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"# 编程入门07：Python函数定义\n",
"你己经接触过很多的函数，现在让我们尝试定义自己的函数——这需要使用def关键字。函数定义好后即可调用，以下是在交互模式中定义并调用了一个非常简单的函数：\n",
"```\n",
"In [1]: def welcome():\n",
" ...: \"\"\"输出欢迎信息\"\"\"\n",
" ...: print(\"欢迎光临！\")\n",
" ...: \n",
"\n",
"In [2]: welcome()\n",
"欢迎光临！\n",
"\n",
"In [3]: type(welcome)\n",
"Out[3]: function\n",
"\n",
"In [4]: help(welcome)\n",
"Help on function welcome in module __main__:\n",
"\n",
"welcome()\n",
" 输出欢迎信息\n",
"```\n",
"你可以发现函数并不神秘，只是把一段程序定义为一个对象以便重复使用而已——另外请注意用三个引号引起来的字符串，这样的字符串允许分行，当放在函数体最前面时则会成为这个函数的“文档字符串”（Docstring），help函数所显示的帮助信息就来自相应对象的文档字符串。你可以使用return语句结束函数并指定返回值，否则默认返回空值。\n",
"```\n",
"In [5]: print(welcome())\n",
"欢迎光临！\n",
"None\n",
"\n",
"In [6]: def iseven(n):\n",
" ...: \"\"\"判断是否偶数\"\"\"\n",
" ...: return n % 2 == 0\n",
" ...: \n",
"\n",
"In [7]: iseven(99)\n",
"Out[7]: False\n",
"```\n",
"\n",
"接下来让我们再用海龟绘图来练习复杂一些的函数：新建文件turtledraw.pyw，定义一个五角星函数star5p，并在之后的代码中调用这个函数：\n",
"```\n",
"\"\"\"自定义的海龟绘图函数集\n",
"\"\"\"\n",
"import turtle as tt\n",
"\n",
"\n",
"def star5p():\n",
" \"\"\"画一颗五角星\n",
" \"\"\"\n",
" t = tt.Turtle() # 生成一个单独的海龟对象\n",
" t.hideturtle()\n",
" t.speed(0)\n",
" t.color(\"purple\")\n",
" t.penup()\n",
" t.begin_fill() # 启用填充区域\n",
" cnt = 0\n",
" while cnt < 5:\n",
" t.forward(20)\n",
" t.left(72)\n",
" t.forward(20)\n",
" t.right(144)\n",
" cnt += 1\n",
" t.end_fill()\n",
"\n",
"\n",
"tt.TurtleScreen._RUNNING = True # 画一个五边形\n",
"tt.color(\"pink\")\n",
"tt.penup()\n",
"tt.begin_fill()\n",
"tt.setpos(-25, -75)\n",
"tt.pendown()\n",
"cnt = 0\n",
"while cnt < 5:\n",
" tt.forward(100)\n",
" tt.left(72)\n",
" cnt += 1\n",
"tt.end_fill()\n",
"star5p() # 调用函数画一颗五角星\n",
"tt.done()\n",
"```\n",
"函数在所属模块空间内分隔出一个子空间——例如star5p函数生成了一个单独的海龟对象变量t。这个变量t只在star5p函数中存在，离开这个函数就不能再使用了，而star5p函数内则可以使用在外面定义的海龟对象变量tt——变量存在并发挥作用的范围称为变量的“作用域”（Scope），如果要修改变量的作用域，可以用global关键字来声明“全局变量”，用nonlocal关键字来声明“非局部变量”。\n",
"\n",
"上面程序中定义的star5p函数很不灵活，不管调用多少次都只从原点开始画一颗同样的五角星，让我们来加以改进——定义函数时加上参数变量列表：x和y变量指定五角星的坐标位置；size变量指定五角星的边长，angle变量指定五角星的倾角，在函数调用时作为参数传入的值将赋给这些变量，参数变量作用域限于函数内部：\n",
"```\n",
"\"\"\"自定义的海龟绘图函数集\n",
"\"\"\"\n",
"import turtle as tt\n",
"\n",
"def star5p(x, y, size=20, angle=0):\n",
" \"\"\"在指定位置画一颗五角星\n",
" \"\"\"\n",
" t = tt.Turtle() # 生成一个单独的海龟对象\n",
" t.hideturtle()\n",
" t.speed(0)\n",
" t.color(\"white\")\n",
" t.penup()\n",
" t.setpos(x, y)\n",
" t.right(angle)\n",
" t.begin_fill() # 启用填充区域\n",
" cnt = 0\n",
" while cnt < 5:\n",
" t.forward(size)\n",
" t.left(72)\n",
" t.forward(size)\n",
" t.right(144)\n",
" cnt += 1\n",
" t.end_fill()\n",
"\n",
"\n",
"def test():\n",
" \"\"\"测试绘图函数：随机画十颗五角星\n",
" \"\"\"\n",
" from random import randint\n",
" tt.TurtleScreen._RUNNING = True\n",
" tt.setup(width=720, height=480, startx=None, starty=None)\n",
" tt.hideturtle()\n",
" tt.speed(0)\n",
" tt.bgcolor(\"purple\")\n",
" tt.penup()\n",
" cnt = 0\n",
" while cnt < 10:\n",
" x = randint(-300, 300)\n",
" y = randint(-200, 200)\n",
" s = randint(10, 30)\n",
" a = randint(0, 72)\n",
" star5p(x, y, s, a) # 带参数调用五角星函数\n",
" cnt += 1\n",
" tt.done()\n",
"\n",
"\n",
"if __name__ == \"__main__\": # 运行模块时调用测试绘图函数\n",
" test()\n",
"```\n",
"现在的star5p函数有4个参数，前两个参数在调用时是必须传入的，后两个参数在定义时赋了默认值，在调用时如果不传入就会使用默认值——如果只传入三个参数，则第三个参数必须使用“变量名=值”的形式例如“angle=10”以避免歧义。上面的程序还定义了一个test函数，实现随机绘制十颗五角星：\n",
"\n",
"\n",
"程序文件turtledraw.pyw实际上就是一个自定义模块，可以使用import语句引入。模块在主名称空间中运行时```__name__```变量的取值是默认的```__main__```，被引入时```__name__```变量的取值则是模块名turtledraw，模块就可以据此判断自己是被运行还是被引入，从而决定是否执行test函数——推荐大家今后也都使用这样的模式，整个程序由独立的函数构成，并在最后调用一个作为入口的“主函数”。\n",
"\n",
"```\n",
"In [1]: import turtledraw as td\n",
"\n",
"In [2]: help(td)\n",
"Help on module turtledraw:\n",
"\n",
"NAME\n",
" turtledraw - 自定义的海龟绘图函数集\n",
"\n",
"FUNCTIONS\n",
" star5p(x, y, size=20, angle=0)\n",
" 在指定位置画一颗五角星\n",
" \n",
" test()\n",
" 测试绘图函数\n",
"\n",
"FILE\n",
" d:\\test\\pystudy\\turtledraw.pyw\n",
"\n",
"\n",
"\n",
"In [3]: td.__name__\n",
"Out[3]: 'turtledraw'\n",
"\n",
"In [4]: \n",
"```\n",
"\n",
"——编程原来是这样……\n",
"\n",
"## 编程小提示：程序项目\n",
"任何IDE都有“工程”或“项目”（Project）的概念，实际上就是建立专门的文件夹来组织相关的程序文件。例如Spyder中创建项目是在主菜单中选择Projects > New Project...，你可以创建一个名为pyStudy的项目，把你编写的所有程序文件都放进去。以后Spyder启动时会自动打开最近的项目并将“工作目录”设为项目所在的文件夹，而不再使用默认的用户文件夹。\n",
"\n",
"\n",
"下一篇：[编程入门08：Python列表类型](08_list.ipynb)"
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