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    "# Exercises: working with modules\n",
    "\n",
    "### Exercise 4.1\n",
    "Write a function that creates an empty file `file_name` in a specified directory `dir_name`.  \n",
    "The function should:\n",
    "* create the directory if needed (support for recursive directory creation is not needed).\n",
    "* not overwrite/delete any existing files.\n",
    "* have a `test_mode` argument that, when set to `True`, cleans-up any newly created file and \n",
    "  deletes the directory `dir_name` if it is empty.\n",
    "\n",
    "To help you get started, the function's definition and docstring are already given below, \n",
    "as well as a few commands to test your function.  \n",
    "What you need to do is write the body of the `create_empty_file()` function."
   ]
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    {
     "ename": "FileNotFoundError",
     "evalue": "[Errno 2] No such file or directory: 'tmp_dir'",
     "output_type": "error",
     "traceback": [
      "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
      "\u001b[0;31mFileNotFoundError\u001b[0m                         Traceback (most recent call last)",
      "\u001b[0;32m<ipython-input-3-3232a3809675>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m\u001b[0m\n\u001b[1;32m     20\u001b[0m \u001b[0;31m# Directory \"dir_name\" should contain 'file_1.txt' and 'file_2.txt', but\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m     21\u001b[0m \u001b[0;31m# not 'file_3.txt' since the later was created in test mode.\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m---> 22\u001b[0;31m \u001b[0mprint\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m\"Content of directory\"\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mdir_name\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;34m\":\"\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0mos\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mlistdir\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0mdir_name\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
      "\u001b[0;31mFileNotFoundError\u001b[0m: [Errno 2] No such file or directory: 'tmp_dir'"
     ]
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    "\n",
    "def create_empty_file(file_name, dir_name, test_mode=False):\n",
    "    \"\"\"Create an empty file named 'file_name' in a directory\n",
    "    named 'dir_name'. If test_mode is True the file is\n",
    "    deleted immediatly after it was created.\n",
    "    \"\"\"\n",
    "    # Write the body of the function here...\n",
    "\n",
    "    \n",
    "    \n",
    "    \n",
    "# Let's test our function.\n",
    "# To verify that the \"create_empty_file()\" function works as expected, we run\n",
    "# it a number of times with test mode on and off, and then look at the content\n",
    "# of \"dir_name\".\n",
    "dir_name = 'tmp_dir'\n",
    "create_empty_file('file_1.txt', dir_name)\n",
    "create_empty_file('file_2.txt', dir_name)\n",
    "create_empty_file('file_3.txt', dir_name, test_mode=True)\n",
    "\n",
    "# Directory \"dir_name\" should contain 'file_1.txt' and 'file_2.txt', but \n",
    "# not 'file_3.txt' since the later was created in test mode.\n",
    "print(\"Content of directory\", dir_name, \":\", os.listdir(dir_name))\n"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<br>\n",
    "\n",
    "## Exercise 4.2\n",
    "What doe this line of code do?  \n",
    "`time.sleep(3)`"
   ]
  },
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "<br>\n",
    "\n",
    "## Exercise 4.3\n",
    "Import the function `is_part_of_set` of the `exercise4_3_module` module, located in the same folder as this notebook.\n",
    "1. What does `is_part_of_set` do ?\n",
    "2. Determine the result of `is_part_of_set` for all values of x and y from -2 to +2 in increment of 0.05.\n",
    "3. How long does this computation takes ?\n",
    "4. How would you represent the result of your work ?\n",
    "\n",
    "> Remember the `help()` function. You can also open the module file as a normal text file to read its code.\n",
    ">\n",
    "> The grid you have to test for contains 6400 points. Try with a smaller number of points first."
   ]
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