Add demo notebook

Author: minrk

Demo.ipynb

@@ -0,0 +1,224 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "# Capturing C-level stdout/stderr with `wurlitzer`\n",
+    "\n",
+    "Sometimes in Python you are calling some C code.\n",
+    "Sometimes that C code makes calls to `printf`,\n",
+    "or otherwise writes to the stdout/stderr of the process."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [],
+   "source": [
+    "import ctypes\n",
+    "libc = ctypes.CDLL(None)\n",
+    "\n",
+    "try:\n",
+    "    c_stderr_p = ctypes.c_void_p.in_dll(libc, 'stderr')\n",
+    "except ValueError:\n",
+    "    # libc.stdout is has a funny name on OS X\n",
+    "    c_stderr_p = ctypes.c_void_p.in_dll(libc, '__stderrp')\n",
+    "\n",
+    "\n",
+    "def printf(msg):\n",
+    "    \"\"\"Call C printf\"\"\"\n",
+    "    libc.printf((msg + '\\n').encode('utf8'))\n",
+    "\n",
+    "def printf_err(msg):\n",
+    "    \"\"\"Cal C fprintf on stderr\"\"\"\n",
+    "    libc.fprintf(c_stderr_p, (msg + '\\n').encode('utf8'))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "IPython forwards the Python-level `sys.stdout` and `sys.stderr`,\n",
+    "but it leaves the process-level file descriptors that C code will write to untouched.\n",
+    "That means that in a context like this notebook, these functions will print to the terminal, because they are not captured:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 2,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "printf(\"Hello?\")\n",
+    "printf_err(\"Stderr? Anybody?\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "With wurlitzer, we can capture these C-level functions:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 3,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "from wurlitzer import pipes, sys_pipes, STDOUT, PIPE"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 4,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "with pipes() as (stdout, stderr):\n",
+    "    printf(\"Hello, stdout!\")\n",
+    "    printf_err(\"Hello, stderr!\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "and redisplay them if we like:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 5,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Hello, stdout!\n"
+     ]
+    },
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "Hello, stderr!\n"
+     ]
+    }
+   ],
+   "source": [
+    "import sys\n",
+    "sys.stdout.write(stdout.read())\n",
+    "sys.stderr.write(stderr.read())"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "Some tools, such as the IPython kernel for Jupyter,\n",
+    "capture the Python-level `sys.stdout` and `sys.stderr` and forward them somewhere.\n",
+    "In the case of Jupyter, this is over a network socket, so that it ends up in the browser.\n",
+    "\n",
+    "If we know that's going on, we can easily hook up the C outputs to the Python-forwarded ones with a single call:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 6,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Hello from C, 0!\n",
+      "Hello from C, 1!\n",
+      "Hello from C, 2!\n",
+      "Hello from C, 3!\n",
+      "Hello from C, 4!\n"
+     ]
+    }
+   ],
+   "source": [
+    "import time\n",
+    "\n",
+    "with sys_pipes():\n",
+    "    for i in range(5):\n",
+    "        time.sleep(1)\n",
+    "        printf(\"Hello from C, %i!\" % i)\n"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "We can also capture the pipes to any writeable streams, such as a `StringIO` object:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 7,
+   "metadata": {
+    "collapsed": false
+   },
+   "outputs": [
+    {
+     "name": "stdout",
+     "output_type": "stream",
+     "text": [
+      "Hello, stdout!\n",
+      "Hello, stderr!\n",
+      "\n"
+     ]
+    }
+   ],
+   "source": [
+    "import io\n",
+    "\n",
+    "stdout = io.StringIO()\n",
+    "with pipes(stdout=stdout, stderr=STDOUT):\n",
+    "    printf(\"Hello, stdout!\")\n",
+    "    printf_err(\"Hello, stderr!\")\n",
+    "\n",
+    "print(stdout.getvalue())"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3",
+   "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": "3.5.1"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 1
+}