getitem and setitem

Author: ostad-ai

P-E-magic methods-getitem-setitem.ipynb

@@ -0,0 +1,165 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "b3c0145c",
+   "metadata": {},
+   "source": [
+    "# Python: Everything\n",
+    "- 24) **Magic methods, __getitem__ and __setitem__:** These methods may be implemented in a class to override the default ones for indexing and assignment, respectively.\n",
+    "<br>----------------------------------------------\n",
+    "<br> https://www.pinterest.com/HamedShahHosseini/programming-languages/\n",
+    "<br>https://github.com/ostad-ai/Python-Everything"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "35474859",
+   "metadata": {},
+   "source": [
+    "Let's apply **indexing** to a list."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "id": "2191568a",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "The list: [2, 3, 5, 7, 11, 13]\n",
+      "List item at index 2 using indexing:5\n",
+      "List item at index 2 using getitem:5\n",
+      "--------------------\n",
+      "The dictionary: {'one': 'hello', 'two': 'world', 'three': 'bye'}\n",
+      "Dict. value at key two, using keying: world\n",
+      "Dict. value at key two, using getitem: world\n"
+     ]
+    }
+   ],
+   "source": [
+    "# both self[key] and self.__getitem__(key) are the same\n",
+    "primes=[2,3,5,7,11,13]\n",
+    "dicts={'one':'hello','two':'world','three':'bye'}\n",
+    "i=2; dkey='two'\n",
+    "print(f'The list: {primes}')\n",
+    "print(f'List item at index {i} using indexing:{primes[i]}')\n",
+    "print(f'List item at index {i} using getitem:{primes.__getitem__(i)}')\n",
+    "print('--------------------')\n",
+    "print(f'The dictionary: {dicts}')\n",
+    "print(f'Dict. value at key {dkey}, using keying: {dicts[dkey]}')\n",
+    "print(f'Dict. value at key {dkey}, using getitem: {dicts.__getitem__(dkey)}')"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "8d2a1b9c",
+   "metadata": {},
+   "source": [
+    "Defining **getitem** method in a class to use indexing on the class instances."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "id": "4d6eeae4",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Student Jim is 27\n"
+     ]
+    }
+   ],
+   "source": [
+    "# we define __getitem__ in our class\n",
+    "class Students:\n",
+    "    def __init__(self,name,age):\n",
+    "        self.records={name: age}\n",
+    "    def __getitem__(self,key):\n",
+    "        return self.records[key]\n",
+    "name,age='Jim',27\n",
+    "std=Students(name,age)\n",
+    "# now we can get an item from the instance by instance[key]\n",
+    "print(f\"Student {name} is {std[name]}\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "e5eece6b",
+   "metadata": {},
+   "source": [
+    "Defining **setitem** method in a class which gets *key* and *value* as its arguments, to use assigment in the class instances."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "id": "07a58317",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Jim is 27 years old with major CS\n",
+      "Julia is 21 years old with major EE\n",
+      "Smith is 32 years old with major CS\n"
+     ]
+    }
+   ],
+   "source": [
+    "# we define both __getitem__ and __setitem__ in our class\n",
+    "class Students2:\n",
+    "    def __init__(self,name,age_major):\n",
+    "        self.records={}\n",
+    "        self.records[name]=age_major\n",
+    "    def __getitem__(self,key):\n",
+    "        return self.records[key]\n",
+    "    def __setitem__(self,key,value):\n",
+    "        self.records[key]=[value[0],value[1]]\n",
+    "name,age_major='Jim',[27,'CS']\n",
+    "std2=Students2(name,age_major)\n",
+    "# now we can assign items by instance[key]=value\n",
+    "std2['Julia']=[21,'EE']\n",
+    "std2['Smith']=[32,'CS']\n",
+    "for key in std2.records.keys():\n",
+    "    print(f'{key} is {std2[key][0]} years old with major {std2[key][1]}')"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "22275727",
+   "metadata": {},
+   "outputs": [],
+   "source": []
+  }
+ ],
+ "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": "3.8.10"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}

README.md

@@ -22,4 +22,5 @@
 20) Exercise: Factorial for non-negative integer and float numbers with *gamma* and *factorial* function of *math module*.
 21) SQLite3 module, introduction: We can use *sqlite3* module in Python for creating and accessing databases without the need to have a separate server. Here, we use basic commands of **SQL** for such purposes. For example, for creating and manipulating tables in our database.
 22) Operator overloading in Python: Using built-in operators, it is possible to change their behaviors via implementing their relevant special methods in a custom class definition. Here, we implement operator == for almost-equal. Also, we implement operator + for vector summation. 
-23) Lambda expressions: We may use lambda keyword to define anonymous functions. Here, we define the syntax for lambda expressions, and then we bring some applications. 
+23) Lambda expressions: We may use lambda keyword to define anonymous functions. Here, we define the syntax for lambda expressions, and then we bring some applications.
+24) Magic methods, __getitem__ and __setitem__: We review getitem and setitem methods that may be used for **indexing** and **assignment** on instances of a class, respectively.