deque-slice.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# 让`deque`支持切片下标"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "import cffi\n",
    "from collections import deque"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Python的标准模块`collections`中的`deque`是一个双向链表结构，它支持在该链表的左右添加元素或者删除元素。它还支持整数下标运算，但是它不支持切片下标："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "9\n"
     ]
    },
    {
     "ename": "TypeError",
     "evalue": "sequence index must be integer, not 'slice'",
     "output_type": "error",
     "traceback": [
      "\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[1;31mTypeError\u001b[0m                                 Traceback (most recent call last)",
      "\u001b[1;32m<ipython-input-5-1d9676b37db5>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m      2\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m      3\u001b[0m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0md\u001b[0m\u001b[1;33m[\u001b[0m\u001b[1;33m-\u001b[0m\u001b[1;36m1\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 4\u001b[1;33m \u001b[0mprint\u001b[0m\u001b[1;33m(\u001b[0m\u001b[0md\u001b[0m\u001b[1;33m[\u001b[0m\u001b[1;33m-\u001b[0m\u001b[1;36m3\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m]\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[1;31mTypeError\u001b[0m: sequence index must be integer, not 'slice'"
     ]
    }
   ],
   "source": [
    "d = deque(range(10))\n",
    "\n",
    "print(d[-1])\n",
    "print(d[-3:])"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "使用`itertools`模块中的`slice()`可以获得对指定的切片迭代的对象，然后再调用`list()`将迭代器转换为列表即可："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[7, 8, 9]"
      ]
     },
     "execution_count": 13,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "from itertools import islice\n",
    "\n",
    "list(islice(d, len(d) - 3, len(d), 1))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "但是`islice()`不支持负数下标，可以使用`slice`对象的`indices()`方法将一个切片对象转换为`start`、`end`和`step`三个整数值。例如："
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 16,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[7, 8, 9]"
      ]
     },
     "execution_count": 16,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "start, end, step = slice(-3, None).indices(len(d))\n",
    "list(islice(d, start, end, step))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "如上所述，我们知道了如何使用切片获取`deque`对象的部分元素，下面让我们看看如何让`deque`支持切片下标。"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "t = ffi.cast(\"ssize_t *\", id(d))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "[1, 140233362912128, 0, 140232829701184, 140232829701184, 32, 31, 0, -1, 0]"
      ]
     },
     "execution_count": 9,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "list(t[0:10])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "dict_keys(['maxlen', '__ge__', '__contains__', 'copy', '__new__', 'extend', '__sizeof__', '__hash__', 'rotate', '__getattribute__', '__copy__', '__lt__', '__iadd__', 'reverse', '__repr__', 'append', '__bool__', '__rmul__', 'extendleft', '__delitem__', '__reversed__', '__setitem__', '__eq__', '__le__', '__iter__', '__mul__', 'appendleft', 'popleft', 'pop', '__init__', '__len__', '__add__', 'insert', 'remove', '__imul__', 'index', '__doc__', 'clear', '__getitem__', '__ne__', '__reduce__', '__gt__', 'count'])\n"
     ]
    }
   ],
   "source": [
    "print(deque.__dict__.keys())"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 32,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "%load_ext cython"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 96,
   "metadata": {},
   "outputs": [],
   "source": [
    "%%cython\n",
    "from cpython.ref cimport PyObject\n",
    "from cpython.sequence cimport PySequence_GetItem\n",
    "from cpython.slice cimport PySlice_GetIndices\n",
    "from itertools import islice\n",
    "from collections import deque\n",
    "DEF BLOCKLEN = 64\n",
    "\n",
    "cdef struct block:\n",
    "    block *leftlink\n",
    "    PyObject * data[BLOCKLEN]\n",
    "    block *rightlink\n",
    "\n",
    "cdef struct dequeobject:\n",
    "    ssize_t ob_refcnt\n",
    "    PyObject * ob_type\n",
    "    ssize_t ob_size\n",
    "    block *leftblock\n",
    "    block *rightblock\n",
    "    ssize_t leftindex\n",
    "    ssize_t rightindex\n",
    "    size_t state\n",
    "    ssize_t maxlen\n",
    "    PyObject * weakreflist\n",
    "\n",
    "cdef struct PyMappingMethods:\n",
    "    void * mp_length\n",
    "    void * mp_subscript\n",
    "    void * mp_ass_subscript\n",
    "\n",
    "cdef ssize_t deque_len(dequeobject * dq):\n",
    "    return dq.ob_size\n",
    "\n",
    "cdef deque_subscript(dequeobject * dq, object item):\n",
    "    cdef ssize_t start, end, step\n",
    "    if isinstance(item, int):\n",
    "        return PySequence_GetItem(<object>dq, <ssize_t>item)\n",
    "    elif isinstance(item, slice):\n",
    "        PySlice_GetIndices(item, dq.ob_size, &start, &end, &step)\n",
    "        return deque(islice(<object>dq, start, end, step))\n",
    "\n",
    "cdef PyMappingMethods deque_mapping_methods = [<void *>deque_len, <void *>deque_subscript, NULL]\n",
    "\n",
    "def addr_of_mapping_methods():\n",
    "    return <ssize_t>&(deque_mapping_methods)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 97,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "4\n",
      "deque([2, 3, 4])\n"
     ]
    },
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "/usr/local/anaconda/envs/py35/lib/python3.5/site-packages/cffi/vengine_cpy.py:188: UserWarning: reimporting '_cffi__xc07c2ec2x260339d1' might overwrite older definitions\n",
      "  % (self.verifier.get_module_name()))\n"
     ]
    }
   ],
   "source": [
    "ffi = cffi.FFI()\n",
    "\n",
    "ffi.cdef(\"\"\"\n",
    "ssize_t tp_as_sequence_offset;\n",
    "ssize_t sq_slice_offset;\n",
    "ssize_t mapping_methods_offset;\n",
    "\"\"\")\n",
    "\n",
    "lib = ffi.verify(\"\"\"\n",
    "\n",
    "ssize_t tp_as_sequence_offset = offsetof(PyTypeObject, tp_as_sequence);\n",
    "ssize_t sq_slice_offset = offsetof(PySequenceMethods, was_sq_slice);\n",
    "ssize_t mapping_methods_offset = offsetof(PyTypeObject, tp_as_mapping);\n",
    "\"\"\", tmpdir=\"/usr/local/home/tmp\")\n",
    "\n",
    "tp_as_mapping = ffi.cast(\"ssize_t *\", id(deque) + lib.mapping_methods_offset)\n",
    "\n",
    "tp_as_mapping[0] = addr_of_mapping_methods()\n",
    "\n",
    "a = deque([1,2,3,4])\n",
    "\n",
    "print(a[3])\n",
    "print(a[1:4])"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 98,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "a.append(5)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 99,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "deque([4, 5])"
      ]
     },
     "execution_count": 99,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "a[-2:]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 27,
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "0"
      ]
     },
     "execution_count": 27,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "ffi.cast(\"ssize_t *\", addr + lib.sq_slice_offset)[0]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 36,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def f():\n",
    "    a = deque([1,2,3])\n",
    "    b = a[2:]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 37,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "from dis import dis"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 79,
   "metadata": {},
   "outputs": [],
   "source": [
    "slice.indices?"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  }
 ],
 "metadata": {
  "anaconda-cloud": {},
  "kernelspec": {
   "display_name": "Python [default]",
   "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": "3.5.2"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 1
}