On the newly-released macOS Sierra 10.12.5, the default web browser which is meant to returned by webbrowser.get() gives an error. Specifically:

    >>> import webbrowser
    >>> br = webbrowser.get()
    >>> br.open("http://python.org")
    0:33: execution error: "http://python.org" doesn’t understand the “open location” message. (-1708)
    False

    >>> ### but this works
    >>> br = webbrowser.get("safari")
    >>> br.open("http://python.org")
True