{  "cells": [   {    "cell_type": "code",    "execution_count": 5,    "metadata": {     "collapsed": false,     "deletable": true,     "editable": true    },    "outputs": [],    "source": [     "import inspect\n",     "\n",     "\n",     "not_cached = object()\n",     "\n",     "\n",     "class lazy:\n",     "    def __init__(self, callable_, *args, **kwargs):\n",     "        self.__callable = callable_\n",     "        self.__args = args\n",     "        self.__kwargs = kwargs\n",     "        self.__cache = not_cached\n",     "        \n",     "    @property\n",     "    def __value(self):\n",     "        if self.__cache is not_cached:\n",     "            self.__cache = self.__callable()\n",     "            \n",     "        return self.__cache\n",     "    \n",     "    \n",     "    def __getattr__(self, name):\n",     "        return getattr(self.__value, name)\n",     "        \n",     "    def __iter__(self):\n",     "        return self\n",     "    \n",     "    def __next__(self):\n",     "        yield from iter(self.__value)\n",     "        \n",     "    def __eq__(self, other):\n",     "        if isinstance(other, lazy):\n",     "            other = other.__value\n",     "        \n",     "        return self.__value == other\n",     "    \n",     "    def __lt__(self, other):\n",     "        if isinstance(other, lazy):\n",     "            other = other.__value\n",     "            \n",     "        return self.__value < other\n",     "    \n",     "    def __and__(self, other):\n",     "        if isinstance(other, lazy):\n",     "            other = other.__value\n",     "            \n",     "        return self.__value & other\n",     "     \n",     "    def __or__(self, other):\n",     "        if isinstance(other, lazy):\n",     "            other = other.__value\n",     "            \n",     "        return self.__value | other\n",     "    \n",     "    def __xor__(self, other):\n",     "        if isinstance(other, lazy):\n",     "            other = other.__value\n",     "            \n",     "        return self.__value ^ other\n",     "    \n",     "    def __add__(self, other):\n",     "        if isinstance(other, lazy):\n",     "            other = other.__value\n",     "            \n",     "        return self.__value + other\n",     "    \n",     "    def __invert__(self, other):\n",     "        return ~self.__value\n",     "    \n",     "    def __str__(self):\n",     "        return str(self.__value)\n",     "    \n",     "    def __repr__(self):\n",     "        return repr(self.__value)\n",     "    \n",     "\n",     "class let:\n",     "    def __init__(self, **equations):\n",     "        self._equations = equations\n",     "        self._cache = {}\n",     "        \n",     "    def __getattr__(self, name):\n",     "        if name not in self._cache:\n",     "            equation = self._equations[name]\n",     "            code = equation.__code__\n",     "            names = set(code.co_names)\n",     "            varnames = set(code.co_varnames)\n",     "            freevars = set(code.co_freevars)\n",     "            \n",     "            globals_ = equation.__globals__\n",     "            globals_set = set(globals_)\n",     "        \n",     "            missing_names = names - (varnames | freevars | globals_set)\n",     "            if (name in missing_names or\n",     "                    not missing_names <= (set(self._equations) | globals_set)):\n",     "                raise ValueError\n",     "            resolved_names = {missing_name: getattr(self, missing_name)\n",     "                                 for missing_name in missing_names\n",     "                                 if missing_name not in globals_}\n",     "            globals_.update(resolved_names)  # mimic closure\n",     "            \n",     "            self._cache[name] = lazy(equation)\n",     "\n",     "        return self._cache[name]\n",     "    \n",     "    def to_locals(self):\n",     "        previous_frame = inspect.stack()[1][0]\n",     "        for equation in self._equations:\n",     "            previous_frame.f_locals[equation] = getattr(self, equation)\n",     "        "    ]   },   {    "cell_type": "code",    "execution_count": 7,    "metadata": {     "collapsed": false,     "deletable": true,     "editable": true    },    "outputs": [     {      "name": "stdout",      "output_type": "stream",      "text": [       "3\n",       "2\n"      ]     }    ],    "source": [     "c = 1\n",     "let(b=lambda: c + 1,\n",     "    a=lambda: b + 1).to_locals()\n",     "\n",     "print(a)\n",     "print(b)"    ]   },   {    "cell_type": "code",    "execution_count": null,    "metadata": {     "collapsed": true,     "deletable": true,     "editable": true    },    "outputs": [],    "source": []   }  ],  "metadata": {   "kernelspec": {    "display_name": "Python 3",    "language": "python",    "name": "python3"   },   "language_info": {    "codemirror_mode": {     "name": "ipython",     "version": 3    },    "file_extension": ".py",    "mimetype": "text/x-python",    "name": "python",    "nbconvert_exporter": "python",    "pygments_lexer": "ipython3",    "version": "3.6.1"   }  },  "nbformat": 4,  "nbformat_minor": 2 }