Tests are how one constrains dynamically typed programs to make sure they work as expected. I have found that adding tests to untyped python code is far better at exposing bugs than adding typing is. I like my IDE to do completion too but I tend to add typing where I think it helps most rather than everywhere.

It just seems that some people like static types and good for them. Dynamic languages are for those of us who are, perhaps, lazy and want to be free to think about how to solve the problem at a slightly higher level without having to get into details.

For me the details activate my perfectionism and that blocks me from advancing with thinking about the goal and the effort of creating an elegant set of types makes me less willing to suddenly throw everything away when I realise there's a better approach. Dynamic languages give me that ability to "sketch" until I have an answer and at that point I have to ask: do I really need this to be in a higher performance language? Once or twice the answer has been yes but working out the approach in a dynamic language allowed me to do it right in the static language first time.