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{
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"cells": [
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"## 变量的作用域\n",
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"\n",
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"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",
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"\n",
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"在Python中，变量的作用域主要有以下几种类型：\n",
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" - 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",
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" - 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",
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" - 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",
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" - 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。 "
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### 局部变量和全局变量\n",
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" - 赋值运算默认创建局部变量"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 24,
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"10\n",
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"10\n"
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]
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}
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],
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"source": [
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"x = 10 # 全局变量\n",
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"\n",
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"def ff():\n",
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" y = 5 # 赋值运算默认创建局部变量\n",
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" print(x) # 可以访问全局变量 x\n",
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"\n",
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"ff() # 调用函数，输出10\n",
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"print(x) # 输出全局变量x的值，仍然是10\n",
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"# 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"
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]
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},
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{
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"cell_type": "raw",
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"metadata": {},
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"source": [
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"''' 特别注意：下面代码发生错误。\n",
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"因为执行到 x = x + 2 ，首先将 x 登记为局部变量，\n",
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"接着计算右边的x + 2时，x还未被定义，所以会报错。\n",
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"'''\n",
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"x = 10\n",
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"\n",
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"def ff(): \n",
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" x = x + 2 # x += 2 等价\n",
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" \n",
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"ff()"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### 函数嵌套\n",
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"\n",
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"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"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 25,
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"2\n",
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"5\n",
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"10\n"
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]
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}
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],
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"source": [
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"x = 10 # 全局变量\n",
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"\n",
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"def outer():\n",
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" x = 5 # outer函数的局部变量\n",
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"\n",
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" def inner():\n",
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" x = 2 # inner函数的局部变量\n",
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" print(x) # 输出局部变量x的值\n",
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"\n",
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" inner() # 输出2\n",
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" print(x) # 输出outer函数的局部变量x的值，即 5\n",
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"\n",
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"outer() # 调用outer函数\n",
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"print(x) # 输出全局变量x的值，即10"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### 如果打算给非局部变量赋值，需要特别申明"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### global 关键字功能\n",
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"\n",
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"想在函数内部修改全局变量，使用global关键字。"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 6,
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"5\n"
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]
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}
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],
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"source": [
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"x = 10 # 全局变量\n",
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"\n",
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"def func():\n",
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" global x # 声明使用全局变量 x\n",
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" x = 5 # 修改全局变量 x 的值\n",
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"\n",
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"func() # 调用函数，修改全局变量x的值\n",
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"print(x) # 输出全局变量x的新值，现在是 5"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"### nonlocal 关键字功能\n",
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"\n",
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"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。"
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]
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},
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{
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"cell_type": "code",
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"execution_count": 14,
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"metadata": {},
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"outputs": [
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{
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"name": "stdout",
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"output_type": "stream",
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"text": [
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"10\n"
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]
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}
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],
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"source": [
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"def outer():\n",
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" x = 5 # outer函数的局部变量\n",
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"\n",
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" def inner():\n",
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" nonlocal x # 声明x是outer函数的局部变量，而不是全局变量\n",
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" x = 10 # 修改outer函数的局部变量x的值\n",
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"\n",
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" inner() # 调用inner函数\n",
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" print(x) # 输出修改后的x值，即10\n",
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"\n",
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"outer() # 调用outer函数"
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]
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}
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],
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"metadata": {
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"hide_input": false,
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"kernelspec": {
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"display_name": "Python 3",
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"language": "python",
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"name": "python3"
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},
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"language_info": {
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"codemirror_mode": {
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"name": "ipython",
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"version": 3
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},
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"file_extension": ".py",
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"mimetype": "text/x-python",
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"name": "python",
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"nbconvert_exporter": "python",
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"pygments_lexer": "ipython3",
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"version": "3.8.3"
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}
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},
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"nbformat": 4,
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"nbformat_minor": 4
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}
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