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:
a = 10
print(a)
There are two other keyboard shortcuts for running code:
Alt-Enterruns the current cell and inserts a new one below.Ctrl-Enterrun 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.
import time
time.sleep(10)
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:
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.
print("hi, stdout")
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.
import time, sys
for i in range(8):
print(i)
time.sleep(0.5)
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:
for i in range(50):
print(i)
Beyond a certain point, output will scroll automatically:
for i in range(500):
print(2**i - 1)