Magics

Magics constitute a simple command language that significantly extends the power of Jupyter notebooks. There are two types of magics:

• Line magics: Commands that are prepended by %, and whose arguments only extend to the end of the line.
• Cell magics: Commands that start with %% and then applies to the whole cell. Must be written on the first line of a cell.

Now list all available magics with %lsmagic (which itself is a magic). You add a question mark to a magic to show the help (e.g. %lsmagic?). Some of them act as shortcuts for commonly used shell commands (%ls, %cp, %cat, ..). Others are useful for debugging and optimizing your code (%timeit, %debug, %prun, ..). For more information see the magics documentation.

A very useful magic, in particular when using shell commands a lot in your work, is %%capture. This will capture the stdout/stderr of any code cell and store them in a Python object. Run %%capture? to display the help and try to understand how it works. Try it out with either some Python code, other magics or shell commands. Here is an example of how you can make it work:

%%capture output
%%bash
echo "Print to stdout"
echo "Print to stderr" >&2

... and in another cell:

print("stdout:" + output.stdout)
print("stderr:" + output.stderr)

The %%script magic is used for specifying a program (bash, perl, ruby, ..) with which to run the code (similar to a shebang). For some languages it's possible to use these shortcuts:

• %%ruby
• %%perl
• %%bash
• %%html
• %%latex
• %%R (here you have to first install the rpy2 extension, for example with Conda, and then load with %load_ext rpy2.ipython)

Try this out if you know any of the languages above. Otherwise you can always try to print the quadratic formula with LaTeX!

\begin{align}
(a+b)^3 &= (a+b)(a+b)^2 \\
        &= (a+b)(a^2+2ab+b^2) \\
        &= a^3+3a^2b+3ab^2+b^3
\end{align}

Another useful magic is %precision which sets the floating point precision in the notebook. As a quick example, add the following to a cell and run it:

float(100/3)

Next set the precision to 4 decimal points by running a cell with:

%precision 4

Now run the cell with float(100/3) again to see the difference.

Running %precision without additional arguments will restore the default.