Why have both dynamic and static typing?

Dynamic typing can be flexible, powerful, convenient and easy. Butit’s not always the best approach; there are good reasons why manydevelopers choose to use statically typed languages or static typingfor Python.

Here are some potential benefits of mypy-style static typing:

• Static typing can make programs easier to understand andmaintain. Type declarations can serve as machine-checkeddocumentation. This is important as code is typically read much moreoften than modified, and this is especially important for large andcomplex programs.
• Static typing can help you find bugs earlier and with less testingand debugging. Especially in large and complex projects this can bea major time-saver.
• Static typing can help you find difficult-to-find bugs before yourcode goes into production. This can improve reliability and reducethe number of security issues.
• Static typing makes it practical to build very useful developmenttools that can improve programming productivity or software quality,including IDEs with precise and reliable code completion, staticanalysis tools, etc.
• You can get the benefits of both dynamic and static typing in asingle language. Dynamic typing can be perfect for a small projector for writing the UI of your program, for example. As your programgrows, you can adapt tricky application logic to static typing tohelp maintenance.

See also the front page of the mypy website.