Strong testing, not strong typing (via @doc55 circa 2003)

The compiler cannot know how you expect the program to behave, so you must "extend" the compiler by adding unit tests (regardless of the language you're using). If you do this, you can make sweeping changes (refactoring code or modifying design) in a rapid manner because you know that your suite of tests will back you up, and immediately fail if there's a problem — just like a compilation fails when there's a syntax problem.

But without a full set of unit tests (at the very least), you can't guarantee the correctness of a program. To claim that the strong, static type checking constraints in C++, Java, or C# will prevent you from writing broken programs is clearly an illusion (you know this from personal experience). In fact, what we need is

Strong testing, not strong typing.

So this, I assert, is an aspect of why Python works. C++ tests happen at compile time (with a few minor special cases). Some Java tests happen at compile time (syntax checking), and some happen at run time (array-bounds checking, for example). Most Python tests happen at runtime rather than at compile time, but they do happen, and that's the important thing (not when). And because I can get a Python program up and running in far less time than it takes you to write the equivalent C++/Java/C# program, I can start running the real tests sooner: unit tests, tests of my hypothesis, tests of alternate approaches, etc.