10)
        data = f.write(str(i)+','+str(j)+'\n')
        print("wrote data")
        time.sleep(1)
        f.flush()


defanimate(i):
    pullData = open("dynamicgraph.txt","r").read()
    dataArray = pullData.split('\n')
    xar = []
    yar = []
    for eachLine in dataArray:
        iflen(eachLine)>1:
            x,y = eachLine.split(',')
            xar.append(int(x))
            yar.append(int(y))
    ax1.clear()
    ax1.plot(xar,yar)

defMain():
    t1=Thread(target=FileWriter)
    t1.start()
    ani = animation.FuncAnimation(fig, animate, interval=1000)
    plt.show()
    print("done")

Main()