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Running Code.ipynb
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Running Code

First and foremost, the IPython Notebook is an interactive environment for writing and running code. IPython is capable of running code in a wide range of languages. However, this notebook, and the default kernel in IPython 2.0, runs Python code.

Code cells allow you to enter and run Python code

Run a code cell using Shift-Enter or pressing the <button class='btn btn-default btn-xs'></button> button in the toolbar above:

In [ ]:
a = 10
In [ ]:

There are two other keyboard shortcuts for running code:

  • Alt-Enter runs the current cell and inserts a new one below.
  • Ctrl-Enter run the current cell and enters command mode.

Managing the IPython Kernel

Code is run in a separate process called the IPython Kernel. The Kernel can be interrupted or restarted. Try running the following cell and then hit the <button class='btn btn-default btn-xs'></button> button in the toolbar above.

In [ ]:
import time

If the Kernel dies you will be prompted to restart it. Here we call the low-level system libc.time routine with the wrong argument via ctypes to segfault the Python interpreter:

In [ ]:
import sys
from ctypes import CDLL
# This will crash a Linux or Mac system
# equivalent calls can be made on Windows
dll = 'dylib' if sys.platform == 'darwin' else 'so.6'
libc = CDLL("libc.%s" % dll) 
libc.time(-1)  # BOOM!!

Cell menu

The "Cell" menu has a number of menu items for running code in different ways. These includes:

  • Run and Select Below
  • Run and Insert Below
  • Run All
  • Run All Above
  • Run All Below

Restarting the kernels

The kernel maintains the state of a notebook's computations. You can reset this state by restarting the kernel. This is done by clicking on the <button class='btn btn-default btn-xs'></button> in the toolbar above.

sys.stdout and sys.stderr

The stdout and stderr streams are displayed as text in the output area.

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print("hi, stdout")
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from __future__ import print_function
print('hi, stderr', file=sys.stderr)

Output is asynchronous

All output is displayed asynchronously as it is generated in the Kernel. If you execute the next cell, you will see the output one piece at a time, not all at the end.

In [ ]:
import time, sys
for i in range(8):

Large outputs

To better handle large outputs, the output area can be collapsed. Run the following cell and then single- or double- click on the active area to the left of the output:

In [ ]:
for i in range(50):

Beyond a certain point, output will scroll automatically:

In [ ]:
for i in range(500):
    print(2**i - 1)