Just one word: typing [1].

Makes life a lot easier.

[1]: https://docs.python.org/3/library/typing.html

The editor support that I get from typing/mypy is fantastic! It feels like writing in a statically typed language. To me this is by far the most important feature of Python3. It's a godsend for larger/complicated code bases.

[1]: https://github.com/python/mypy