learn-python3/notebooks/beginner/file_io.ipynb

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  {
   "cell_type": "markdown",
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   "source": [
    "# [File I/O](https://docs.python.org/3/tutorial/inputoutput.html#reading-and-writing-files)\n",
    "Reading and writing files."
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Working with paths"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "import os\n",
    "\n",
    "current_file = os.path.realpath('file_io.ipynb')  \n",
    "print('current file: {}'.format(current_file))\n",
    "# Note: in .py files you can get the path of current file by __file__\n",
    "\n",
    "current_dir = os.path.dirname(current_file)  \n",
    "print('current directory: {}'.format(current_dir))\n",
    "# Note: in .py files you can get the dir of current file by os.path.dirname(__file__)\n",
    "\n",
    "data_dir = os.path.join(current_dir, 'data')\n",
    "print('data directory: {}'.format(data_dir))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "### Checking if path exists"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "print('exists: {}'.format(os.path.exists(data_dir)))\n",
    "print('is file: {}'.format(os.path.isfile(data_dir)))\n",
    "print('is directory: {}'.format(os.path.isdir(data_dir)))"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Reading files"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "file_path = os.path.join(data_dir, 'simple_file.txt')\n",
    "\n",
    "with open(file_path, 'r') as simple_file:\n",
    "    for line in simple_file:\n",
    "        print(line.strip())"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The [`with`](https://docs.python.org/3/reference/compound_stmts.html#the-with-statement) statement is for obtaining a [context manager](https://docs.python.org/3/reference/datamodel.html#with-statement-context-managers) that will be used as an execution context for the commands inside the `with`. Context managers guarantee that certain operations are done when exiting the context. \n",
    "\n",
    "In this case, the context manager guarantees that `simple_file.close()` is implicitly called when exiting the context. This is a way to make developers life easier: you don't have to remember to explicitly close the file you openened nor be worried about an exception occuring while the file is open. Unclosed file maybe a source of a resource leak. Thus, prefer using `with open()` structure always with file I/O.\n",
    "\n",
    "To have an example, the same as above without the `with`."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "file_path = os.path.join(data_dir, 'simple_file.txt')\n",
    "\n",
    "# THIS IS NOT THE PREFERRED WAY\n",
    "simple_file = open(file_path, 'r')\n",
    "for line in simple_file:\n",
    "    print(line.strip())\n",
    "simple_file.close()  # This has to be called explicitly "
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Writing files"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "new_file_path = os.path.join(data_dir, 'new_file.txt')\n",
    "\n",
    "with open(new_file_path, 'w') as my_file:\n",
    "    my_file.write('This is my first file that I wrote with Python.')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Now go and check that there is a new_file.txt in the data directory. After that you can delete the file by:"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "if os.path.exists(new_file_path):  # make sure it's there\n",
    "    os.remove(new_file_path)"
   ]
  }
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