Recently, a person here asked a basic question about how to compute in Python all permutations of elements from a list. As for most questions asked by students, I haven't provided the actual source code in my answer, but rather explained how to approach the problem, which essentially ended up in a basic presentation of test driven development.

Thinking about the problem itself, I remembered all the similar problems I did have to solve when I was myself studying programming in university. We were never taught about test driven development, nor any comparable method, but nevertheless were constantly asked to write algorithms which sort elements from a list, or solve Towers of Hanoi puzzle, etc. Nor do all the students who come on SoftwareEngineering.SE seem to know how TDD (Test Driven Development) would help them solving the problems they are given: if they were, they would have formulated their questions very differently.

The fact is, without TDD, all those problems are indeed quite challenging. I consider myself a professional developer, but I would still spend some time writing a list-ordering algorithm without TDD, if asked, and I would be mostly clueless if I were asked to compute all the permutations of the elements in a list, still without using TDD. With TDD, on the other hand, I solved the permutations problem in a matter of minutes when writing the answer to the student.

What is the reason TDD is either not taught at all or at least very late in universities? In other words, would it be problematic to explain the TDD approach before asking students to write list sorting algorithms and similar stuff?

Following the remarks in two of the questions:

In your question, you appear to present TDD as a "problem solving device" [...] In the past, TDD has been presented as a "solution discovery mechanism," but even Bob Martin (the principal advocate of TDD) concedes that you must bring a significant amount of prior knowledge to the technique.

and especially:

I'm curious why you think TDD makes a tricky algorithm problem with a well-defined spec easier.

I find it necessary to explain a bit more what is, in my opinion, so magical about TDD when it comes to solving problems.

In high school and in the university, I didn't have any specific techniques to solve problems, and this applied to both programming and mathematics. In retrospective, I suppose that one of such techniques is to review the current/last lesson/lecture and seek relation with the exercice. If the lesson is about integrals, there are chances the problem the teacher asked to solve requires to use integrals. If the lecture was about recursion, there are chances that the puzzle given to the students could be solved using recursion. I'm also sure there are well formalized approaches to solving problems in mathematics, and those approaches can be applied to programming as well; however, I never learned any.

This means that in practice, my approach was simply to poke the problem around, trying to guess how should it be solved. Back then, if I was given the challenge of generating permutations of elements from a list, I wouldn't start with an empty list as input, but rather an illustrative example, such as [4, 9, 2] , trying to figure out why are there six possible permutations, and how can I generate them through code. From there, I need a lot of thinking to find a possible way to solve the problem. This is essentially what the author in the original question did, ending up using random . Similarly, when I was a student, no other students of my age would start with [] : all would immediately rush to the case with two or three elements, and then remain stuck for half an hour, sometimes ending up with code which doesn't compile or doesn't run.

The TDD approach, for me, appears to be counter-intuitive. I mean, it works very well, but I would have never figured out myself, before reading a few articles about TDD, that (1) I should start with the simplest case, (2) write the test before writing code, and (3) never rush, trying to fulfil several scenarios in code. Looking at how beginner programmers think, I have an impression that I'm not the only one finding it counter-intuitive. It may be, I believe, more intuitive for programmers who have a good understanding of a functional language. I suppose that in Haskell, for example, it would be natural to handle the permutations problem by considering first a case of an empty list, then a case of a list with one element, and then a case of a list of multiple elements. In languages where recursive algorithms are possible, but not as natural as in Haskell, such approach is however much less natural, unless, of course, one practices TDD.