Program not writing to list from while loop?

Question

I'm trying to write values to my VideoFlag list when the pixel intensity difference is higher than a predefined threshold. On output however my output 'flag.txt' file is empty, and I'm not sure why. Does anyone know in what way my code is wrong?

Thanks!

import cv2
import tkinter as tk
from tkinter.filedialog import askopenfilename
import numpy as np 
import os
import matplotlib.pyplot as plt 

MIList =[]
VideoFlag=[]

def frame_diff(prev_frame, cur_frame, next_frame):
    diff_frames1 = cv2.absdiff(next_frame, cur_frame)

    diff_frames2 = cv2.absdiff(cur_frame, prev_frame)

    return cv2.bitwise_and(diff_frames1, diff_frames2)

def get_frame(cap):
    ret, frame = cap.read()
    if ret == True:
        scaling_factor = 1
        frame = cv2.resize(frame, None, fx = scaling_factor, fy = scaling_factor, interpolation = cv2.INTER_AREA)
        return cv2.cvtColor(frame, cv2.COLOR_RGB2GRAY)

def moving_average(MIList, n=30) :
    ret = np.cumsum(MIList, dtype=float)
    ret[n:] = ret[n:] - ret[:-n]
    return ret[n - 1:] / n

def main():

    root = tk.Tk()
    root.withdraw()

    selectedvideo = askopenfilename()
    cap = cv2.VideoCapture(selectedvideo)
    length = cap.get(cv2.CAP_PROP_FRAME_COUNT)
    intlength = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))
    currentframenumber = cap.get(cv2.CAP_PROP_POS_FRAMES)
    intcurrentframenumber = int(cap.get(cv2.CAP_PROP_POS_FRAMES))
    scaling_factor = 1
    fourcc = cv2.VideoWriter_fourcc(*'XVID')
    out = cv2.VideoWriter((selectedvideo + 'motionindexed.avi'),fourcc, 60.0, (640,478), isColor=False)
    with open((selectedvideo + 'threshold' + '.txt'), 'r') as readthreshold:
        threshold = float(readthreshold.readline())

    prev_frame = get_frame(cap)
    cur_frame = get_frame(cap)
    next_frame = get_frame(cap)

    while (cap.isOpened()):

        try:
            cv2.imshow("Object Movement", frame_diff(prev_frame, cur_frame, next_frame))
            prev_frame = cur_frame
            cur_frame = next_frame
            next_frame = get_frame(cap)
            differencesquared = (next_frame-cur_frame)**2
            interframedifference = np.sum(differencesquared)
            MIList.append(interframedifference)
            print(interframedifference)
            if interframedifference >= threshold:
                out.write(cur_frame)
                VideoFlag.append(str(intcurrentframenumber + '|' + 1))
                print(VideoFlag)
            elif interframedifference < threshold:
                VideoFlag.append(str(intcurrentframenumber + '|' + 0))
                print(VideoFlag)

            key = cv2.waitKey(1)
            if key == ord('q'):
                break
        except:
            break

    with open((selectedvideo + 'flag' + '.txt'), 'w') as f:
        for item in VideoFlag:
            f.write(str(item))


    cap.release()
    cv2.destroyAllWindows()

if __name__ == '__main__':
    # this is called if this code was not imported ... ie it was directly run
    # if this is called, that means there is no GUI already running, so we need to create a root
    root = tk.Tk()
    root.withdraw()
    main()

Answer 1

Replacing

with open((selectedvideo + 'flag' + '.txt'), 'w') as f:
    for item in VideoFlag:
        f.write(str(item))

with

# Note that you need to append('a') data to the file instead of writing('w') to it for each iteration. 
# The last line will be empty string and that is what contains finally.
with open((selectedvideo + 'flag' + '.txt'), 'a') as f: 
    for item in VideoFlag:
        f.write(str(item))

shall resolve the problem.

Answer 2

I think I've solved my problem - so for whatever reason it wasn't running due to my types in my append method - I'd forgotten to convert one of my integers to a string I think, I've reworked it and I think that's solved my issue! Cheers for the input guys!

import cv2
import tkinter as tk
from tkinter.filedialog import askopenfilename
import numpy as np 
import os
import matplotlib.pyplot as plt 

def frame_diff(prev_frame, cur_frame, next_frame):
    diff_frames1 = cv2.absdiff(next_frame, cur_frame)

    diff_frames2 = cv2.absdiff(cur_frame, prev_frame)

    return cv2.bitwise_and(diff_frames1, diff_frames2)

def get_frame(cap):
    ret, frame = cap.read()
    if ret == True:
        scaling_factor = 1
        frame = cv2.resize(frame, None, fx = scaling_factor, fy = scaling_factor, interpolation = cv2.INTER_AREA)
        return cv2.cvtColor(frame, cv2.COLOR_RGB2GRAY)

def main():

    root = tk.Tk()
    root.withdraw()

    MIList = []
    VideoFlag = []
    selectedvideo = askopenfilename()
    cap = cv2.VideoCapture(selectedvideo)
    length = cap.get(cv2.CAP_PROP_FRAME_COUNT)
    intlength = int(cap.get(cv2.CAP_PROP_FRAME_COUNT))
    currentframenumber = cap.get(cv2.CAP_PROP_POS_FRAMES)
    intcurrentframenumber = int(cap.get(cv2.CAP_PROP_POS_FRAMES))
    scaling_factor = 1
    fourcc = cv2.VideoWriter_fourcc(*'XVID')
    out = cv2.VideoWriter((selectedvideo + 'motionindexed.avi'),fourcc, 60.0, (640,478), isColor=False)
    with open((selectedvideo + 'threshold' + '.txt'), 'r') as readthreshold:
        threshold = float(readthreshold.readline())

    prev_frame = get_frame(cap)
    cur_frame = get_frame(cap)
    next_frame = get_frame(cap)

    while (cap.isOpened()):

        try:
            cv2.imshow("Object Movement", frame_diff(prev_frame, cur_frame, next_frame))
            prev_frame = cur_frame
            cur_frame = next_frame
            next_frame = get_frame(cap)
            differencesquared = (next_frame-cur_frame)**2
            interframedifference = np.sum(differencesquared)
            MIList.append(interframedifference)
            print(interframedifference)
            if interframedifference >= threshold:
                out.write(cur_frame)
                VideoFlag.append((str(intcurrentframenumber) + ' ' + '|' + ' ' + '1' + '\n' ))


            elif interframedifference < threshold:
                VideoFlag.append((str(intcurrentframenumber) + ' ' + '|' + ' ' + '0' + ' \n'))



            key = cv2.waitKey(1)
            if key == ord('q'):
                break
        except:
            break

    with open((selectedvideo + 'flag' + '.txt'), 'w') as f:
        for item in VideoFlag:
            f.write((str(item) + '\n'))
        print(VideoFlag)


    cap.release()
    cv2.destroyAllWindows()

if __name__ == '__main__':
    # this is called if this code was not imported ... ie it was directly run
    # if this is called, that means there is no GUI already running, so we need to create a root
    root = tk.Tk()
    root.withdraw()
    main()