python's `__init__` and `__del__` vs `__enter__` and `__exit__` methods

del_vs_exit.ipynb

{
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  {
   "cell_type": "markdown",
   "metadata": {
    "collapsed": true
   },
   "source": [
    "## `__del__` vs. `__exit__`\n"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "DEBUG:root:test\n"
     ]
    }
   ],
   "source": [
    "import logging\n",
    "rl = logging.getLogger()\n",
    "rl.setLevel(logging.DEBUG)\n",
    "logging.debug(\"test\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
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   "source": [
    "class Thing(object): # Basically always want to explicitly inherit from `object` in Python 2\n",
    "    \n",
    "    def __init__(self, tid):\n",
    "        self.tid = tid\n",
    "        logging.info(\"Thing __init__'d: {}\".format(self))\n",
    "    \n",
    "    def __del__(self):\n",
    "        logging.info(\"Thing __del__'d: {}\".format(self))\n",
    "    \n",
    "    def __enter__(self):\n",
    "        logging.info(\"Thing __enter__'ing: {}\".format(self))\n",
    "        return self\n",
    "    \n",
    "    def __exit__(self, type, value, traceback):\n",
    "        logging.info(\"Thing __exit__'ing: {}\\n exception info: ({})\".format(\n",
    "            self, (type, value, traceback)))\n",
    "    \n",
    "    def __str__(self):\n",
    "        return \"Thing(tid={}) @ {}\".format(self.__dict__.get(\"tid\", \"<not set>\"), id(self))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:Thing __init__'d: Thing(tid=1) @ 64515312\n"
     ]
    }
   ],
   "source": [
    "t1=Thing(1)"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Here, we call `t1.__enter__` by using `t1` in a `with` statement; the `enterRetVal` variable that references the return value from `t1.__enter__` is not deleted at the end of the `with` block, but `t1`'s `__exit__` method is called:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:Thing __enter__'ing: Thing(tid=1) @ 64515312\n",
      "INFO:root:t1 is enterRetVal: True\n",
      "INFO:root:Thing __exit__'ing: Thing(tid=1) @ 64515312\n",
      " exception info: ((None, None, None))\n"
     ]
    }
   ],
   "source": [
    "with t1 as enterRetVal:\n",
    "    logging.info(\"t1 is enterRetVal: {}\".format(t1 is enterRetVal))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 5,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "data": {
      "text/plain": [
       "True"
      ]
     },
     "execution_count": 5,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "enterRetVal is t1 # `enterRetVal` is still in scope, and still references the same that `t1` does:"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "calling `del` on `enterRetVal` or `t1` will decrement the reference count on `Thing(tid=1)`, but it won't be collected because there's still another reference"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 6,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "del enterRetVal"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "calling `del` on `t1` now will cause `Thing(tid=1)` to be collected:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 7,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:Thing __del__'d: Thing(tid=1) @ 64515312\n"
     ]
    }
   ],
   "source": [
    "del t1"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "even tho there are no more references to `Thing(tid=1)`, we may not see the logging message from its `__del__` method yet: nothing in the spec says that `__del__` has to be called right away (and in other python implementations like IronPython, it's never called right away), and in some cases it's never called at all: https://docs.python.org/2/reference/datamodel.html#object.__del__"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 8,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def doStuff():\n",
    "    t2 = Thing(2)\n",
    "    with t2 as enterRetVal:\n",
    "        logging.info(\"t2 is enterRetVal: {}\".format(t2 is enterRetVal))\n",
    "    # Things will be cleaned up automatically when they go out of scope .."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:Thing __init__'d: Thing(tid=2) @ 44176208\n",
      "INFO:root:Thing __enter__'ing: Thing(tid=2) @ 44176208\n",
      "INFO:root:t2 is enterRetVal: True\n",
      "INFO:root:Thing __exit__'ing: Thing(tid=2) @ 44176208\n",
      " exception info: ((None, None, None))\n",
      "INFO:root:Thing __del__'d: Thing(tid=2) @ 44176208\n"
     ]
    }
   ],
   "source": [
    "doStuff()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 10,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def doMoreStuff():\n",
    "    with Thing(3) as enterRetVal:\n",
    "        logging.info(\"only 1 reference to {} (i.e., `enterRetVal`)\".format(enterRetVal))\n",
    "    # Things will be cleaned up automatically when they go out of scope .."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 11,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:Thing __init__'d: Thing(tid=3) @ 44176816\n",
      "INFO:root:Thing __enter__'ing: Thing(tid=3) @ 44176816\n",
      "INFO:root:only 1 reference to Thing(tid=3) @ 44176816 (i.e., `enterRetVal`)\n",
      "INFO:root:Thing __exit__'ing: Thing(tid=3) @ 44176816\n",
      " exception info: ((None, None, None))\n",
      "INFO:root:Thing __del__'d: Thing(tid=3) @ 44176816\n"
     ]
    }
   ],
   "source": [
    "doMoreStuff()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 12,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": [
    "def doStuff4():\n",
    "    with Thing(4) as enterRetVal:\n",
    "        logging.info(\"only 1 reference to {} (i.e., `enterRetVal`)\".format(enterRetVal))\n",
    "    return enterRetVal"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 13,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:Thing __init__'d: Thing(tid=4) @ 44175856\n",
      "INFO:root:Thing __enter__'ing: Thing(tid=4) @ 44175856\n",
      "INFO:root:only 1 reference to Thing(tid=4) @ 44175856 (i.e., `enterRetVal`)\n",
      "INFO:root:Thing __exit__'ing: Thing(tid=4) @ 44175856\n",
      " exception info: ((None, None, None))\n"
     ]
    }
   ],
   "source": [
    "t4 = doStuff4()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 14,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:t4 references Thing(tid=4) @ 44175856\n"
     ]
    }
   ],
   "source": [
    "logging.info(\"t4 references {}\".format(t4))"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 15,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:Thing __enter__'ing: Thing(tid=4) @ 44175856\n",
      "INFO:root:Thing __exit__'ing: Thing(tid=4) @ 44175856\n",
      " exception info: ((None, None, None))\n"
     ]
    }
   ],
   "source": [
    "with t4:\n",
    "    pass"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 16,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:Thing __del__'d: Thing(tid=4) @ 44175856\n"
     ]
    }
   ],
   "source": [
    "del t4"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 17,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:nothing referencing `Thing(tid=4)` anymore ..\n"
     ]
    }
   ],
   "source": [
    "logging.info(\"nothing referencing `Thing(tid=4)` anymore ..\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Sometimes `__del__` can be called even tho `__init__` hasn't been called; `__exit__` can never be called without `__enter__`:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 18,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stderr",
     "output_type": "stream",
     "text": [
      "INFO:root:Thing __del__'d: Thing(tid=<not set>) @ 44176720\n"
     ]
    },
    {
     "ename": "TypeError",
     "evalue": "__init__() takes exactly 2 arguments (1 given)",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m\u001b[0m",
      "\u001b[0;31mTypeError\u001b[0mTraceback (most recent call last)",
      "\u001b[0;32m<ipython-input-18-48b865074d18>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m()\u001b[0m\n\u001b[0;32m----> 1\u001b[0;31m \u001b[0mt\u001b[0m\u001b[1;33m=\u001b[0m\u001b[0mThing\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mTypeError\u001b[0m: __init__() takes exactly 2 arguments (1 given)"
     ]
    }
   ],
   "source": [
    "t=Thing()"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {
    "collapsed": true
   },
   "outputs": [],
   "source": []
  }
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