PyLzuCSEdu/3 变量作用域.ipynb

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   "cell_type": "markdown",
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   "source": [
    "## 变量的作用域\n",
    "\n",
    "变量的作用域（scope）决定了变量可以被访问和使用的代码区域。\n",
    "\n",
    "在Python中，变量的作用域主要有以下几种类型：\n",
    " - 局部作用域 (Local Scope):函数内部定义的变量, 仅在函数内部可见可用。\n",
    " - 嵌套作用域 (Enclosing Scope):在嵌套函数中，外部函数的变量对内部函数可见。\n",
    " - 全局作用域 (Global Scope): 在所有函数外部定义的变量，可以在整个程序中被访问。\n",
    " - 内置作用域 (Built-in Scope): 包含Python内置函数、变量和异常等。最外层作用域。   "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 局部变量和全局变量\n",
    " - 赋值运算默认创建局部变量"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 24,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "10\n",
      "10\n"
     ]
    }
   ],
   "source": [
    "x = 10  # 全局变量\n",
    "\n",
    "def ff():\n",
    "    y = 5  # 赋值运算默认创建局部变量\n",
    "    print(x)  # 可以访问全局变量 x\n",
    "\n",
    "ff()  # 调用函数，输出10\n",
    "print(x)  # 输出全局变量x的值，仍然是10\n",
    "# print(y)  # 出错，y 在 ff 内部定义，是局部变量，在函数外部无法访问"
   ]
  },
  {
   "cell_type": "raw",
   "metadata": {},
   "source": [
    "''' 特别注意：下面代码发生错误。\n",
    "因为执行到 x = x + 2 ，首先将 x 登记为局部变量，\n",
    "接着计算右边的x + 2时，x还未被定义，所以会报错。\n",
    "'''\n",
    "x  = 10\n",
    "\n",
    "def ff():    \n",
    "   x = x + 2   #  x += 2 等价\n",
    "    \n",
    "ff()"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 函数嵌套\n",
    "\n",
    "Python 变量访问优先级规则(LEGB)，这也是变量的遮蔽顺序：Local（局部作用域） -> Enclosing（嵌套作用域） -> Global（全局作用域） -> Built-in（内置作用域）\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 25,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "2\n",
      "5\n",
      "10\n"
     ]
    }
   ],
   "source": [
    "x = 10  # 全局变量\n",
    "\n",
    "def outer():\n",
    "    x = 5  # outer函数的局部变量\n",
    "\n",
    "    def inner():\n",
    "        x = 2  # inner函数的局部变量\n",
    "        print(x)  # 输出局部变量x的值\n",
    "\n",
    "    inner()  # 输出2\n",
    "    print(x)  # 输出outer函数的局部变量x的值，即 5\n",
    "\n",
    "outer()  # 调用outer函数\n",
    "print(x)  # 输出全局变量x的值，即10"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### 如果打算给非局部变量赋值，需要特别申明"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "###  global 关键字功能\n",
    "\n",
    "想在函数内部修改全局变量，使用global关键字。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "5\n"
     ]
    }
   ],
   "source": [
    "x = 10  # 全局变量\n",
    "\n",
    "def func():\n",
    "    global x  # 声明使用全局变量 x\n",
    "    x = 5    # 修改全局变量 x 的值\n",
    "\n",
    "func()  # 调用函数，修改全局变量x的值\n",
    "print(x)  # 输出全局变量x的新值，现在是 5"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### nonlocal 关键字功能\n",
    "\n",
    "函数嵌套时，如果想修改外层函数的局部变量，需要使用nonlocal关键字。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "10\n"
     ]
    }
   ],
   "source": [
    "def outer():\n",
    "    x = 5  # outer函数的局部变量\n",
    "\n",
    "    def inner():\n",
    "        nonlocal x  # 声明x是outer函数的局部变量，而不是全局变量\n",
    "        x = 10     # 修改outer函数的局部变量x的值\n",
    "\n",
    "    inner()  # 调用inner函数\n",
    "    print(x)  # 输出修改后的x值，即10\n",
    "\n",
    "outer()  # 调用outer函数"
   ]
  }
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