Like import from ImageMagick, but for video. Because someone wondered if this exists - I don't know if it did, but now it does. No options. Requires ffmpeg and python-xlib.

importv.py

import Xlib.display
from Xlib import X, Xcursorfont

def XSelectWindow(dpy):
        screen = dpy.screen()
        root = screen.root
        gc = root.create_gc(background=screen.black_pixel, foreground=screen.white_pixel, function=X.GXinvert, subwindow_mode=X.IncludeInferiors)
        font = dpy.open_font('cursor')
        cursor = font.create_glyph_cursor(font, Xcursorfont.crosshair, Xcursorfont.crosshair+1, (65535, 65535, 65535), (0, 0, 0))
        try:
                root.grab_pointer(0, X.ButtonPressMask|X.ButtonReleaseMask|X.ButtonMotionMask, X.GrabModeSync, X.GrabModeAsync, root, cursor, X.CurrentTime)
                try:
                        dpy.grab_server()
                        start = None
                        w = h = 0
                        pressed = 0
                        while start is None or pressed:
                                rect = w*w + h*h > 9
                                # Draw rectangle
                                if rect:
                                        root.rectangle(gc, x, y, w, h)

                                dpy.allow_events(X.SyncPointer, X.CurrentTime)
                                # Fetch a button press/release or the most recent MotionNotify
                                ev = dpy.next_event()
                                for i in range(dpy.pending_events()):
                                        if ev.type in (X.ButtonPress, X.ButtonRelease):
                                                break
                                        # I'm not sure if we can receive any other events, but it doesn't hurt to be safe
                                        ev_ = dpy.next_event()
                                        if ev_.type in (X.ButtonPress, X.ButtonRelease, X.MotionNotify):
                                                ev = ev_

                                # Clear rectangle
                                if rect:
                                        root.rectangle(gc, x, y, w, h)

                                if ev.type == X.ButtonPress:
                                        start = x, y = ev.event_x, ev.event_y
                                        pressed += 1
                                elif ev.type == X.ButtonRelease:
                                        pressed -= 1
                                elif ev.type == X.MotionNotify:
                                        (x, w), (y, h) = sorted((start[0], ev.event_x)), sorted((start[1], ev.event_y))
                                        w -= x
                                        h -= y
                        # Make sure rectangle clear goes through (not quite sure why the rectangle persists [at least for me] without this. python-xlib doesn't flush on quit?)
                        dpy.flush()
                finally:
                        dpy.ungrab_server()
        finally:
                dpy.ungrab_pointer(X.CurrentTime)
        return x, y, w+1, h+1

if __name__ == '__main__':
        import sys, os

        if len(sys.argv) != 2:
                print('usage: %s FILE' % sys.argv[0], file=sys.stderr)
                sys.exit(-1)

        dpy = Xlib.display.Display()
        x, y, w, h = XSelectWindow(dpy)

        display = dpy.get_display_name()
        fname = sys.argv[1]

        os.execvp('ffmpeg', ['ffmpeg', '-video_size', '%dx%d' % (w, h), '-framerate', '25', '-f', 'x11grab', '-i', '%s+%d,%d' % (display, x, y), fname])