W5: Begin intermission on decorators

Author: tomasff

week5/w5.ipynb

@@ -376,7 +376,7 @@
     "We're repeating the code for the constructor, introduce and, eat. The main difference is the profession specific method!\n",
     "What if we want to change how a person should be introduced? We'd have to update the `introduce` method in `Person`, `ComputerScientist`, `Doctor`.\n",
     "\n",
-    "We can use *inheritance* to simplify our code, improve its maintainability, reduce development costs (avoids the scenario above which could introduce bugs!),\n",
+    "We can use *inheritance* to improve its maintainability, reduce development costs (avoids the scenario above which could introduce bugs!),\n",
     "and avoid repeating code."
    ]
   },
@@ -465,27 +465,147 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "**[INSERT diagram for class vs object]**"
+    "### Intermission: Decorators\n",
+    "Before we continue exploring object-oriented programming, let's have a look at decorators.\n",
+    "\n",
+    "Congratulations, you've just been hired by the *National X-Soc Co-op Bank Ltd* to develop their ATM system!\n",
+    "\n",
+    "They want you to implement the depositing and withdrawal functionality and you define the\n",
+    "functions `deposit_monies` and `withdraw_monies`.\n",
+    "These operations are, of course, very sensitive, so you check if the username matches\n",
+    "the username of the authorised personnel."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "def deposit_monies(user):\n",
+    "    if user not in {\"admin\", \"important_banker\"}:\n",
+    "        print(\"Unauthorised!\")\n",
+    "        return\n",
+    "    \n",
+    "    print(f\"Brrr brr very important operation. Depositing $44b into an account\")\n",
+    "\n",
+    "def withdraw_monies(user):\n",
+    "    if user not in {\"admin\", \"important_banker\"}:\n",
+    "        print(\"Unauthorised!\")\n",
+    "        return\n",
+    "    \n",
+    "    print(f\"Brrr brr very important operation. Withdrawing $44b from an account\")\n",
+    "\n",
+    "withdraw_monies(\"admin\")\n",
+    "withdraw_monies(\"donald knuth\")\n",
+    "\n",
+    "deposit_monies(\"admin\")\n",
+    "deposit_monies(\"donald knuth\")"
    ]
   },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "TBD:\n",
-    "- polymorphism/abstraction/encapsulation a little bit\n",
-    "- attributes (class and instance atttr)\n",
-    "- inheritance example\n",
-    "- then intermission on anotations\n",
-    "- static and class methods"
+    "However, after an FCA audit, the auditors notice that the authorisation code is repeated in each operation.\n",
+    "If we want to update our authorisation scheme, we'd have to update the code of every operation. This is error prone and could introduce vulnerabilities into the ATM system\n",
+    "so the FCA asked the *National X-Soc Co-op Bank Ltd* to investigate this issue.\n",
+    "\n",
+    "We could start, by introducing a function which \"adds\" this authorisation code before an operation.\n",
+    "As we've learned, **everything** in Python is an object, including functions, so you can pass a function\n",
+    "as an argument of a function or even return a function!\n",
+    "\n",
+    "Essentially, we define a function, which will return another function. The new function\n",
+    "checks if the user is authorised, and if the user is authorised, we execute the operation!"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "def requires_authorisation(operation):\n",
+    "    def secure_operation(user):\n",
+    "        if user not in {\"admin\", \"important_banker\"}:\n",
+    "            print(\"Unauthorised!\")\n",
+    "            return\n",
+    "        \n",
+    "        operation(user)\n",
+    "\n",
+    "    return secure_operation\n",
+    "\n",
+    "def deposit_monies(user):\n",
+    "    print(f\"Brrr brr very important operation. Depositing $44b into an account\")\n",
+    "\n",
+    "def withdraw_monies(user):\n",
+    "    print(f\"Brrr brr very important operation. Withdrawing $44b from an account\")\n",
+    "\n",
+    "\n",
+    "secure_withdraw_monies = requires_authorisation(withdraw_monies)\n",
+    "\n",
+    "secure_withdraw_monies(\"admin\")\n",
+    "secure_withdraw_monies(\"donald knuth\")\n",
+    "\n",
+    "secure_deposit_monies = requires_authorisation(deposit_monies)\n",
+    "\n",
+    "secure_deposit_monies(\"admin\")\n",
+    "secure_deposit_monies(\"donald knuth\")"
    ]
   },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### Intermission: Decorators\n",
-    "Before we continue exploring classes...\n"
+    "However, we can go one step further and make our code a bit nicer with *syntatic sugar*"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "def requires_authorisation(operation):\n",
+    "    def secure_operation(user):\n",
+    "        if user not in {\"admin\", \"important_banker\"}:\n",
+    "            print(\"Unauthorised!\")\n",
+    "            return\n",
+    "        \n",
+    "        operation(user)\n",
+    "\n",
+    "    return secure_operation\n",
+    "\n",
+    "@requires_authorisation\n",
+    "def deposit_monies(user):\n",
+    "    print(f\"Brrr brr very important operation. Depositing $44b into an account\")\n",
+    "\n",
+    "@requires_authorisation\n",
+    "def withdraw_monies(user):\n",
+    "    print(f\"Brrr brr very important operation. Withdrawing $44b from an account\")\n",
+    "\n",
+    "withdraw_monies(\"admin\")\n",
+    "withdraw_monies(\"donald knuth\")\n",
+    "\n",
+    "deposit_monies(\"admin\")\n",
+    "deposit_monies(\"donald knuth\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**[INSERT diagram for class vs object]**"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "TBD:\n",
+    "- polymorphism/abstraction/encapsulation a little bit\n",
+    "- then intermission on anotations\n",
+    "- static and class methods"
    ]
   },
   {