[duped]

Typing allows you to specify the expected behavior in terms of input/output structure of algorithms in such a way that they can be statically verified without writing unit tests or manual checking code in the source, allowing your unit tests to check behavior by value rather than by value and structure. The equivalent of type checking is not unit testing, but fuzzing.

Python is not easy to grok at all, when you consider you have to grok implementations to understand what they are supposed to do, and require extensive runtime debugging to figure out if it is behaving as expected before you can even write unit tests.

Compare to decent statically typed languages, which have quicker write/debug cycles since checking type definitions is faster than checking code behavior, and the structural unit testing is covered automatically by the compiler.

It's like getting more than 50% of your programs' test coverage, for free!