4.6 定义函数 Defining Functions

We can create a function that writes the Fibonacci series to an arbitrary boundary:

我们可以编写一个函数来生成给定上界的菲波那契数列：

>>> def fib(n):    # write Fibonacci series up to n
...     """Print a Fibonacci series up to n."""
...     a, b = 0, 1
...     while b < n:
...         print b,
...         a, b = b, a+b
...
>>> # Now call the function we just defined:
... fib(2000)
1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597

The keyword def introduces a function definition. It must be followed by the function name and the parenthesized list of formal parameters. The statements that form the body of the function start at the next line, and must be indented. The first statement of the function body can optionally be a string literal; this string literal is the function's documentation string, or docstring.

关键字 def 引入了一个函数定义。在其后必须跟有函数名和包括形式参数的圆括号。函数体语句从下一行开始，必须是缩进的。函数体的第一行可以是一个字符串值，这个字符串是该函数的 (文档字符串（documentation string）)，也可称作 docstring 。

There are tools which use docstrings to automatically produce online or printed documentation, or to let the user interactively browse through code; it's good practice to include docstrings in code that you write, so try to make a habit of it.

有些文档字符串工具可以在线处理或打印文档，或让用户交互的浏览代码;在代码中加入文档字符串是一个好的作法，应该养成这个习惯。

The execution of a function introduces a new symbol table used for the local variables of the function. More precisely, all variable assignments in a function store the value in the local symbol table; whereas variable references first look in the local symbol table, then in the global symbol table, and then in the table of built-in names. Thus, global variables cannot be directly assigned a value within a function (unless named in a global statement), although they may be referenced.

调用函数时会为局部变量引入一个新的符号表。所有的局部变量都存储在这个局部符号表中。引用参数时，会先从局部符号表中查找，然后是全局符号表，然后是内置命名表。因此，全局参数虽然可以被引用，但它们不能在函数中直接赋值（除非它们用 global 语句命名）。

The actual parameters (arguments) to a function call are introduced in the local symbol table of the called function when it is called; thus, arguments are passed using call by value (where the value is always an object reference, not the value of the object).^4.1 When a function calls another function, a new local symbol table is created for that call.

函数引用的实际参数在函数调用时引入局部符号表，因此，实参总是传值调用（这里的值总是一个对象引用，而不是该对象的值）。4.1 一个函数被另一个函数调用时，一个新的局部符号表在调用过程中被创建。

A function definition introduces the function name in the current symbol table. The value of the function name has a type that is recognized by the interpreter as a user-defined function. This value can be assigned to another name which can then also be used as a function. This serves as a general renaming mechanism:

函数定义在当前符号表中引入函数名。作为用户定义函数，函数名有一个为解释器认可的类型值。这个值可以赋给其它命名，使其能够作为一个函数来使用。这就像一个重命名机制：

>>> fib
<function object at 10042ed0>
>>> f = fib
>>> f(100)
1 1 2 3 5 8 13 21 34 55 89

You might object that fib is not a function but a procedure. In Python, like in C, procedures are just functions that don't return a value. In fact, technically speaking, procedures do return a value, albeit a rather boring one. This value is called None (it's a built-in name). Writing the value None is normally suppressed by the interpreter if it would be the only value written. You can see it if you really want to:

你可能认为fib不是一个函数（ function ），而是一个过程（ procedure ）。Python 和 C 一样，过程只是一个没有返回值的函数。实际上，从技术上讲，过程也有一个返回值，虽然是一个不讨人喜欢的。这个值被称为 None （这是一个内置命名）。如果一个值只是 None 的话，通常解释器不会写一个 None 出来，如果你真想要看它的话，可以这样做：

>>> print fib(0)
None

It is simple to write a function that returns a list of the numbers of the Fibonacci series, instead of printing it:

以下示列演示了如何从函数中返回一个包含菲波那契数列的数值链表，而不是打印它：

>>> def fib2(n): # return Fibonacci series up to n
...     """Return a list containing the Fibonacci series up to n."""
...     result = []
...     a, b = 0, 1
...     while b < n:
...         result.append(b)    # see below
...         a, b = b, a+b
...     return result
...
>>> f100 = fib2(100)    # call it
>>> f100                # write the result
[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

This example, as usual, demonstrates some new Python features:

和以前一样，这个例子演示了一些新的 Python 功能：

The return statement returns with a value from a function. return without an expression argument returns None. Falling off the end of a procedure also returns None.
return 语句从函数中返回一个值，不带表达式的 return 返回 None 。过程结束后也会返回 None 。
The statement result.append(b) calls a method of the list object result. A method is a function that `belongs' to an object and is named obj.methodname, where obj is some object (this may be an expression), and methodname is the name of a method that is defined by the object's type. Different types define different methods. Methods of different types may have the same name without causing ambiguity. (It is possible to define your own object types and methods, using classes, as discussed later in this tutorial.) The method append() shown in the example, is defined for list objects; it adds a new element at the end of the list. In this example it is equivalent to "result = result + [b]", but more efficient.
语句 result.append(b) 称为链表对象 result 的一个(方法（ method ）)。方法是一个“属于”某个对象的函数，它被命名为 (obj.methodename) ，这里的 (obj) 是某个对象（可能是一个表达式），(methodename) 是某个在该对象类型定义中的方法的命名。不同的类型定义不同的方法。不同类型可能有同样名字的方法，但不会混淆。（当你定义自己的对象类型和方法时，可能会出现这种情况，本指南后面的章节会介绍如何使用）。示例中演示的 append() 方法由链表对象定义，它向链表中加入一个新元素。在示例中它等同于""("result = result + [b]")，不过效率更高。

注脚

... object).^4.1: Actually, call by object reference would be a better description, since if a mutable object is passed, the caller will see any changes the callee makes to it (items inserted into a list).