sitc/python/1_9_Errors_Exceptions.ipynb

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    "![](http://www.upm.es/sfs/Rectorado/Gabinete%20del%20Rector/Logos/UPM/EscPolitecnica/EscUpmPolit_p.gif \"UPM\")"
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   "metadata": {},
   "source": [
    "# Course Notes for Learning Intelligent Systems"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Department of Telematic Engineering Systems, Universidad Politécnica de Madrid, © 2016 Carlos A. Iglesias"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Errors and Exceptions"
   ]
  },
  {
   "cell_type": "markdown",
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   "source": [
    "## Errors\n",
    "\n",
    "Errors (or syntax errors) are errors because the syntax is incorrect."
   ]
  },
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   "cell_type": "code",
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   "outputs": [],
   "source": [
    "# Example SyntaxError - missing semicolon in while\n",
    "while True\n",
    "    print \"hi\""
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "## Exceptions\n",
    "Even when the syntax is correct, we can have other types of errors."
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# Example TypeError - wrong use of '+' with different types\n",
    "3 + 'a'"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# Example NameError:  variable not defined\n",
    "a \n"
   ]
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   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Exceptions can be handled with the statement **try-except-else-finally**. \n",
    "\n",
    "* **try**: code block when an exception can occur\n",
    "* **except**: code block executed if the exception ocurrs (catches the exception)\n",
    "* **else**:  (optional) code block executed if no exception occurs\n",
    "* **finally**: (optional) code block executed always (both if there is an exception or not)"
   ]
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   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "# Example\n",
    "# Try to enter a wrong input (e.g. a) and then a number (e.g. 0)\n",
    "while True:\n",
    "    try:\n",
    "        num = int(input(\"Please enter a number: \"))\n",
    "        break\n",
    "    except ValueError:\n",
    "        print(\"Oops!  That was no valid number.  Try again...\")"
   ]
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   "cell_type": "code",
   "execution_count": null,
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   "outputs": [],
   "source": [
    "# Example with finally\n",
    "while True:\n",
    "    try:\n",
    "        num = int(input(\"Please enter a number: \"))\n",
    "        break\n",
    "    except ValueError:\n",
    "        print(\"Oops!  That was no valid number.  Try again...\")\n",
    "    finally:\n",
    "        print(\"End of input\")"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "# Example with else and finally\n",
    "while True:\n",
    "    try:\n",
    "        num = int(input(\"Please enter a number: \"))\n",
    "    except ValueError:\n",
    "        print(\"Oops!  That was no valid number.\")\n",
    "        continue\n",
    "    else:\n",
    "        print(\"No exception\")\n",
    "        break\n",
    "    finally:\n",
    "        print(\"Number entered\")"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "Exception can be thrown with 'raise'"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "metadata": {},
   "outputs": [],
   "source": [
    "def add(a, b):\n",
    "    try:\n",
    "        return a + b\n",
    "    except:\n",
    "        raise TypeError(\"Both arguments should be numbers or strings\")\n",
    "        \n",
    "add(3, 'a')"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "# Licence"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The notebook is freely licensed under under the [Creative Commons Attribution Share-Alike license](https://creativecommons.org/licenses/by/2.0/).  \n",
    "\n",
    "© 2016 Carlos A. Iglesias, Universidad Politécnica de Madrid."
   ]
  }
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