IPython: beyond plain Python¶
When executing code in IPython, all valid Python syntax works as-is, but IPython provides a number of features designed to make the interactive experience more fluid and efficient.
First things first: running code, getting help¶
In the notebook, to run a cell of code, hit Shift-Enter. This executes the cell and puts the cursor in the next cell below, or makes a new one if you are at the end. Alternately, you can use:
Alt-Enterto force the creation of a new cell unconditionally (useful when inserting new content in the middle of an existing notebook).Control-Enterexecutes the cell and keeps the cursor in the same cell, useful for quick experimentation of snippets that you don't need to keep permanently.
print "Hi"
Getting help:
?
Typing object_name? will print all sorts of details about any object, including docstrings, function definition lines (for call arguments) and constructor details for classes.
import collections
collections.namedtuple?
collections.Counter??
*int*?
An IPython quick reference card:
%quickref
Tab completion¶
Tab completion, especially for attributes, is a convenient way to explore the structure of any object you’re dealing with. Simply type object_name.<TAB> to view the object’s attributes. Besides Python objects and keywords, tab completion also works on file and directory names.
collections.
The interactive workflow: input, output, history¶
2+10
_+10
You can suppress the storage and rendering of output if you append ; to the last cell (this comes in handy when plotting with matplotlib, for example):
10+20;
_
The output is stored in _N and Out[N] variables:
_10 == Out[10]
And the last three have shorthands for convenience:
print 'last output:', _
print 'next one :', __
print 'and next :', ___
In[11]
_i
_ii
print 'last input:', _i
print 'next one :', _ii
print 'and next :', _iii
%history -n 1-5
Exercise
Write the last 10 lines of history to a file named log.py.
Accessing the underlying operating system¶
!pwd
files = !ls
print "My current directory's files:"
print files
!echo $files
!echo {files[0].upper()}
Note that all this is available even in multiline blocks:
import os
for i,f in enumerate(files):
if f.endswith('ipynb'):
!echo {"%02d" % i} - "{os.path.splitext(f)[0]}"
else:
print '--'
Beyond Python: magic functions¶
The IPyhton 'magic' functions are a set of commands, invoked by prepending one or two % signs to their name, that live in a namespace separate from your normal Python variables and provide a more command-like interface. They take flags with -- and arguments without quotes, parentheses or commas. The motivation behind this system is two-fold:
To provide an orthogonal namespace for controlling IPython itself and exposing other system-oriented functionality.
To expose a calling mode that requires minimal verbosity and typing while working interactively. Thus the inspiration taken from the classic Unix shell style for commands.
%magic
Line vs cell magics:
%timeit range(10)
%%timeit
range(10)
range(100)
Line magics can be used even inside code blocks:
for i in range(5):
size = i*100
print 'size:',size,
%timeit range(size)
Magics can do anything they want with their input, so it doesn't have to be valid Python:
%%bash
echo "My shell is:" $SHELL
echo "My memory status is:"
free
Another interesting cell magic: create any file you want locally from the notebook:
%%writefile test.txt
This is a test file!
It can contain anything I want...
And more...
!cat test.txt
Let's see what other magics are currently defined in the system:
%lsmagic
Running normal Python code: execution and errors¶
Not only can you input normal Python code, you can even paste straight from a Python or IPython shell session:
>>> # Fibonacci series:
... # the sum of two elements defines the next
... a, b = 0, 1
>>> while b < 10:
... print b
... a, b = b, a+b
In [1]: for i in range(10):
...: print i,
...:
And when your code produces errors, you can control how they are displayed with the %xmode magic:
%%writefile mod.py
def f(x):
return 1.0/(x-1)
def g(y):
return f(y+1)
Now let's call the function g with an argument that would produce an error:
import mod
mod.g(0)
%xmode plain
mod.g(0)
%xmode verbose
mod.g(0)
The default %xmode is "context", which shows additional context but not all local variables. Let's restore that one for the rest of our session.
%xmode context
Running code in other languages with special %% magics¶
%%perl
@months = ("July", "August", "September");
print $months[0];
%%ruby
name = "world"
puts "Hello #{name.capitalize}!"
Exercise¶
Write a cell that executes in Bash and prints your current working directory as well as the date.
Apologies to Windows users who may not have Bash available, not sure how to obtain the equivalent result with cmd.exe or Powershell.
%load soln/bash-script
Raw Input in the notebook¶
Since 1.0 the IPython notebook web application support raw_input which for example allow us to invoke the %debug magic in the notebook:
mod.g(0)
%debug
Don't foget to exit your debugging session. Raw input can of course be use to ask for user input:
enjoy = raw_input('Are you enjoying this tutorial ?')
print 'enjoy is :', enjoy
Plotting in the notebook¶
This magic configures matplotlib to render its figures inline:
%matplotlib inline
import numpy as np
import matplotlib.pyplot as plt
x = np.linspace(0, 2*np.pi, 300)
y = np.sin(x**2)
plt.plot(x, y)
plt.title("A little chirp")
fig = plt.gcf() # let's keep the figure object around for later...
The IPython kernel/client model¶
%connect_info
We can connect automatically a Qt Console to the currently running kernel with the %qtconsole magic, or by typing ipython console --existing <kernel-UUID> in any terminal:
%qtconsole