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‎notebooks/01.04-Input-Output-History.ipynb

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{
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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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{
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"cell_type": "markdown",
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"metadata": {},
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"source": [
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"# Input and Output History\n",
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"\n",
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"Previously we saw that the IPython shell allows you to access previous commands with the up and down arrow keys, or equivalently the Ctrl-p/Ctrl-n shortcuts.\n",
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"Additionally, in both the shell and the notebook, IPython exposes several ways to obtain the output of previous commands, as well as string versions of the commands themselves.\n",
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"We'll explore those here."
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]
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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's ``In`` and ``Out`` Objects\n",
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"\n",
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"By now I imagine you're quite familiar with the ``In [1]:``/``Out[1]:`` style prompts used by IPython.\n",
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"But it turns out that these are not just pretty decoration: they give a clue as to how you can access previous inputs and outputs in your current session.\n",
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"Imagine you start a session that looks like this:\n",
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"\n",
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"```ipython\n",
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"In [1]: import math\n",
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"\n",
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"In [2]: math.sin(2)\n",
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"Out[2]: 0.9092974268256817\n",
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"\n",
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"In [3]: math.cos(2)\n",
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"Out[3]: -0.4161468365471424\n",
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"```"
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]
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},
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"We've imported the built-in ``math`` package, then computed the sine and the cosine of the number 2.\n",
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"These inputs and outputs are displayed in the shell with ``In``/``Out`` labels, but there's more–IPython actually creates some Python variables called ``In`` and ``Out`` that are automatically updated to reflect this history:\n",
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"\n",
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"```ipython\n",
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"In [4]: print(In)\n",
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"['', 'import math', 'math.sin(2)', 'math.cos(2)', 'print(In)']\n",
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"\n",
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"In [5]: Out\n",
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"Out[5]: {2: 0.9092974268256817, 3: -0.4161468365471424}\n",
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"```"
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]
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},
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{
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"source": [
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"The ``In`` object is a list, which keeps track of the commands in order (the first item in the list is a place-holder so that ``In[1]`` can refer to the first command):\n",
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"\n",
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"```ipython\n",
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"In [6]: print(In[1])\n",
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"import math\n",
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"```\n",
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"\n",
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"The ``Out`` object is not a list but a dictionary mapping input numbers to their outputs (if any):\n",
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"\n",
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"```ipython\n",
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"In [7]: print(Out[2])\n",
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"0.9092974268256817\n",
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"```\n",
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"\n",
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"Note that not all operations have outputs: for example, ``import`` statements and ``print`` statements don't affect the output.\n",
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"The latter may be surprising, but makes sense if you consider that ``print`` is a function that returns ``None``; for brevity, any command that returns ``None`` is not added to ``Out``.\n",
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"\n",
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"Where this can be useful is if you want to interact with past results.\n",
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"For example, let's check the sum of ``sin(2) ** 2`` and ``cos(2) ** 2`` using the previously-computed results:\n",
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"\n",
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"```ipython\n",
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"In [8]: Out[2] ** 2 + Out[3] ** 2\n",
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"Out[8]: 1.0\n",
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"```\n",
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"\n",
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"The result is ``1.0`` as we'd expect from the well-known trigonometric identity.\n",
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"In this case, using these previous results probably is not necessary, but it can become very handy if you execute a very expensive computation and want to reuse the result!"
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]
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},
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{
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"cell_type": "markdown",
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"source": [
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"## Underscore Shortcuts and Previous Outputs\n",
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"\n",
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"The standard Python shell contains just one simple shortcut for accessing previous output; the variable ``_`` (i.e., a single underscore) is kept updated with the previous output; this works in IPython as well:\n",
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"\n",
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"```ipython\n",
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"In [9]: print(_)\n",
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"1.0\n",
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"```\n",
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"\n",
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"But IPython takes this a bit further—you can use a double underscore to access the second-to-last output, and a triple underscore to access the third-to-last output (skipping any commands with no output):\n",
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"\n",
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"```ipython\n",
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"In [10]: print(__)\n",
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"-0.4161468365471424\n",
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"\n",
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"In [11]: print(___)\n",
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"0.9092974268256817\n",
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"```\n",
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"\n",
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"IPython stops there: more than three underscores starts to get a bit hard to count, and at that point it's easier to refer to the output by line number.\n",
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"\n",
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"There is one more shortcut we should mention, however–a shorthand for ``Out[X]`` is ``_X`` (i.e., a single underscore followed by the line number):\n",
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"\n",
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"```ipython\n",
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"In [12]: Out[2]\n",
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"Out[12]: 0.9092974268256817\n",
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"\n",
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"In [13]: _2\n",
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"Out[13]: 0.9092974268256817\n",
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"```"
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]
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},
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{
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"cell_type": "markdown",
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"metadata": {},
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"## Suppressing Output\n",
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"Sometimes you might wish to suppress the output of a statement (this is perhaps most common with the plotting commands that we'll explore in [Introduction to Matplotlib](04.00-Introduction-To-Matplotlib.ipynb)).\n",
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"Or maybe the command you're executing produces a result that you'd prefer not like to store in your output history, perhaps so that it can be deallocated when other references are removed.\n",
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"The easiest way to suppress the output of a command is to add a semicolon to the end of the line:\n",
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"\n",
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"```ipython\n",
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"In [14]: math.sin(2) + math.cos(2);\n",
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"```\n",
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"\n",
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"Note that the result is computed silently, and the output is neither displayed on the screen or stored in the ``Out`` dictionary:\n",
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"\n",
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"```ipython\n",
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"In [15]: 14 in Out\n",
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"Out[15]: False\n",
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"```"
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]
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},
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{
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"cell_type": "markdown",
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"source": [
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"## Related Magic Commands\n",
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"For accessing a batch of previous inputs at once, the ``%history`` magic command is very helpful.\n",
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"Here is how you can print the first four inputs:\n",
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"\n",
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"```ipython\n",
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"In [16]: %history -n 1-4\n",
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" 1: import math\n",
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" 2: math.sin(2)\n",
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" 3: math.cos(2)\n",
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" 4: print(In)\n",
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"```\n",
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"\n",
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"As usual, you can type ``%history?`` for more information and a description of options available.\n",
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"Other similar magic commands are ``%rerun`` (which will re-execute some portion of the command history) and ``%save`` (which saves some set of the command history to a file).\n",
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"For more information, I suggest exploring these using the ``?`` help functionality discussed in [Help and Documentation in IPython](01.01-Help-And-Documentation.ipynb)."
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]
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