write a video file opencv pycharm mp4

edit

i'm trying to save my video to an mp4 file but at the moment to see the properties of the file saved it has only a few bytes. I proved to save in format .avi and in the folder always has 5.55 Kb. In the directory of pyCharm it shows an interrogation sign (?) I'm working with opencv-python 4.2. Sorry, i'm new in opencv.

import cv2
import numpy as np
import matplotlib.pyplot as plt

img = cv2.VideoCapture('SofiPepsi.mp4')
out = cv2.VideoWriter('output.avi', cv2.VideoWriter_fourcc(*'XVID'), 25, (320,240))

while(1):
    ret, frame = img.read()
    gray = cv2.cvtColor(frame, cv2.COLOR_BGR2GRAY)
    out.write(frame)
    cv2.imshow('gray', gray)
    cv2.imshow('frame', frame)
    if cv2.waitKey(25) & 0xFF == ord('q'):
        break


img.release()
out.release()
cv2.destroyAllWindows()