adds bit about class variables

Author: kyleniemeyer

classes-objects.ipynb

@@ -181,6 +181,56 @@
     "print(higgs.roar)\n",
     "```\n",
     "\n",
+    "::: {.content-visible when-format=\"html\"}\n",
+    "::: aside \n",
+    "Class variables can get a bit confusing; if you modify the variable associated with an object, it becomes an instance variable. For example:\n",
+    "\n",
+    "```python\n",
+    "class Particle(object):\n",
+    "    spin = 0\n",
+    "\n",
+    "    def init(self, charge, mass):\n",
+    "        self.charge = charge\n",
+    "        self.mass = mass\n",
+    "```\n",
+    "\n",
+    "Here, `spin` is a class variable while `charge` and `mass` are instance variables. Creating two particles:\n",
+    "```python\n",
+    "p1 = Particle(1, 1)\n",
+    "p2 = Particle(-1, 1)\n",
+    "\n",
+    "print(p1.spin)\n",
+    "print(p2.spin)\n",
+    "```\n",
+    "\n",
+    "we'll get\n",
+    "```\n",
+    "0\n",
+    "0\n",
+    "```\n",
+    "\n",
+    "If we change `spin` for `p1` only, it modifies the value just for that instance:\n",
+    "```python\n",
+    "p1.spin = 0.5\n",
+    "print(p1.spin)\n",
+    "print(p2.spin)\n",
+    "```\n",
+    "gives\n",
+    "0.5\n",
+    "0\n",
+    "However, in general spin is still a class variable, and we can modify it for the whole class:\n",
+    "Particle.spin = -0.5\n",
+    "print(p1.spin)\n",
+    "print(p2.spin)\n",
+    "which gives\n",
+    "0.5\n",
+    "-0.5\n",
+    "So for p1, spin has become an instance variable, but  p2  is still pointing to the class variable.\n",
+    "```\n",
+    "\n",
+    ":::\n",
+    ":::\n",
+    "\n",
     "## Instance variables\n",
     "\n",
     "::: {.fragment}\n",