{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# 不可变集合"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "对应于元组（`tuple`）与列表（`list`）的关系，对于集合（`set`），**Python**提供了一种叫做不可变集合（`frozen set`）的数据结构。\n",
    "\n",
    "使用 `frozenset` 来进行创建："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "frozenset({1, 2, 3, 'a'})"
      ]
     },
     "execution_count": 1,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s = frozenset([1, 2, 3, 'a', 1])\n",
    "s"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "与集合不同的是，不可变集合一旦创建就不可以改变。\n",
    "\n",
    "不可变集合的一个主要应用是用来作为字典的键，例如用一个字典来记录两个城市之间的距离："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "{frozenset({'Austin', 'New York'}): 1515,\n",
       " frozenset({'Austin', 'Los Angeles'}): 1233,\n",
       " frozenset({'Los Angeles', 'New York'}): 2498}"
      ]
     },
     "execution_count": 2,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "flight_distance = {}\n",
    "city_pair = frozenset(['Los Angeles', 'New York'])\n",
    "flight_distance[city_pair] = 2498\n",
    "flight_distance[frozenset(['Austin', 'Los Angeles'])] = 1233\n",
    "flight_distance[frozenset(['Austin', 'New York'])] = 1515\n",
    "flight_distance"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "由于集合不分顺序，所以不同顺序不会影响查阅结果："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "1515"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "flight_distance[frozenset(['New York','Austin'])]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "1515"
      ]
     },
     "execution_count": 4,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "flight_distance[frozenset(['Austin','New York'])]"
   ]
  }
 ],
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  "kernelspec": {
   "display_name": "Python 2",
   "language": "python",
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   "codemirror_mode": {
    "name": "ipython",
    "version": 2
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   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython2",
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