def install_except_hook(except_hook=_hooks.except_hook):
    """
    Install an except hook that will show the crash report dialog when an
    unhandled exception has occured.

    :param except_hook: except_hook function that will be called on the main
        thread whenever an unhandled exception occured. The function takes
        two parameters: the exception object and the traceback string.
    """
    if not _backends:
        raise ValueError('no backends found, you must at least install one '
                         'backend before calling this function')
    global _except_hook
    _except_hook = _hooks.QtExceptHook(except_hook)