protect entry point

notebooks/02-routing-speed-time.ipynb

@@ -252,7 +252,13 @@
    "source": [
     "## 4. Multiprocessing\n",
     "\n",
-    "Calculating lots of shortest paths can be slow, but OSMnx has built-in shortest path solver parallelization and multiprocessing. With the `shortest_path` function, you can pass in a single origin-destination pair to solve the one shortest path, or you can pass in lists of origins and destinations to solve each shortest path between the pairs. If you're solving shortest paths for multiple origins/destinations, the `cpus` argument determines how many CPU cores to utilize for parallelized solving. Multiprocessing adds some overhead, so it's only faster if you're solving a lot of paths. It also has substantial RAM requirements (as it must copy the graph into each sub-process), so be careful with your RAM when setting the `cpus` argument."
+    "Calculating lots of shortest paths can be slow, but OSMnx has built-in shortest path solver parallelization and multiprocessing. With the `shortest_path` function, you can pass in a single origin-destination pair to solve the one shortest path, or you can pass in lists of origins and destinations to solve each shortest path between the pairs.\n",
+    "\n",
+    "If you're solving shortest paths for multiple origins/destinations, the `cpus` argument determines how many CPU cores to utilize for parallelized solving. Multiprocessing adds some overhead, but it's faster if you're solving a lot of paths. It also has higher RAM requirements (as it must copy data into each sub-process), so be careful with your RAM when setting the `cpus` argument.\n",
+    "\n",
+    "If you are multiprocessing, always remember to protect your entry point. From the [Python docs](https://docs.python.org/3/library/multiprocessing.html#multiprocessing-programming):\n",
+    "\n",
+    "> Make sure that the main module can be safely imported by a new Python interpreter without causing unintended side effects (such as starting a new process)... one should protect the “entry point” of the program by using `if __name__ == '__main__':`"
    ]
   },
   {
@@ -273,20 +279,13 @@
    "metadata": {},
    "outputs": [],
    "source": [
-    "# how many CPUs do you have\n",
-    "mp.cpu_count()"
-   ]
-  },
-  {
-   "cell_type": "code",
-   "execution_count": null,
-   "metadata": {},
-   "outputs": [],
-   "source": [
-    "# %%time\n",
     "# it takes 2.3 seconds to solve all the routes using all the cores on my computer\n",
     "# I have a 24-thread AMD 5900x: performance will depend on your specific CPU\n",
-    "# routes = ox.routing.shortest_path(G, origs, dests, weight=\"travel_time\", cpus=None)"
+    "if __name__ == \"__main__\":\n",
+    "    # how many CPU cores do you have\n",
+    "    print(mp.cpu_count())\n",
+    "    routes = ox.routing.shortest_path(G, origs, dests, weight=\"travel_time\", cpus=None)\n",
+    "    print(len(routes))"
    ]
   },
   {