# The Art of the Function

## 如何编写良好的函数

* 如何编写良好的函数
  * 每个函数都应该只做一个任务
    * 任务使用短小的名称来定义，使用一行文本来标识
  * 不要重复劳动（DRY）是软件工程的中心法则。
    * 多个代码段不应该描述重复的逻辑
  * 函数应该定义得泛型
    * 任何情况下能可以使用

## 文档字符串

* 描述这个函数的文档，叫做文档字符串
  * 使用三个引号
    * 第一行描述函数的任务
    * 描述参数
* [文档字符串准则](https://www.python.org/dev/peps/pep-0257/)

```python
def pressure(v, t, n):
    """Compute the pressure in pascals of an ideal gas.

    Applies the ideal gas law: http://en.wikipedia.org/wiki/Ideal_gas_law

    v -- volume of gas, in cubic meters
    t -- absolute temperature in degrees kelvin
    n -- particles of gas
    """
    k = 1.38e-23  # Boltzmann's constant
    return n * k * t / v

help(pressure) # 查看文档字符串
```

## 参数默认值

```python
k_b=1.38e-23  # Boltzmann's constant
def pressure(v, t, n=6.022e23):
    return n * k_b * t / v
```

* 函数体的大多数数据
  * 应该表示为具名参数的默认值
* 永远不会改变的值
  * 就像基本常数k\_b，应该定义在全局帧中。

## python 可以有多个 return 值

```python
def divide_exact(n,d):
    return n // d, n % d
quotient, remainder = divide_exact(2013, 10)
>>> quotient
201
>>> remainder 
3
```

* 实际上返回的是一个元组 tuple


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