import tkinter as tk
WIDTH, HEIGHT = 400, 300  # Defines aspect ratio of window.

def maintain_aspect_ratio(event, aspect_ratio):
    """ Event handler to override root window resize events to maintain the
        specified width to height aspect ratio.
    """
    if event.widget.master:  # Not root window?
        return  # Ignore.

    # <Configure> events contain the widget's new width and height in pixels.
    new_aspect_ratio = event.width / event.height

    # Decide which dimension controls.
    if new_aspect_ratio > aspect_ratio:
        # Use width as the controlling dimension.
        desired_width = event.width
        desired_height = int(event.width / aspect_ratio)
    else:
        # Use height as the controlling dimension.
        desired_height = event.height
        desired_width = int(event.height * aspect_ratio)

    # Override if necessary.
    if event.width != desired_width or event.height != desired_height:
        # Manually give it the proper dimensions.
        event.widget.geometry(f'{desired_width}x{desired_height}')
        return "break"  # Block further processing of this event.

root = tk.Tk()
root.geometry(f'{WIDTH}x{HEIGHT}')
root.bind('<Configure>', lambda event: maintain_aspect_ratio(event, WIDTH/HEIGHT))
root.mainloop()