docs/examples/notebooks/Capturing Output.ipynb

{
 "metadata": {
  "name": "Capturing Output"
 },
 "nbformat": 3,
 "nbformat_minor": 0,
 "worksheets": [
  {
   "cells": [
    {
     "cell_type": "heading",
     "level": 1,
     "metadata": {},
     "source": [
      "Capturing Output with <tt>%%capture</tt>"
     ]
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "One of IPython's new cell magics is `%%capture`, which captures stdout/err for a cell,\n",
      "and discards them or stores them in variables in your namespace."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "import sys"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "By default, it just swallows it up.  This is a simple way to suppress unwanted output."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "%%capture\n",
      "print 'hi, stdout'\n",
      "print >> sys.stderr, 'hi, stderr'"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "If you specify a name, then stdout and stderr will be stored in an object in your namespace."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "%%capture captured\n",
      "print 'hi, stdout'\n",
      "print >> sys.stderr, 'hi, stderr'"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "captured"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "Calling the object writes the output to stdout/err as appropriate."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "captured()"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "captured.stdout"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "captured.stderr"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "`%%capture` only captures stdout/err, not displaypub, so you can still do plots and use the display protocol inside %%capture"
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "%pylab inline"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "%%capture wontshutup\n",
      "\n",
      "print \"setting up X\"\n",
      "x = np.linspace(0,5,1000)\n",
      "print \"step 2: constructing y-data\"\n",
      "y = np.sin(x)\n",
      "print \"step 3: display info about y\"\n",
      "plt.plot(x,y)\n",
      "print \"okay, I'm done now\""
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "wontshutup()"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "markdown",
     "metadata": {},
     "source": [
      "And you can selectively disable capturing stdout or stderr by passing `--no-stdout/err`."
     ]
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "%%capture cap --no-stderr\n",
      "print 'hi, stdout'\n",
      "print >> sys.stderr, \"hello, stderr\""
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "cap.stdout"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    },
    {
     "cell_type": "code",
     "collapsed": false,
     "input": [
      "cap.stderr"
     ],
     "language": "python",
     "metadata": {},
     "outputs": []
    }
   ],
   "metadata": {}
  }
 ]
}