第261页 | Learning Python | 阅读 ‧ 电子书库

What Python Removes

Although Python requires the extra colon character, there are three things programmers in C-like languages must include that you don’t generally have to in Python.

Parentheses are optional

The first of these is the set of parentheses around the tests at the top of the statement:

if (x < y)

The parentheses here are required by the syntax of many C-like languages. In Python, though, they are not—we simply omit the parentheses, and the statement works the same way:

if x < y

Technically speaking, because every expression can be enclosed in parentheses, including them will not hurt in this Python code, and they are not treated as an error if present. But don’t do that: you’ll be wearing out your keyboard needlessly, and broadcasting to the world that you’re an ex-C programmer still learning Python (I was once, too). The Python way is to simply omit the parentheses in these kinds of statements altogether.

End of line is end of statement

The second and more significant syntax component you won’t find in Python code is the semicolon. You don’t need to terminate statements with semicolons in Python the way you do in C-like languages:

x = 1;

In Python, the general rule is that the end of a line automatically terminates the statement that appears on that line. In other words, you can leave off the semicolons, and it works the same way:

x = 1

There are some ways to work around this rule, as you’ll see in a moment. But, in general, you write one statement per line for the vast majority of Python code, and no semicolon is required.

Here, too, if you are pining for your C programming days (if such a state is possible...) you can continue to use semicolons at the end of each statement—the language lets you get away with them if they are present. But don’t do that either (really!); again, doing so tells the world that you’re still a C programmer who hasn’t quite made the switch to Python coding. The Pythonic style is to leave off the semicolons altogether.

End of indentation is end of block

The third and final syntax component that Python removes, and the one that may seem the most unusual to soon-to-be-ex-C programmers (until they’ve used it for 10 minutes and realize it’s actually a feature), is that you do not type anything explicit in your code to syntactically mark the beginning and end of a nested block of code. You don’t need to include begin/end, then/endif, or braces around the nested block, as you do in C-like languages:

if (x > y) {
    x = 1;
    y = 2;
}

Instead, in Python, we consistently indent all the statements in a given single nested block the same distance to the right, and Python uses the statements’ physical indentation to determine where the block starts and stops:

if x > y:
    x = 1
    y = 2

By indentation, I mean the blank whitespace all the way to the left of the two nested statements here. Python doesn’t care how you indent (you may use either spaces or tabs), or how much you indent (you may use any number of spaces or tabs). In fact, the indentation of one nested block can be totally different from that of another. The syntax rule is only that for a given single nested block, all of its statements must be indented the same distance to the right. If this is not the case, you will get a syntax error, and your code will not run until you repair its indentation to be consistent.