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sitc/python/1_9_Errors_Exceptions.ipynb
cif2cif f71e7e167b First commit notebooks for python
2016-02-01 13:05:58 +01:00

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{
"cells": [
{
"cell_type": "markdown",
"metadata": {},
"source": [
"![](http://www.upm.es/sfs/Rectorado/Gabinete%20del%20Rector/Logos/UPM/EscPolitecnica/EscUpmPolit_p.gif \"UPM\")"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# Course Notes for Learning Intelligent Systems"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"Department of Telematic Engineering Systems, Universidad Politécnica de Madrid, © 2015 Carlos A. Iglesias"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"# Errors and Exceptions"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Errors\n",
"\n",
"Errors (or syntax errors) are errors because the syntax is incorrect."
]
},
{
"cell_type": "code",
"execution_count": 1,
"metadata": {
"collapsed": false
},
"outputs": [
{
"ename": "SyntaxError",
"evalue": "invalid syntax (<ipython-input-1-16b482c78ccd>, line 2)",
"output_type": "error",
"traceback": [
"\u001b[1;36m File \u001b[1;32m\"<ipython-input-1-16b482c78ccd>\"\u001b[1;36m, line \u001b[1;32m2\u001b[0m\n\u001b[1;33m while True\u001b[0m\n\u001b[1;37m ^\u001b[0m\n\u001b[1;31mSyntaxError\u001b[0m\u001b[1;31m:\u001b[0m invalid syntax\n"
]
}
],
"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": 2,
"metadata": {
"collapsed": false
},
"outputs": [
{
"ename": "TypeError",
"evalue": "unsupported operand type(s) for +: 'int' and 'str'",
"output_type": "error",
"traceback": [
"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[1;32m<ipython-input-2-fb8fe04af7b6>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m 1\u001b[0m \u001b[1;31m# Example TypeError - wrong use of '+' with different types\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 2\u001b[1;33m \u001b[1;36m3\u001b[0m \u001b[1;33m+\u001b[0m \u001b[1;34m'a'\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m: unsupported operand type(s) for +: 'int' and 'str'"
]
}
],
"source": [
"# Example TypeError - wrong use of '+' with different types\n",
"3 + 'a'"
]
},
{
"cell_type": "code",
"execution_count": 3,
"metadata": {
"collapsed": false,
"scrolled": true
},
"outputs": [
{
"ename": "NameError",
"evalue": "name 'a' is not defined",
"output_type": "error",
"traceback": [
"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[1;31mNameError\u001b[0m Traceback (most recent call last)",
"\u001b[1;32m<ipython-input-3-82179588c5b9>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m 1\u001b[0m \u001b[1;31m# Example NameError: variable not defined\u001b[0m\u001b[1;33m\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 2\u001b[1;33m \u001b[0ma\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[1;31mNameError\u001b[0m: name 'a' is not defined"
]
}
],
"source": [
"# Example NameError: variable not defined\n",
"a \n"
]
},
{
"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)"
]
},
{
"cell_type": "code",
"execution_count": 4,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Please enter a number: a\n",
"Oops! That was no valid number. Try again...\n",
"Please enter a number: 1\n"
]
}
],
"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...\")"
]
},
{
"cell_type": "code",
"execution_count": 1,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Please enter a number: a\n",
"Oops! That was no valid number. Try again...\n",
"End of input\n",
"Please enter a number: 1\n",
"End of input\n"
]
}
],
"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": 2,
"metadata": {
"collapsed": false
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Please enter a number: a\n",
"Oops! That was no valid number.\n",
"Number entered\n",
"Please enter a number: 1\n",
"No exception\n",
"Number entered\n"
]
}
],
"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": 5,
"metadata": {
"collapsed": false
},
"outputs": [
{
"ename": "TypeError",
"evalue": "Both arguments should be numbers or strings",
"output_type": "error",
"traceback": [
"\u001b[1;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[1;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[1;32m<ipython-input-5-1e4048e7e923>\u001b[0m in \u001b[0;36madd\u001b[1;34m(a, b)\u001b[0m\n\u001b[0;32m 2\u001b[0m \u001b[1;32mtry\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 3\u001b[1;33m \u001b[1;32mreturn\u001b[0m \u001b[0ma\u001b[0m \u001b[1;33m+\u001b[0m \u001b[0mb\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m 4\u001b[0m \u001b[1;32mexcept\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
"\u001b[1;31mTypeError\u001b[0m: unsupported operand type(s) for +: 'int' and 'str'",
"\nDuring handling of the above exception, another exception occurred:\n",
"\u001b[1;31mTypeError\u001b[0m Traceback (most recent call last)",
"\u001b[1;32m<ipython-input-5-1e4048e7e923>\u001b[0m in \u001b[0;36m<module>\u001b[1;34m()\u001b[0m\n\u001b[0;32m 5\u001b[0m \u001b[1;32mraise\u001b[0m \u001b[0mTypeError\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m\"Both arguments should be numbers or strings\"\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m 6\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 7\u001b[1;33m \u001b[0madd\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;36m3\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;34m'a'\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[1;32m<ipython-input-5-1e4048e7e923>\u001b[0m in \u001b[0;36madd\u001b[1;34m(a, b)\u001b[0m\n\u001b[0;32m 3\u001b[0m \u001b[1;32mreturn\u001b[0m \u001b[0ma\u001b[0m \u001b[1;33m+\u001b[0m \u001b[0mb\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m 4\u001b[0m \u001b[1;32mexcept\u001b[0m\u001b[1;33m:\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[1;32m----> 5\u001b[1;33m \u001b[1;32mraise\u001b[0m \u001b[0mTypeError\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;34m\"Both arguments should be numbers or strings\"\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0m\u001b[0;32m 6\u001b[0m \u001b[1;33m\u001b[0m\u001b[0m\n\u001b[0;32m 7\u001b[0m \u001b[0madd\u001b[0m\u001b[1;33m(\u001b[0m\u001b[1;36m3\u001b[0m\u001b[1;33m,\u001b[0m \u001b[1;34m'a'\u001b[0m\u001b[1;33m)\u001b[0m\u001b[1;33m\u001b[0m\u001b[0m\n",
"\u001b[1;31mTypeError\u001b[0m: Both arguments should be numbers or strings"
]
}
],
"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",
"© 2015 Carlos A. Iglesias, Universidad Politécnica de Madrid."
]
}
],
"metadata": {
"kernelspec": {
"display_name": "Python 3",
"language": "python",
"name": "python3"
},
"language_info": {
"codemirror_mode": {
"name": "ipython",
"version": 3
},
"file_extension": ".py",
"mimetype": "text/x-python",
"name": "python",
"nbconvert_exporter": "python",
"pygments_lexer": "ipython3",
"version": "3.5.1"
}
},
"nbformat": 4,
"nbformat_minor": 0
}
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