Python urllib.request.urlopen() only return when given 'timeout' parameter, otherwise freeze forever

I'd encountered a strange problem when using urllib.request.urlopen function. When I call the function without given the timeout parameter, it just freeze forever (or really really long time), but once I provide the timeout parameter, eg. timeout = 1, the function just return exactly after the interval of timeout without any problem.

As shown in the gif below. (I was using Ipython prompt)