{ "cells": [ { "cell_type": "markdown", "id": "12b02b53", "metadata": {}, "source": [ "

Exceptions & Errors

\n", "
\n", "

Basic types of exceptions and errors

\n", "
When we write code we often get error messages, these can be of some different types\n", "
\n", "\n", "
\n", "Lets look att some of these" ] }, { "cell_type": "code", "execution_count": 1, "id": "c913485b", "metadata": {}, "outputs": [ { "ename": "SyntaxError", "evalue": "unterminated string literal (detected at line 2) (2743400583.py, line 2)", "output_type": "error", "traceback": [ "\u001b[0;36m Cell \u001b[0;32mIn[1], line 2\u001b[0;36m\u001b[0m\n\u001b[0;31m print(\"hello)\u001b[0m\n\u001b[0m ^\u001b[0m\n\u001b[0;31mSyntaxError\u001b[0m\u001b[0;31m:\u001b[0m unterminated string literal (detected at line 2)\n" ] } ], "source": [ "#Syntax Error\n", "print(\"hello)" ] }, { "cell_type": "code", "execution_count": 2, "id": "eee2b687", "metadata": {}, "outputs": [ { "ename": "NameError", "evalue": "name 'hello' is not defined", "output_type": "error", "traceback": [ "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m", "\u001b[0;31mNameError\u001b[0m Traceback (most recent call last)", "\u001b[1;32m/Users/randlerm2/Documents/UU och Campus/Undervisning UU/Kurser 23-24/23 Coding in Python /Lectures/L5/Exceptions and Errors.ipynb Cell 3\u001b[0m line \u001b[0;36m2\n\u001b[1;32m 1\u001b[0m \u001b[39m#Name error\u001b[39;00m\n\u001b[0;32m----> 2\u001b[0m \u001b[39mprint\u001b[39m(hello)\n", "\u001b[0;31mNameError\u001b[0m: name 'hello' is not defined" ] } ], "source": [ "#Name error\n", "print(hello)" ] }, { "cell_type": "code", "execution_count": 3, "id": "9cd0226e", "metadata": {}, "outputs": [ { "ename": "ZeroDivisionError", "evalue": "division by zero", "output_type": "error", "traceback": [ "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m", "\u001b[0;31mZeroDivisionError\u001b[0m Traceback (most recent call last)", "\u001b[1;32m/Users/randlerm2/Documents/UU och Campus/Undervisning UU/Kurser 23-24/23 Coding in Python /Lectures/L5/Exceptions and Errors.ipynb Cell 4\u001b[0m line \u001b[0;36m2\n\u001b[1;32m 1\u001b[0m \u001b[39m#Zero division\u001b[39;00m\n\u001b[0;32m----> 2\u001b[0m \u001b[39mprint\u001b[39m(\u001b[39m1\u001b[39m\u001b[39m/\u001b[39m\u001b[39m0\u001b[39m)\n", "\u001b[0;31mZeroDivisionError\u001b[0m: division by zero" ] } ], "source": [ "#Zero division\n", "print(1/0)" ] }, { "cell_type": "code", "execution_count": 4, "id": "ff7bba7a", "metadata": {}, "outputs": [ { "ename": "TypeError", "evalue": "unsupported operand type(s) for +: 'int' and 'str'", "output_type": "error", "traceback": [ "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m", "\u001b[0;31mTypeError\u001b[0m Traceback (most recent call last)", "\u001b[1;32m/Users/randlerm2/Documents/UU och Campus/Undervisning UU/Kurser 23-24/23 Coding in Python /Lectures/L5/Exceptions and Errors.ipynb Cell 5\u001b[0m line \u001b[0;36m2\n\u001b[1;32m 1\u001b[0m \u001b[39m#Type error\u001b[39;00m\n\u001b[0;32m----> 2\u001b[0m \u001b[39mprint\u001b[39m(\u001b[39m1\u001b[39m\u001b[39m+\u001b[39m\u001b[39m\"\u001b[39m\u001b[39m2\u001b[39m\u001b[39m\"\u001b[39m)\n", "\u001b[0;31mTypeError\u001b[0m: unsupported operand type(s) for +: 'int' and 'str'" ] } ], "source": [ "#Type error\n", "print(1+\"2\")" ] }, { "cell_type": "code", "execution_count": 5, "id": "874c4879", "metadata": {}, "outputs": [ { "ename": "ValueError", "evalue": "could not convert string to float: 'hello'", "output_type": "error", "traceback": [ "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m", "\u001b[0;31mValueError\u001b[0m Traceback (most recent call last)", "\u001b[1;32m/Users/randlerm2/Documents/UU och Campus/Undervisning UU/Kurser 23-24/23 Coding in Python /Lectures/L5/Exceptions and Errors.ipynb Cell 6\u001b[0m line \u001b[0;36m2\n\u001b[1;32m 1\u001b[0m \u001b[39m#Value error\u001b[39;00m\n\u001b[0;32m----> 2\u001b[0m \u001b[39mprint\u001b[39m(\u001b[39mfloat\u001b[39m(\u001b[39m\"\u001b[39m\u001b[39mhello\u001b[39m\u001b[39m\"\u001b[39m))\n", "\u001b[0;31mValueError\u001b[0m: could not convert string to float: 'hello'" ] } ], "source": [ "#Value error\n", "print(float(\"hello\"))" ] }, { "cell_type": "code", "execution_count": 6, "id": "9e940ee8", "metadata": {}, "outputs": [ { "ename": "IndexError", "evalue": "list index out of range", "output_type": "error", "traceback": [ "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m", "\u001b[0;31mIndexError\u001b[0m Traceback (most recent call last)", "\u001b[1;32m/Users/randlerm2/Documents/UU och Campus/Undervisning UU/Kurser 23-24/23 Coding in Python /Lectures/L5/Exceptions and Errors.ipynb Cell 7\u001b[0m line \u001b[0;36m3\n\u001b[1;32m 1\u001b[0m \u001b[39m#Index error\u001b[39;00m\n\u001b[1;32m 2\u001b[0m list1 \u001b[39m=\u001b[39m [\u001b[39m1\u001b[39m,\u001b[39m2\u001b[39m,\u001b[39m3\u001b[39m,\u001b[39m4\u001b[39m,\u001b[39m5\u001b[39m]\n\u001b[0;32m----> 3\u001b[0m \u001b[39mprint\u001b[39m(list1[\u001b[39m5\u001b[39m])\n", "\u001b[0;31mIndexError\u001b[0m: list index out of range" ] } ], "source": [ "#Index error\n", "list1 = [1,2,3,4,5]\n", "print(list1[5])" ] }, { "cell_type": "markdown", "id": "ecc8b0be", "metadata": {}, "source": [ "Sometimes we don't get any error message until we run the code - runtime errors" ] }, { "cell_type": "code", "execution_count": 7, "id": "4d26e723", "metadata": {}, "outputs": [ { "ename": "ValueError", "evalue": "invalid literal for int() with base 10: 'fhfhg'", "output_type": "error", "traceback": [ "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m", "\u001b[0;31mValueError\u001b[0m Traceback (most recent call last)", "\u001b[1;32m/Users/randlerm2/Documents/UU och Campus/Undervisning UU/Kurser 23-24/23 Coding in Python /Lectures/L5/Exceptions and Errors.ipynb Cell 9\u001b[0m line \u001b[0;36m3\n\u001b[1;32m 1\u001b[0m \u001b[39m#Try this but give \"five\" as input instead of an integer\u001b[39;00m\n\u001b[1;32m 2\u001b[0m \u001b[39m#This will give a runtime ValueError\u001b[39;00m\n\u001b[0;32m----> 3\u001b[0m x \u001b[39m=\u001b[39m \u001b[39mint\u001b[39m(\u001b[39minput\u001b[39m(\u001b[39m\"\u001b[39m\u001b[39mWhat\u001b[39m\u001b[39m'\u001b[39m\u001b[39ms x? \u001b[39m\u001b[39m\"\u001b[39m))\n\u001b[1;32m 4\u001b[0m \u001b[39mprint\u001b[39m(\u001b[39mf\u001b[39m\u001b[39m\"\u001b[39m\u001b[39mx is \u001b[39m\u001b[39m{\u001b[39;00mx\u001b[39m}\u001b[39;00m\u001b[39m\"\u001b[39m)\n", "\u001b[0;31mValueError\u001b[0m: invalid literal for int() with base 10: 'fhfhg'" ] } ], "source": [ "#Try this but give \"five\" as input instead of an integer\n", "#This will give a runtime ValueError\n", "x = int(input(\"What's x? \"))\n", "print(f\"x is {x}\")" ] }, { "cell_type": "markdown", "id": "6c15b5de", "metadata": {}, "source": [ "

Handling exceptions

\n", "Lets try dealing with this kind of error in order for our code to handle them gracefully" ] }, { "cell_type": "code", "execution_count": null, "id": "d93e851c", "metadata": {}, "outputs": [], "source": [ "#Standard code snippet for getting a number as an input using try-except-else\n", "while True:\n", " try:\n", " x = int(input(\"Please enter a number: \"))\n", " except ValueError:\n", " print(\"Oops! That was no valid number. Try again...\")\n", " else:\n", " break\n", "print(f\"The integer is: {x} \")" ] }, { "cell_type": "code", "execution_count": 8, "id": "122712ba", "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Oops! That was no valid integer. Try again...\n", "The integer is: 3 \n" ] } ], "source": [ "#A slightly shorter version skipping else\n", "while True:\n", " try:\n", " x = int(input(\"Please enter a integer: \"))\n", " break\n", " except ValueError:\n", " print(\"Oops! That was no valid integer. Try again...\")\n", "print(f\"The integer is: {x} \")" ] }, { "cell_type": "code", "execution_count": 9, "id": "3877e062", "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "The integer is: 5 \n" ] } ], "source": [ "#Version with no message, just asking again\n", "while True:\n", " try:\n", " x = int(input(\"Please enter a integer: \"))\n", " break\n", " except ValueError:\n", " pass\n", "print(f\"The integer is: {x} \")" ] }, { "cell_type": "markdown", "id": "34e5a0c2", "metadata": {}, "source": [ "Python will tell us what's wrong in the standard cases, but sometimes we might need a bit more control raising our own error messages." ] }, { "cell_type": "code", "execution_count": 12, "id": "0cde0953", "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Not an integer\n" ] }, { "ename": "AssertionError", "evalue": "Not an even integer", "output_type": "error", "traceback": [ "\u001b[0;31m---------------------------------------------------------------------------\u001b[0m", "\u001b[0;31mAssertionError\u001b[0m Traceback (most recent call last)", "\u001b[1;32m/Users/randlerm2/Documents/UU och Campus/Undervisning UU/Kurser 23-24/23 Coding in Python /Lectures/L5/Exceptions and Errors.ipynb Cell 15\u001b[0m line \u001b[0;36m5\n\u001b[1;32m 3\u001b[0m \u001b[39mtry\u001b[39;00m:\n\u001b[1;32m 4\u001b[0m x \u001b[39m=\u001b[39m \u001b[39mint\u001b[39m(\u001b[39minput\u001b[39m(\u001b[39m\"\u001b[39m\u001b[39mPlease enter an even integer: \u001b[39m\u001b[39m\"\u001b[39m))\n\u001b[0;32m----> 5\u001b[0m \u001b[39massert\u001b[39;00m x\u001b[39m%\u001b[39m\u001b[39m2\u001b[39m \u001b[39m==\u001b[39m \u001b[39m0\u001b[39m, \u001b[39m\"\u001b[39m\u001b[39mNot an even integer\u001b[39m\u001b[39m\"\u001b[39m\n\u001b[1;32m 6\u001b[0m \u001b[39mif\u001b[39;00m x\u001b[39m%\u001b[39m\u001b[39m2\u001b[39m:\n\u001b[1;32m 7\u001b[0m \u001b[39mraise\u001b[39;00m \u001b[39mValueError\u001b[39;00m()\n", "\u001b[0;31mAssertionError\u001b[0m: Not an even integer" ] } ], "source": [ "#Raising an Assertionerror message with \"assert\"\n", "while True:\n", " try:\n", " x = int(input(\"Please enter an even integer: \"))\n", " assert x%2 == 0, \"Not an even integer\"\n", " if x%2:\n", " raise ValueError()\n", " break\n", " except ValueError:\n", " #pass\n", " if isinstance(x, int):\n", " print(x)\n", " print(\"Integer but not even\")\n", " x = \"\"\n", " else:\n", " print(\"Not an integer\")\n", "print(f\"The even integer is: {x} \")\n" ] }, { "cell_type": "markdown", "id": "1b8e04a2", "metadata": {}, "source": [ "

Going a bit forward we can create our own custom exception type

" ] }, { "cell_type": "code", "execution_count": 13, "id": "3fe6b002", "metadata": {}, "outputs": [ { "name": "stdout", "output_type": "stream", "text": [ "Not an integer\n", "Not an even integer\n", "The even integer is: 6 \n" ] } ], "source": [ "#Creating custom exception\n", "class NotEvenError(Exception):\n", " pass\n", "#The actual code using the custom exception\n", "while True:\n", " try:\n", " x = int(input(\"Please enter an even integer: \"))\n", " if x%2:\n", " raise NotEvenError()\n", " break\n", " except ValueError:\n", " print(\"Not an integer\")\n", " except NotEvenError:\n", " print(\"Not an even integer\")\n", "print(f\"The even integer is: {x} \")" ] } ], "metadata": { "kernelspec": { "display_name": "Python 3 (ipykernel)", "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": "3.11.4" } }, "nbformat": 4, "nbformat_minor": 5 }