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‎notebooks/01.03-Magic-Commands.ipynb

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"<img align=\"left\" style=\"padding-right:10px;\" src=\"figures/PDSH-cover-small.png\">\n",
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"*This notebook contains an excerpt from the [Python Data Science Handbook](http://shop.oreilly.com/product/0636920034919.do) by Jake VanderPlas; the content is available [on GitHub](https://github.com/jakevdp/PythonDataScienceHandbook).*\n",
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"\n",
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"*The text is released under the [CC-BY-NC-ND license](https://creativecommons.org/licenses/by-nc-nd/3.0/us/legalcode), and code is released under the [MIT license](https://opensource.org/licenses/MIT). If you find this content useful, please support the work by [buying the book](http://shop.oreilly.com/product/0636920034919.do)!*"
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]
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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# IPython Magic Commands\n",
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"\n",
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"The previous two sections showed how IPython lets you use and explore Python efficiently and interactively.\n",
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"Here we'll begin discussing some of the enhancements that IPython adds on top of the normal Python syntax.\n",
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"These are known in IPython as *magic commands*, and are prefixed by the ``%`` character.\n",
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"These magic commands are designed to succinctly solve various common problems in standard data analysis.\n",
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"Magic commands come in two flavors: *line magics*, which are denoted by a single ``%`` prefix and operate on a single line of input, and *cell magics*, which are denoted by a double ``%%`` prefix and operate on multiple lines of input.\n",
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"We'll demonstrate and discuss a few brief examples here, and come back to more focused discussion of several useful magic commands later in the chapter."
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]
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{
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"## Pasting Code Blocks: ``%paste`` and ``%cpaste``\n",
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"\n",
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"When working in the IPython interpreter, one common gotcha is that pasting multi-line code blocks can lead to unexpected errors, especially when indentation and interpreter markers are involved.\n",
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"A common case is that you find some example code on a website and want to paste it into your interpreter.\n",
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"Consider the following simple function:\n",
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"\n",
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"``` python\n",
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">>> def donothing(x):\n",
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"... return x\n",
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"\n",
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"```\n",
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"The code is formatted as it would appear in the Python interpreter, and if you copy and paste this directly into IPython you get an error:\n",
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"\n",
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"```ipython\n",
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"In [2]: >>> def donothing(x):\n",
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" ...: ... return x\n",
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" ...: \n",
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" File \"<ipython-input-20-5a66c8964687>\", line 2\n",
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" ... return x\n",
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" ^\n",
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"SyntaxError: invalid syntax\n",
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"```\n",
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"\n",
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"In the direct paste, the interpreter is confused by the additional prompt characters.\n",
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"But never fear–IPython's ``%paste`` magic function is designed to handle this exact type of multi-line, marked-up input:\n",
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"\n",
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"```ipython\n",
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"In [3]: %paste\n",
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">>> def donothing(x):\n",
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"... return x\n",
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"\n",
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"## -- End pasted text --\n",
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"```\n",
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"\n",
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"The ``%paste`` command both enters and executes the code, so now the function is ready to be used:\n",
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"\n",
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"```ipython\n",
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"In [4]: donothing(10)\n",
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"Out[4]: 10\n",
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"```\n",
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"\n",
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"A command with a similar intent is ``%cpaste``, which opens up an interactive multiline prompt in which you can paste one or more chunks of code to be executed in a batch:\n",
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"\n",
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"```ipython\n",
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"In [5]: %cpaste\n",
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"Pasting code; enter '--' alone on the line to stop or use Ctrl-D.\n",
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":>>> def donothing(x):\n",
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":... return x\n",
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":--\n",
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"```\n",
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"\n",
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"These magic commands, like others we'll see, make available functionality that would be difficult or impossible in a standard Python interpreter."
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]
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},
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{
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"cell_type": "markdown",
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"## Running External Code: ``%run``\n",
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"As you begin developing more extensive code, you will likely find yourself working in both IPython for interactive exploration, as well as a text editor to store code that you want to reuse.\n",
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"Rather than running this code in a new window, it can be convenient to run it within your IPython session.\n",
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"This can be done with the ``%run`` magic.\n",
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"\n",
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"For example, imagine you've created a ``myscript.py`` file with the following contents:\n",
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"\n",
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"```python\n",
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"#-------------------------------------\n",
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"# file: myscript.py\n",
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"\n",
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"def square(x):\n",
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" \"\"\"square a number\"\"\"\n",
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" return x ** 2\n",
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"\n",
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"for N in range(1, 4):\n",
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" print(N, \"squared is\", square(N))\n",
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"```\n",
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"\n",
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"You can execute this from your IPython session as follows:\n",
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"\n",
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"```ipython\n",
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"In [6]: %run myscript.py\n",
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"1 squared is 1\n",
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"2 squared is 4\n",
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"3 squared is 9\n",
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"```\n",
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"\n",
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"Note also that after you've run this script, any functions defined within it are available for use in your IPython session:\n",
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"\n",
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"```ipython\n",
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"In [7]: square(5)\n",
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"Out[7]: 25\n",
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"```\n",
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"\n",
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"There are several options to fine-tune how your code is run; you can see the documentation in the normal way, by typing **``%run?``** in the IPython interpreter."
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]
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"cell_type": "markdown",
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"## Timing Code Execution: ``%timeit``\n",
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"Another example of a useful magic function is ``%timeit``, which will automatically determine the execution time of the single-line Python statement that follows it.\n",
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"For example, we may want to check the performance of a list comprehension:\n",
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"\n",
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"```ipython\n",
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"In [8]: %timeit L = [n ** 2 for n in range(1000)]\n",
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"1000 loops, best of 3: 325 µs per loop\n",
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"```\n",
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"\n",
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"The benefit of ``%timeit`` is that for short commands it will automatically perform multiple runs in order to attain more robust results.\n",
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"For multi line statements, adding a second ``%`` sign will turn this into a cell magic that can handle multiple lines of input.\n",
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"For example, here's the equivalent construction with a ``for``-loop:\n",
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"\n",
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"```ipython\n",
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"In [9]: %%timeit\n",
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" ...: L = []\n",
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" ...: for n in range(1000):\n",
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" ...: L.append(n ** 2)\n",
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" ...: \n",
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"1000 loops, best of 3: 373 µs per loop\n",
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"```\n",
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"\n",
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"We can immediately see that list comprehensions are about 10% faster than the equivalent ``for``-loop construction in this case.\n",
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"We'll explore ``%timeit`` and other approaches to timing and profiling code in [Profiling and Timing Code](01.07-Timing-and-Profiling.ipynb)."
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]
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},
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"cell_type": "markdown",
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"## Help on Magic Functions: ``?``, ``%magic``, and ``%lsmagic``\n",
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"\n",
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"Like normal Python functions, IPython magic functions have docstrings, and this useful\n",
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"documentation can be accessed in the standard manner.\n",
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"So, for example, to read the documentation of the ``%timeit`` magic simply type this:\n",
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"\n",
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"```ipython\n",
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"In [10]: %timeit?\n",
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"```\n",
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"\n",
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"Documentation for other functions can be accessed similarly.\n",
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"To access a general description of available magic functions, including some examples, you can type this:\n",
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"\n",
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"```ipython\n",
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"In [11]: %magic\n",
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"```\n",
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"\n",
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"For a quick and simple list of all available magic functions, type this:\n",
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"\n",
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"```ipython\n",
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"In [12]: %lsmagic\n",
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"```\n",
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"\n",
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"Finally, I'll mention that it is quite straightforward to define your own magic functions if you wish.\n",
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"We won't discuss it here, but if you are interested, see the references listed in [More IPython Resources](01.08-More-IPython-Resources.ipynb)."
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]
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