Reading Code: A Computer Science Curriculum

Date: 19 August 2008 Tags: computing, python

I developed the following ideas about how to teach computer programming during a recent conversation with Daniel Rocco, a professor at the University of West Georgia, and Georgia Tech grad student Derek Richardson, and I wanted to expand on the ideas here on my blog. I have been heavily influenced by reading Greg Wilson, and, for all I know, he or someone else may have already put these ideas together into something like the outline below.

The first day of class

To teach computer programming, a professor ought to stride into the room on the semester's first day, explain that a computer program is a text file full of instructions for the computer to follow, and then proceed to check out an example from version control. His desktop should be displayed on a big screen in front of the class. He should read through the example program with them, discuss it — it should be a Python program that does something requiring two or three dozen lines, like opening a window and displaying the message, “Hello, world!” — and then run it. He should demonstrate how to edit the program so that it prints something else instead (perhaps “Hello, professor!”), run the altered version for them, and, finally, check the modified program into version control.

The class now has the tools they need to complete their first assignment. The students are told that a version-control repository has been created for each of them; that they have each been subscribed to their repository's email notification list; and that, upon returning home from class, their email inbox will already hold a message informing them that their first assignment — the “Hello, world!” program itself — has been checked into their repository by the course software. By the next class period they are to have checked the assignment out, altered it so that it prints “Hello” followed by their own name, and submitted the assignment by checking the program back in.

Hopefully the university will have provided a lab period for the course, in which the TA can show the students how to get Python and the version-control software installed on their own desktops or laptops. If not, then the professor will have to cover those details in class. In either case, of course, the same information should be available online on the course web site. But the important thing is that, from their very first assignment, the students are using the same tools for software development that they will use as professionals out in the wider world.

Test suites, and debugging

That first day of class taught the students to view several dozen lines of code — that, in our example, initialized and opened a window with some text written inside of it — and to dive in and modify a single feature that they understand: the string constant containing the words “Hello, world!” The next several class periods will, as usual in computer science, move onward from simple string and numeric constants to teach students about expressions, variables, and control structures. But the manner in which these concepts are taught will immediately induct the young programmers into habits that they will follow for the rest of their lives.

The crucial thing is the introduction, hopefully in the second class period, of the idea of a test suite. This will prepare them for their second homework assignment, which, as usual, will be automatically committed to their repositories right after class is over. While they verified the correctness of their first homework assignment merely by running it (“Look, it printed Hello, <my name> in a window!”), every subsequent assignment will be based, instead, on test suites. They should be doctests, so that they will look just like the professor's sessions with the Python prompt, but with some accompanying paragraphs for explanation. One test suite, named before, will succeed against the example program as it is first delivered to their repository. The second one, named after, will initially fail against the code. The student completes the exercise by adjusting the program, using their newly learned concept, so that it begins to succeed against the second test suite. And, of course, they commit the program to their repository when they are ready to turn the assignment in.

For fun — well, actually, not just for fun, because it will provide important cognitive cues as well — the before and after pair of tests should tell a small story about changing requirements. Imagine an example program that reads in credit card debits, and prints out how much money was spent; but, the after test explains, Sally now wants it to process her credit card payments as well (which will be prefixed with a plus sign in the input) and then print out the current balance in her account. Ambitious instructors might even create exercises in which a whole series of tests lead the student through several incremental improvements to a single program file.

How should class be conducted? I imagine that the actual instruction should include three important ingredients.

First, each concept will first be introduced by explaining why we need it. I cannot emphasize enough the pedagogical value of motivating each language feature with an explanation of the need that it fills! My understanding of Taylor polynomials in Calculus III would have been revolutionized if it had occurred to the professor that we might first need an explanation of why Taylor polynomials exist before we would be interested in the proof that they work! The explanation of a concept so often needs to start with simple information like “variables exist because programs need to remember things,” “control structures help us when we do not know in advance how many times our program might have to repeat a task,” and so forth.

Second, the professor will illustrate the new language feature by alternating between the whiteboard and the Python prompt — between leading the students to think about the idea, and actually letting them see it work.

Third, short example programs will be shown that do something useful with the new concept. These programs will often include code that the student does not yet understand, but, in the middle, include a pleasantly glaring and obvious example of the new concept in use. The professor should not only run these programs from the command line, which, honestly, leaves the students straining to imagine what complex sequence of steps has resulted in the output, but he should run them repeatedly in pdb so that the students can actually watch the variables changing their value, the control statements repeating a loop, and so forth. It would be most useful if the professor attaches the debugger to an editor that displays the whole program while highlighting the line currently being executed.

Invariants, and further progress

I should pause, and emphasize that the conviction which stands behind my design of this course is that a student is simply not competent to write a Python function until they have read at least, say, one hundred well-designed and fully documented functions written by others. I mean this with complete seriousness: students should spend weeks reading and modifying good code before they are trusted to produce functions or classes on their own.

By the time a student is asked to write a function from the ground up, they should be so accustomed to docstrings, helpful comments, and informative variable names, that they put these into their own functions as though they were the most natural things in the world. In fact, the instructor should ideally produce students who are not even aware that Python functions can execute if they do not include a docstring and informative comments!

The same idea lies behind teaching a professional slate of tools — version control, test suites, the Python prompt, and a debugger — right from the start. We thereby hope to create students that, if ever faced with a project or situation from which these tools are absent, will immediately know that they have been placed at a disadvantage. They might not, in that future environment, be able to remedy the situation; but they will at least feel where their wasted hours are going, and know which tool to introduce if they have the opportunity.

The reason, incidentally, that doctests are critically important to this style of instruction is that the student only needs to understand the Python prompt itself to read and comprehend the doctest. By contrast, even the simplest unit tests usually require quite complicated features, like the import statement and function calls, that could delay the introduction of testing by several class periods at least. Only doctests allow testing to be introduced early enough for it to be the backbone of even their earliest assignments.

Refactoring and test writing

I would probably keep the students busy modifying (and, therefore, mostly reading) code through the entire first half of the introductory curriculum. They would ascend through adjusting constants and expressions to adding control statements and calculations of their own; then, learning how to write or rewrite entire functions; learning input and output programming both to files and the screen; and, perhaps, if it suited the curriculum and the time available, they would be shown object-based programming, including using and writing object methods.

I would probably make refactoring the final stage of that first half of the course, which might actually be a bit fun for the students because the structure of the assignments would shift. Through all of the other homeworks, students received both a before and after test suite, and had to make the program run successfully against the latter. But now, to learn refactoring, will be given only one test suite, that must work on both the initial version of the program and on the improved version that they will create through applying basic refactoring techniques. Instead of taking a flying leap from a before behavior to a program that they hope now exhibits the after behavior, they can make steps that are as tiny and incremental as they please, checking to make sure that they still satisfy the test suite after each little change.

Incidentally, as I described those “tiny steps” in the previous paragraph, I unconsciously was imagining the pleasant task of making little changes to a program and checking each change into version control after checking that the tests still pass. It is the ability to back up one step if the procedure suddenly goes haywire that can make refactoring leisurely. Without version control, a series of refactoring steps can become a nightmare wilderness in which all of one's work has to be thrown away because, at the last step, you bungle the change, cannot figure out what you have done, and can never quite get the program working again without going back to the start.

All of which raises the question: how will the students learn that version control is useful for anything other than handing in assignments? I recommend that the professor smuggle a useful version-control stunt into every single class period: he should always plan to illustrate some mistake, or show some failed approach to a problem, then revert the change with version control before proceeding. The students must be made to feel, from repeated illustrations, that frequently checking in code forms a safety net beneath them that prevents them from losing their work if an inadvertent destructive change (not to mention something like a hard drive crash!) forces them to retrace their steps and start part of an assignment over again.

Test writing should probably also be practiced in the first half of the course — and the student, of course, will be expected to produce tests like the very many that he has already read: each telling a little coherent story that explains, as well as demonstrates, how the program behaves. I would probably, in each homework set, provide one exercise in which they are given a program that, in fact, has a bug, despite passing its before test! The exercise will tell a little story about a customer for whom the program is failing, and the student will be required both to write a test case that exercises the bug and reproduces the failure, and then to fix the program itself.

The second half

This curriculum's second half — which I would imagine being the second semester of a two-course series, but with bright students (or carefully limited goals) might fit into the closing weeks of a single semester — is when students would begin to be eased into the blasted wasteland which is the world of software which they will too soon enter.

What wasteland? Why, the parched and mottled landscape of uncommented, untested, uncontrolled code with which we have strewn the earth because of the utterly inadequate techniques by which nearly everyone is “taught to program” under the current system.

But, again, we must ease the students into how terrible the world is. First would come an assignment in which, inexplicably, the program they were modifying entirely lacked comments; the students, if they are properly prepared, will actually gasp when the professor shows them the code. Perhaps the homework will simply be to check out the program and provide it with comments!

Again, an assignment will appear that entirely lacks a test for some critical feature; they will have to provide the missing test. Then a small library will be thrown at them that, unimaginably, lacks tests entirely; their assignment will be to write them. You can see that, with each of these rescue-mission assignments, the students are learning a skill which will come in handy as soon as they enter the real world and have to perform exactly such a rescue operation on a code base for which they are made responsible.

Then would come a bit of a relief — a more normal situation; actually, a wonderful situation to encounter out in the real world! They will receive, simply, a complete test suite for which no program exists. For the first time, they will be expected to write an entire code base from the ground up that satisfies the set of tests. It is really at this point that the instructor will be able to email me and tell me whether this curriculum has done any kind of good at all: do the students write, in response to this assignment, the kind of sham, idiosyncratic, and entirely undocumented code that is typical of first-year programmers? Or — hope beyond hopes! — are some of them actually producing docstrings, and writing comments, and using normal Python idioms, as a result of seeing more than ten thousand lines of code before being entrusted with this most sacred of duties: inflicting entirely new code of their own upon the world?

The instructor will have at least three questions as he watches the students committing to their repositories. First, how many students are committing code frequently, instead of committing days of work at once? Second, are comments and documentation being checked in with code the first time, or just being added before the code is graded? Finally, are test suites being written up front, or thrown together at the last minute once they think that their code is working?

A final assignment

How should we conclude this curriculum?

Obviously, by making each student write, all by themselves, both a suitably complex program or library and its entire test suite. This could even be given a social dimension: the last assignments could be partitioned so as to require cooperative work among the students, or even interaction among the whole class, with some students writing test suites for libraries their code requires, and other students creating the code to satisfy those needs. Each student will thus learn something about the social dimension of programming: both what it is like to deliver and elaborate a test suite to someone tasked with figuring out what you need, and what it is like to receive a test suite, have to demand clarifications, and then write to it as your specification.

Lessons from teaching language

After reading back over the ideas outlined above, I realize that this computer programming curriculum is startlingly similar to the way that human languages like French or Chinese are taught in modern universities. And on reflection, I very much like the analogy! I will, in fact, make the strong assertion that the extent to which programming courses differ from all normal language instruction is, in fact, the extent to which they are misguided.

There is nothing so evident in teaching French than that the student needs to start by reading much more French than they are asked to write. Can you imagine the disaster if a French teacher taught only the rules of French, tested the students to see if they could write convincing French sentences yet, and responded to their abject failure by drilling them on yet more rules?

Yet this is exactly how we teach programming! In the early 1990s, I was a TA for one of Georgia Tech's introductory programming class, and the code that students produced was uniformly terrible! But so long as it somehow worked, they were given a passing grade — in fact, an A! — and hustled on to the next assignment, with absolutely no opportunity to learn that the code they were writing was utter trash — completely disjoint and incomprehensible. They were, in other words, being taught all of the grammar of computer programming, but without seeing any literature — well-written, clear, fully documented code — that they could imitate.

Perhaps that is the central insight that I want to communicate in this blog post: that writing code is imitation far more than it is the application of rules. After more than a decade of writing Python code by myself, it was a shock to start working on the Grok project and, in the Zope code repository, begin to see how real professionals wrote their Python code. I learned more techniques and common idioms in one year than in my entire previous decade!

I should also note that my idea of routinely exposing students to language features that they have not learned yet — like that first assignment where they hunt for the “Hello, world!” string amidst code that opens a window with a text widget — corresponds directly to the modern educational concept of untidy input, where you show students small amounts of “real French” even from the first day of class. By showing them the full complexity of the target language, even at the early stage where all they can do is “find and circle the subject of each sentence”, you are beginning to teach them what their writing will look like on that distant day when what they produce is full-fledged speech. And, critically, you are not allowing them to mistake the crude and lopsided sentences they create in their homework for the real, fluid, and graceful language that is French.

The end of the matter

I should finish with this simple plea: we must stop virtually forcing students to produce ugly, untested, uncommented, and undocumented routines as part of their introductory curriculum. And we must, at all costs, stop encouraging the delusion that, if they do so, they have written anything that deserves to be called a computer program.

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