Why does the drawn picture disappear when using win32gui.SetPixel?

Question

Python

There is a simple program:

import win32gui as win
import win32api

comp = win.GetDC(0)
color = win32api.RGB(255,255,255)

for i in range(300, 320):
    for t in range(300, 320):
        win.SetPixel(comp, i, t, color)

But when she draws a square, it disappears after a few seconds, what should I do?