I would say it is best to do both TDD and have a statically typed language with explicit types written down for every variable. The latter mainly for readability. Let us look for a minute at how mathematicians write things. It is like 'let n be a natural number, let f be a function from the reals to the reals and so on...'. It would be very inconvenient if they wrote things like 'let us have some symbols n and f and you will now have to read the rest of the text to try to figure out what they are....'. It is quite telling that in python it now became customary to include a kind of docstrings to document every parameter. However, no automated process is checking these so they may not be true. So then we start using mypy and, voila, there we are again with static type annotations for every parameter. The whole dynamic language thing was just a detour that did not make anything better. They saved the author a few seconds at the cost of the person trying to read the code taking a few minutes to find out what the type of the parameter was. I also think one should use the 'auto' keyword in C++ quite sparingly.