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01.03 Magic Commands

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The previous two sections showed how IPython lets you use and explore Python efficiently and interactively. Here we'll begin discussing some of the enhancements that IPython adds on top of the normal Python syntax. These are known in IPython as magic commands, and are prefixed by the % character. These magic commands are designed to succinctly solve various common problems in standard data analysis. 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. 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.

Pasting Code Blocks: %paste and %cpaste

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. A common case is that you find some example code on a website and want to paste it into your interpreter. Consider the following simple function:
>>> def donothing(x):
...     return x
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:
In [2]: >>> def donothing(x):
   ...:     ...     return x
   ...:     
  File "<ipython-input-20-5a66c8964687>", line 2
    ...     return x
                 ^
SyntaxError: invalid syntax
In the direct paste, the interpreter is confused by the additional prompt characters. But never fear–IPython's %paste magic function is designed to handle this exact type of multi-line, marked-up input:
In [3]: %paste
>>> def donothing(x):
...     return x

## -- End pasted text --
The %paste command both enters and executes the code, so now the function is ready to be used:
In [4]: donothing(10)
Out[4]: 10
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:
In [5]: %cpaste
Pasting code; enter '--' alone on the line to stop or use Ctrl-D.
:>>> def donothing(x):
:...     return x
:--
These magic commands, like others we'll see, make available functionality that would be difficult or impossible in a standard Python interpreter.

Running External Code: %run

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. Rather than running this code in a new window, it can be convenient to run it within your IPython session. This can be done with the %run magic.
For example, imagine you've created a myscript.py file with the following contents:
python
#-------------------------------------
# file: myscript.py

def square(x):
    """square a number"""
    return x ** 2

for N in range(1, 4):
    print(N, "squared is", square(N))

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