{
"cells": [
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# 函数"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## 定义函数"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"函数`function`，通常接受输入参数，并有返回值。\n",
"\n",
"它负责完成某项特定任务，而且相较于其他代码，具备相对的独立性。"
]
},
{
"cell_type": "code",
"execution_count": 1,
"metadata": {
"collapsed": true
},
"outputs": [],
"source": [
"def add(x, y):\n",
" \"\"\"Add two numbers\"\"\"\n",
" a = x + y\n",
" return a"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"函数通常有一下几个特征：\n",
"- 使用 `def` 关键词来定义一个函数。\n",
"- `def` 后面是函数的名称，括号中是函数的参数，不同的参数用 `,` 隔开， `def foo():` 的形式是必须要有的，参数可以为空；\n",
"- 使用缩进来划分函数的内容；\n",
"- `docstring` 用 `\"\"\"` 包含的字符串，用来解释函数的用途，可省略；\n",
"- `return` 返回特定的值，如果省略，返回 `None` 。"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## 使用函数"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"使用函数时，只需要将参数换成特定的值传给函数。\n",
"\n",
"**Python**并没有限定参数的类型，因此可以使用不同的参数类型："
]
},
{
"cell_type": "code",
"execution_count": 2,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"5\n",
"foobar\n"
]
}
],
"source": [
"print add(2, 3)\n",
"print add('foo', 'bar')"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"在这个例子中，如果传入的两个参数不可以相加，那么**Python**会将报错："
]
},
{
"cell_type": "code",
"execution_count": 3,
"metadata": {
"collapsed": false
},
"outputs": [
{
"ename": "TypeError",
"evalue": "unsupported operand type(s) for +: 'int' and 'str'",
"output_type": "error",
"traceback": [
"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[1;32m<ipython-input-3-6f8dcf7eb280>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[1;32mprint\u001b[0m \u001b[0madd\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;36m2\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;34m\"foo\"\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[1;32m<ipython-input-1-e831943cfaf2>\u001b[0m in \u001b[0;36madd\u001b[1;34m(x, y)\u001b[0m\n\u001b[0;32m 1\u001b[0m \u001b[1;32mdef\u001b[0m \u001b[0madd\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0mx\u001b[0m\u001b[1;33m,\u001b[0m \u001b[0my\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m 2\u001b[0m \u001b[1;34m\"\"\"Add two numbers\"\"\"\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 3\u001b[1;33m \u001b[0ma\u001b[0m \u001b[1;33m=\u001b[0m \u001b[0mx\u001b[0m \u001b[1;33m+\u001b[0m \u001b[0my\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m 4\u001b[0m \u001b[1;32mreturn\u001b[0m \u001b[0ma\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
"\u001b[1;31mTypeError\u001b[0m: unsupported operand type(s) for +: 'int' and 'str'"
]
}
],
"source": [
"print add(2, \"foo\")"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"如果传入的参数数目与实际不符合，也会报错："
]
},
{
"cell_type": "code",
"execution_count": 4,
"metadata": {
"collapsed": false
},
"outputs": [
{
"ename": "TypeError",
"evalue": "add() takes exactly 2 arguments (3 given)",
"output_type": "error",
"traceback": [
"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[1;32m<ipython-input-4-ed7bae31fc7d>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[1;32mprint\u001b[0m \u001b[0madd\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;36m1\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;36m2\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;36m3\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m: add() takes exactly 2 arguments (3 given)"
]
}
],
"source": [
"print add(1, 2, 3)"
]
},
{
"cell_type": "code",
"execution_count": 5,
"metadata": {
"collapsed": false
},
"outputs": [
{
"ename": "TypeError",
"evalue": "add() takes exactly 2 arguments (1 given)",
"output_type": "error",
"traceback": [
"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[1;32m<ipython-input-5-a954233d3b0d>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[1;32mprint\u001b[0m \u001b[0madd\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;36m1\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m: add() takes exactly 2 arguments (1 given)"
]
}
],
"source": [
"print add(1)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"传入参数时，Python提供了两种选项，第一种是上面使用的按照位置传入参数，另一种则是使用关键词模式，显式地指定参数的值："
]
},
{
"cell_type": "code",
"execution_count": 6,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"5\n",
"barfoo\n"
]
}
],
"source": [
"print add(x=2, y=3)\n",
"print add(y=\"foo\", x=\"bar\")"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"可以混合这两种模式："
]
},
{
"cell_type": "code",
"execution_count": 7,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"5\n"
]
}
],
"source": [
"print add(2, y=3)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## 设定参数默认值"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"可以在函数定义的时候给参数设定默认值，例如："
]
},
{
"cell_type": "code",
"execution_count": 8,
"metadata": {
"collapsed": true
},
"outputs": [],
"source": [
"def quad(x, a=1, b=0, c=0):\n",
" return a*x**2 + b*x + c"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"可以省略有默认值的参数："
]
},
{
"cell_type": "code",
"execution_count": 9,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"4.0\n"
]
}
],
"source": [
"print quad(2.0)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"可以修改参数的默认值："
]
},
{
"cell_type": "code",
"execution_count": 10,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"10.0\n"
]
}
],
"source": [
"print quad(2.0, b=3)"
]
},
{
"cell_type": "code",
"execution_count": 11,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"12.0\n"
]
}
],
"source": [
"print quad(2.0, 2, c=4)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"这里混合了位置和指定两种参数传入方式，第二个2是传给 `a` 的。\n",
"\n",
"注意，在使用混合语法时，要注意不能给同一个值赋值多次，否则会报错，例如："
]
},
{
"cell_type": "code",
"execution_count": 12,
"metadata": {
"collapsed": false
},
"outputs": [
{
"ename": "TypeError",
"evalue": "quad() got multiple values for keyword argument 'a'",
"output_type": "error",
"traceback": [
"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[1;32m<ipython-input-12-101d0c090bbb>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[1;32m----> 1\u001b[1;33m \u001b[1;32mprint\u001b[0m \u001b[0mquad\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;36m2.0\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;36m2\u001b[0m\u001b[1;33m,\u001b[0m \u001b[0ma\u001b[0m\u001b[1;33m=\u001b[0m\u001b[1;36m2\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m: quad() got multiple values for keyword argument 'a'"
]
}
],
"source": [
"print quad(2.0, 2, a=2)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## 接收不定参数"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"使用如下方法，可以使函数接受不定数目的参数："
]
},
{
"cell_type": "code",
"execution_count": 13,
"metadata": {
"collapsed": true
},
"outputs": [],
"source": [
"def add(x, *args):\n",
" total = x\n",
" for arg in args:\n",
" total += arg\n",
" return total"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"这里，`*args` 表示参数数目不定，可以看成一个元组，把第一个参数后面的参数当作元组中的元素。"
]
},
{
"cell_type": "code",
"execution_count": 14,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"10\n",
"3\n"
]
}
],
"source": [
"print add(1, 2, 3, 4)\n",
"print add(1, 2)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"这样定义的函数不能使用关键词传入参数，要使用关键词，可以这样："
]
},
{
"cell_type": "code",
"execution_count": 15,
"metadata": {
"collapsed": true
},
"outputs": [],
"source": [
"def add(x, **kwargs):\n",
" total = x\n",
" for arg, value in kwargs.items():\n",
" print \"adding \", arg\n",
" total += value\n",
" return total"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"这里， `**kwargs` 表示参数数目不定，相当于一个字典，关键词和值对应于键值对。"
]
},
{
"cell_type": "code",
"execution_count": 16,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"adding y\n",
"adding z\n",
"adding w\n",
"46\n"
]
}
],
"source": [
"print add(10, y=11, z=12, w=13)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"再看这个例子，可以接收任意数目的位置参数和键值对参数："
]
},
{
"cell_type": "code",
"execution_count": 17,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"(2, 3) {'x': 'bar', 'z': 10}\n"
]
}
],
"source": [
"def foo(*args, **kwargs):\n",
" print args, kwargs\n",
"\n",
"foo(2, 3, x='bar', z=10)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"不过要按顺序传入参数，先传入位置参数 `args` ，在传入关键词参数 `kwargs` 。"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## 返回多个值"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"函数可以返回多个值："
]
},
{
"cell_type": "code",
"execution_count": 18,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"5.0 0.927295218002\n"
]
}
],
"source": [
"from math import atan2\n",
"\n",
"def to_polar(x, y):\n",
" r = (x**2 + y**2) ** 0.5\n",
" theta = atan2(y, x)\n",
" return r, theta\n",
"\n",
"r, theta = to_polar(3, 4)\n",
"print r, theta"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"事实上，**Python**将返回的两个值变成了元组："
]
},
{
"cell_type": "code",
"execution_count": 19,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"(5.0, 0.9272952180016122)\n"
]
}
],
"source": [
"print to_polar(3, 4)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"因为这个元组中有两个值，所以可以使用\n",
"\n",
" r, theta = to_polar(3, 4)\n",
"\n",
"给两个值赋值。\n",
"\n",
"列表也有相似的功能："
]
},
{
"cell_type": "code",
"execution_count": 20,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"1 2 3\n"
]
}
],
"source": [
"a, b, c = [1, 2, 3]\n",
"print a, b, c"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"事实上，不仅仅返回值可以用元组表示，也可以将参数用元组以这种方式传入："
]
},
{
"cell_type": "code",
"execution_count": 21,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"5\n"
]
}
],
"source": [
"def add(x, y):\n",
" \"\"\"Add two numbers\"\"\"\n",
" a = x + y\n",
" return a\n",
" \n",
"z = (2, 3)\n",
"print add(*z)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"这里的`*`必不可少。"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"事实上，还可以通过字典传入参数来执行函数："
]
},
{
"cell_type": "code",
"execution_count": 22,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"5\n"
]
}
],
"source": [
"def add(x, y):\n",
" \"\"\"Add two numbers\"\"\"\n",
" a = x + y\n",
" return a\n",
"\n",
"w = {'x': 2, 'y': 3}\n",
"print add(**w)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## map 方法生成序列"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"可以通过 `map` 的方式利用函数来生成序列："
]
},
{
"cell_type": "code",
"execution_count": 23,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"[4, 9, 16]\n"
]
}
],
"source": [
"def sqr(x): \n",
" return x ** 2\n",
"\n",
"a = [2,3,4]\n",
"print map(sqr, a)"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"其用法为：\n",
" \n",
" map(aFun, aSeq)\n",
"\n",
"将函数 `aFun` 应用到序列 `aSeq` 上的每一个元素上，返回一个列表，不管这个序列原来是什么类型。\n",
"\n",
"事实上，根据函数参数的多少，`map` 可以接受多组序列，将其对应的元素作为参数传入函数："
]
},
{
"cell_type": "code",
"execution_count": 24,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"[12, 8, 7]\n"
]
}
],
"source": [
"def add(x, y): \n",
" return x + y\n",
"\n",
"a = (2,3,4)\n",
"b = [10,5,3]\n",
"print map(add,a,b)"
]
}
],
"metadata": {
"kernelspec": {
"display_name": "Python 2",
"language": "python",
"name": "python2"
},
"language_info": {
"codemirror_mode": {
"name": "ipython",
"version": 2
},
"file_extension": ".py",
"mimetype": "text/x-python",
"name": "python",
"nbconvert_exporter": "python",
"pygments_lexer": "ipython2",
"version": "2.7.10"
}
},
"nbformat": 4,
"nbformat_minor": 0
}