Stop a while loop by escape key?

Unfortunately it's pretty difficult to listen for keypresses unless you expect one for every iteration of your loop, which for what you're doing is impractical. I think Joel has it right in the comments, you should be using

Ctrl+C

and catching the KeyboardInterrupt like this

from PIL import ImageGrab
import numpy as np
import cv2
import ctypes
user32 = ctypes.windll.user32
screensize = user32.GetSystemMetrics(0), user32.GetSystemMetrics(1)

def record_screen():

    fourcc = cv2.VideoWriter_fourcc(*'XVID')
    out = cv2.VideoWriter('ResultFile.avi', fourcc, 25.0, screensize)

    while True:
        try:
            img = ImageGrab.grab()
            img_np = np.array(img)
            frame = cv2.cvtColor(img_np, cv2.COLOR_BGR2RGB)
            out.write(frame)
            print('recording....')
        except KeyboardInterrupt:
            break

    out.release()
    cv2.destroyAllWindows()


record_screen()

This way, the KeyboardInterrupt won't terminate the program, it'll just end the while loop, allowing you to release your writer and cleanup the rest of the cv2 resources.

Because you're using PyCharm as your IDE, Ctrl+C might not work for you - try Ctrl+F2 instead.

You should make a variable called running! Set 'running = True' before the while loop. And instead of the while loop being 'while True:', make it 'while running = True:'. Finally, in the while loop, if you hit ESC, set 'running = False'

Here is an example with pygame:

import pygame
pygame.init()

def record():

    # init
    running = True

    while running == True:

        # record

        events = pygame.event.get()

        for event in events:

            if event.type == pygame.K_ESCAPE:

                running = False

    quit()

record()