Why Python’s whitespace rule is right

Python is famous among programming languages for its fairly unique syntax: rather than being delimited by curly braces or “begin/end” keywords, blocks are delimited by indentation. Indenting a line is like adding an opening curly brace, and de-denting is like a closing curly brace. When people criticise Python, it is usually the first complaint: “why would I want to use a language which requires me to indent code?” Indeed, while programmers are very used to indenting their code, they are very un-used to being forced to do so, and I can understand why they may take it as an insult that a language tells them how to write code. I don’t usually like to get into syntax arguments, because I find them very superficial — it is much more important to discuss the semantics of a language than its syntax. But this is such a common argument among Python detractors, I wanted to address it. Python is right, and it’s just about the only language that is.

I think the rub is that programmers like to think of languages as a tool, and tools should be as flexible as possible. I think in general it is a good principle for programming languages not to enforce conventions. Languages that do tend to annoy people who don’t subscribe to the same conventions. For example, the Go programming language enforces the “One True Brace Style” — every opening curly brace must appear on the same line as the function header or control statement. This irritates me because that’s not my preferred convention. But the indentation convention is so universal that it is considered bad programming practice to not indent in all cases. (There is disagreement over tabs vs spaces, the number of spaces, etc, but we all agree that indentation is good.) There is not a single situation in any country, in any programming language, or at any skill level, in which is it acceptable to not indent your code the way Python requires it. Therefore, it is technically redundant to have a language that is not whitespace-sensitive. Any language that is not whitespace-sensitive requires (by universal convention) that programmers communicate the scoping of the code in two distinct manners for every single line of code: braces (or begin/end) and indentation. You are required to make sure that these two things match up, and if you don’t, then you have a program that doesn’t work the way it looks like it works, and the compiler isn’t going to tell you.

There are two solutions to this problem. 1: Make the compiler tell you. Force the programmer to indent and put in curly braces, and have the compiler check the indentation and give either a warning or error if they don’t match up. Now you’ve solved the problem of accidentally getting it wrong, but now what is the point of requiring curly braces at all? The programmer would just be doing extra work to please the compiler. We may as well go with 2: take out the curly braces and just have the compiler determine the blocks based on indentation.

When you really analyse it, Python’s whitespace sensitivity is actually the only logical choice for a programming language, because you only communicate your intent one way, and that intent is read the same way by humans and computers. The only reason to use a whitespace-insensitive language is that that’s the way we’ve always done things, and that’s never a good reason. That is why my programming language, Mars, has the same indentation rule as Python.

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An interesting aside: there is a related syntax rule in Python which doesn’t seem quite so logical: you are required to place a colon at the end of any line preceding an indent. I haven’t fully tested this, but I’m pretty sure there is no technical reason for that (the parser could still work unambiguously without that colon), and it doesn’t seem to add much to the readability either. I slavishly followed this rule in Mars too, because as a Python programmer it “feels right” to me. But perhaps it would be better to drop it.

Still totally undecided about implementation inheritance

Everyone keeps saying it’s a bad thing. They cite the fragile base class problem and other various issues which I don’t really have a problem with. (The FBC problem as far as I can tell almost always is caused when someone violates the Liskov substitution principle.)

I recently went to a Go talk. Go is another in a string of languages which extol the virtues of interface inheritance and not implementation inheritance. But in most of the OO code I write (including the code I encourage and expect my students to write), I use class hierarchies with a lot of data and code re-use in the child classes, I think to great effect. Am I mistaken?

To this end I shall undergo an experiment: Having never used Go, I will rewrite the Java project I gave my students in Go, making extensive use of interface inheritance and see if there’s a natural “Go-ism” for achieving code and data re-use where my existing code uses implementation inheritance — I assume using ordinary composition and functions.