Another good reason for delaying an import is when the import is only needed for testing.

In module_x
...
def callable_x(parent)  # htest #
    from tkinter import Toplevel
    box = Toplevel(parent)
    ...
if __name__ == '__main__':
    from unittest import main
    main('idlelib.idle_test.test_module_x', verbosity=2)

    from idlelib.idle_test.htest import run
    run(callable_x)

This should be part of 'Import Standards' in idlelib.README.