Fail connecting to DBus

[tj5527]

I am new to DBus, and am practicing using python dbus for fun. Following the code at https://stackoverflow.com/questions/33250864/getting-signals-working-on-pulseaudios-dbus-interface#33776607 When executing python script it complains

    dbus.exceptions.DBusException: org.freedesktop.DBus.Error.FileNotFound: Failed to connect to socket /run/user/1001/pulse/dbus-socket: No such file or directory

What might cause this error and how to fix that?

My environment is Ubuntu 16.04 LTS, python 3.5.2.

Thanks
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