Examples creating and using custom context managers in Python

context_managers.ipynb

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    "## Context Manager Examples"
   ]
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      "some external work\n",
      "initializing context\n",
      "entering context\n",
      "some internal work\n",
      "5\n",
      "some more internal work\n",
      "exiting context\n",
      "some more external work\n"
     ]
    }
   ],
   "source": [
    "class C:\n",
    "    def __init__(self):\n",
    "        print(\"initializing context\")\n",
    "        self.x = 0\n",
    "\n",
    "    def __enter__(self):\n",
    "        print(\"entering context\")\n",
    "        self.x = 5\n",
    "        return self\n",
    "    \n",
    "    def add(self, y):\n",
    "        self.x + y\n",
    "        \n",
    "    def subtract(self, y):\n",
    "        self.x - y\n",
    "\n",
    "            \n",
    "    def __exit__(self, exit_type, exit_val, exit_tb):\n",
    "        print(\"exiting context\")\n",
    "    \n",
    "    \n",
    "print(\"some external work\")\n",
    "with C() as c: # both init and enter happen here\n",
    "    print(\"some internal work\")\n",
    "    print(c.x)\n",
    "    print(\"some more internal work\")\n",
    "    # exiting happen at the end\n",
    "print(\"some more external work\")\n"
   ]
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   "cell_type": "code",
   "execution_count": 8,
   "metadata": {},
   "outputs": [
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     "name": "stdout",
     "output_type": "stream",
     "text": [
      "__init__ code here\n",
      "__enter__ code goes here\n",
      "something\n",
      "__exit__ cleanup goes here\n"
     ]
    }
   ],
   "source": [
    "from contextlib import contextmanager\n",
    "\n",
    "@contextmanager\n",
    "def context(boolean):\n",
    "    print(\"__init__ code here\")\n",
    "    try:\n",
    "        print(\"__enter__ code goes here\")\n",
    "        yield object()\n",
    "    except Exception as e:\n",
    "        print(\"errors handled here\")\n",
    "        if not boolean:\n",
    "            raise e\n",
    "    finally:\n",
    "        print(\"__exit__ cleanup goes here\")\n",
    "        \n",
    "        \n",
    "with context(True) as c:\n",
    "    print(\"something\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 9,
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "some external work\n",
      "initializing context\n",
      "entering context\n",
      "some internal work\n",
      "5\n",
      "some more internal work\n",
      "exiting context\n",
      "some more external work\n"
     ]
    }
   ],
   "source": [
    "from contextlib import contextmanager\n",
    "\n",
    "@contextmanager\n",
    "def C(boolean=False):\n",
    "    print(\"initializing context\")    \n",
    "    try:\n",
    "        print(\"entering context\")\n",
    "        data = {\n",
    "            \"x\": 5\n",
    "        }\n",
    "        obj = type('', (object,), data)()\n",
    "        yield obj\n",
    "    except Exception as e:\n",
    "        print(\"errors handled here\")\n",
    "        if not boolean:\n",
    "            raise e\n",
    "    finally:\n",
    "        print(\"exiting context\")\n",
    "        \n",
    "\n",
    "        \n",
    "print(\"some external work\")\n",
    "with C() as c: # both init and enter happen here\n",
    "    print(\"some internal work\")\n",
    "    print(c.x)\n",
    "    print(\"some more internal work\")\n",
    "    # exiting happen at the end\n",
    "print(\"some more external work\")        \n",
    "        "
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": []
  }
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Note that in the __exit__ method using the class way:

def __exit__(self, exit_type, exit_val, exit_tb):
    print("exiting context")

exit_type is the exit type
exit_val is the exit value
exit_tb is the exit traceback

Try printed them out if you want to see what they look like.