Why Indentation Syntax?

The indentation rule may seem unusual at first glance to programmers accustomed to C-like languages, but it is a deliberate feature of Python, and it’s one of the main ways that Python almost forces programmers to produce uniform, regular, and readable code. It essentially means that you must line up your code vertically, in columns, according to its logical structure. The net effect is to make your code more consistent and readable (unlike much of the code written in C-like languages).

To put that more strongly, aligning your code according to its logical structure is a major part of making it readable, and thus reusable and maintainable, by yourself and others. In fact, even if you never use Python after reading this book, you should get into the habit of aligning your code for readability in any block-structured language. Python forces the issue by making this a part of its syntax, but it’s an important thing to do in any programming language, and it has a huge impact on the usefulness of your code.

广告：个人专属 VPN，独立 IP，无限流量，多机房切换，还可以屏蔽广告和恶意软件，每月最低仅 5 美元

Your experience may vary, but when I was still doing development on a full-time basis, I was mostly paid to work on large old C++ programs that had been worked on by many programmers over the years. Almost invariably, each programmer had his or her own style for indenting code. For example, I’d often be asked to change a while loop coded in the C++ language that began like this:

while (x > 0) {

Before we even get into indentation, there are three or four ways that programmers can arrange these braces in a C-like language, and organizations often have political debates and write standards manuals to address the options (which seems more than a little off-topic for the problem to be solved by programming). Ignoring that, here’s the scenario I often encountered in C++ code. The first person who worked on the code indented the loop four spaces:

while (x > 0) {
    --------;
    --------;

That person eventually moved on to management, only to be replaced by someone who liked to indent further to the right:

while (x > 0) {
    --------;
    --------;
           --------;
           --------;

That person later moved on to other opportunities, and someone else picked up the code who liked to indent less:

while (x > 0) {
    --------;
    --------;
           --------;
           --------;
--------;
--------;
}

And so on. Eventually, the block is terminated by a closing brace (}), which of course makes this “block-structured code” (he says, sarcastically). In any block-structured language, Python or otherwise, if nested blocks are not indented consistently, they become very difficult for the reader to interpret, change, or reuse, because the code no longer visually reflects its logical meaning. Readability matters, and indentation is a major component of readability.

Here is another example that may have burned you in the past if you’ve done much programming in a C-like language. Consider the following statement in C:

if (x)
     if (y)
          statement1;
else
     statement2;

Which if does the else here go with? Surprisingly, the else is paired with the nested if statement (if (y)), even though it looks visually as though it is associated with the outer if (x). This is a classic pitfall in the C language, and it can lead to the reader completely misinterpreting the code and changing it incorrectly in ways that might not be uncovered until the Mars rover crashes into a giant rock!

This cannot happen in Python—because indentation is significant, the way the code looks is the way it will work. Consider an equivalent Python statement:

if x:
     if y:
          statement1
else:
     statement2

In this example, the if that the else lines up with vertically is the one it is associated with logically (the outer if x). In a sense, Python is a WYSIWYG language—what you see is what you get because the way code looks is the way it runs, regardless of who coded it.

If this still isn’t enough to underscore the benefits of Python’s syntax, here’s another anecdote. Early in my career, I worked at a successful company that developed systems software in the C language, where consistent indentation is not required. Even so, when we checked our code into source control at the end of the day, this company ran an automated script that analyzed the indentation used in the code. If the script noticed that we’d indented our code inconsistently, we received an automated email about it the next morning—and so did our managers!

The point is that even when a language doesn’t require it, good programmers know that consistent use of indentation has a huge impact on code readability and quality. The fact that Python promotes this to the level of syntax is seen by most as a feature of the language.

Also keep in mind that nearly every programmer-friendly text editor has built-in support for Python’s syntax model. In the IDLE Python GUI, for example, lines of code are automatically indented when you are typing a nested block; pressing the Backspace key backs up one level of indentation, and you can customize how far to the right IDLE indents statements in a nested block. There is no universal standard on this: four spaces or one tab per level is common, but it’s up to you to decide how and how much you wish to indent. Indent further to the right for further nested blocks, and less to close the prior block.

As a rule of thumb, you probably shouldn’t mix tabs and spaces in the same block in Python, unless you do so consistently; use tabs or spaces in a given block, but not both (in fact, Python 3.0 now issues an error for inconsistent use of tabs and spaces, as we’ll see in Chapter 12). But you probably shouldn’t mix tabs or spaces in indentation in any structured language—such code can cause major readability issues if the next programmer has his or her editor set to display tabs differently than yours. C-like languages might let coders get away with this, but they shouldn’t: the result can be a mangled mess.

I can’t stress enough that regardless of which language you code in, you should be indenting consistently for readability. In fact, if you weren’t taught to do this earlier in your career, your teachers did you a disservice. Most programmers—especially those who must read others’ code—consider it a major asset that Python elevates this to the level of syntax. Moreover, generating tabs instead of braces is no more difficult in practice for tools that must output Python code. In general, if you do what you should be doing in a C-like language anyhow, but get rid of the braces, your code will satisfy Python’s syntax rules.