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-Enter to force the creation of a new cell unconditionally (useful when inserting new content in the middle of an existing notebook).
  • Control-Enter executes the cell and keeps the cursor in the same cell, useful for quick experimentation of snippets that you don't need to keep permanently.

In [1]:
print "Hi"


Getting help:

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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.

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import collections

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An IPython quick reference card:

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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.

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The interactive workflow: input, output, history

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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):

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The output is stored in _N and Out[N] variables:

In [11]:
_10 == Out[10]


And the last three have shorthands for convenience:

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print 'last output:', _
print 'next one   :', __
print 'and next   :', ___

last output: True
next one   : 22
and next   : 22

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u'_10 == Out[10]'

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In [16]:
print 'last input:', _i
print 'next one  :', _ii
print 'and next  :', _iii

last input: _ii
next one  : _i
and next  : In[11]

In [17]:
%history -n 1-5

   1: print "Hi"
   2: ?
import collections
   4: collections.Counter??
   5: *int*?


Write the last 10 lines of history to a file named log.py.

Accessing the underlying operating system

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files = !ls
print "My current directory's files:"
print files

My current directory's files:
['Beyond Plain Python.ipynb', 'Converting Notebooks With nbconvert.ipynb', 'Custom Display Logic.ipynb', 'images', 'Notebook Basics.ipynb', 'Rich Output.ipynb', 'Using Interact.ipynb', 'Working With Markdown Cells.html', 'Working With Markdown Cells.ipynb', 'Working With Markdown Cells.pdf', 'Working With Markdown Cells.tex']

In [20]:
!echo $files

[Beyond Plain Python.ipynb, Converting Notebooks With nbconvert.ipynb, Custom Display Logic.ipynb, images, Notebook Basics.ipynb, Rich Output.ipynb, Using Interact.ipynb, Working With Markdown Cells.html, Working With Markdown Cells.ipynb, Working With Markdown Cells.pdf, Working With Markdown Cells.tex]

In [21]:
!echo {files[0].upper()}


Note that all this is available even in multiline blocks:

In [22]:
import os
for i,f in enumerate(files):
    if f.endswith('ipynb'):
        !echo {"%02d" % i} - "{os.path.splitext(f)[0]}"
        print '--'

00 - Beyond Plain Python
01 - Converting Notebooks With nbconvert
02 - Custom Display Logic
04 - Notebook Basics
05 - Rich Output
06 - Using Interact
08 - Working With Markdown Cells

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.

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Line vs cell magics:

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%timeit range(10)

1000000 loops, best of 3: 195 ns per loop

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1000000 loops, best of 3: 832 ns per loop

Line magics can be used even inside code blocks:

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for i in range(5):
    size = i*100
    print 'size:',size, 
    %timeit range(size)

size: 010000000 loops, best of 3: 122 ns per loop
 size: 1001000000 loops, best of 3: 651 ns per loop
 size: 2001000000 loops, best of 3: 1.1 µs per loop
 size: 3001000000 loops, best of 3: 1.74 µs per loop
 size: 400100000 loops, best of 3: 2.66 µs per loop

Magics can do anything they want with their input, so it doesn't have to be valid Python:

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echo "My shell is:" $SHELL
echo "My memory status is:"

My shell is: /bin/bash
My memory status is:
             total       used       free     shared    buffers     cached
Mem:       7870496    7077772     792724     265364     442596    2153668
-/+ buffers/cache:    4481508    3388988
Swap:      3905532      20552    3884980

Another interesting cell magic: create any file you want locally from the notebook:

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%%writefile test.txt
This is a test file!
It can contain anything I want...

And more...

Writing test.txt

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!cat test.txt

This is a test file!
It can contain anything I want...

And more...

Let's see what other magics are currently defined in the system:

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Available line magics:
%alias  %alias_magic  %autocall  %automagic  %autosave  %bookmark  %cat  %cd  %cl  %clear  %clk  %colors  %config  %connect_info  %cp  %d  %dd  %debug  %dhist  %dirs  %dl  %doctest_mode  %dx  %ed  %edit  %env  %gui  %hist  %history  %install_default_config  %install_ext  %install_profiles  %killbgscripts  %ldir  %less  %lf  %lk  %ll  %load  %load_ext  %loadpy  %logoff  %logon  %logstart  %logstate  %logstop  %ls  %lsmagic  %lx  %macro  %magic  %man  %matplotlib  %mkdir  %more  %mv  %notebook  %page  %pastebin  %pdb  %pdef  %pdoc  %pfile  %pinfo  %pinfo2  %popd  %pprint  %precision  %profile  %prun  %psearch  %psource  %pushd  %pwd  %pycat  %pylab  %qtconsole  %quickref  %recall  %rehashx  %reload_ext  %rep  %rerun  %reset  %reset_selective  %rm  %rmdir  %run  %save  %sc  %store  %sx  %system  %tb  %time  %timeit  %unalias  %unload_ext  %who  %who_ls  %whos  %xdel  %xmode

Available cell magics:
%%!  %%HTML  %%SVG  %%bash  %%capture  %%debug  %%file  %%html  %%javascript  %%latex  %%perl  %%prun  %%pypy  %%python  %%python2  %%python3  %%ruby  %%script  %%sh  %%svg  %%sx  %%system  %%time  %%timeit  %%writefile

Automagic is ON, % prefix IS NOT needed for line magics.

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:

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>>> # Fibonacci series:
... # the sum of two elements defines the next
... a, b = 0, 1
>>> while b < 10:
...     print b
...     a, b = b, a+b


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In [1]: for i in range(10):
   ...:     print i,

0 1 2 3 4 5 6 7 8 9

And when your code produces errors, you can control how they are displayed with the %xmode magic:

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%%writefile mod.py

def f(x):
    return 1.0/(x-1)

def g(y):
    return f(y+1)

Writing mod.py

Now let's call the function g with an argument that would produce an error:

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import mod

ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-34-a54c5799f57e> in <module>()
      1 import mod
----> 2 mod.g(0)

/home/fperez/teach/astro-hack-week/day1/ipython/mod.py in g(y)
      5 def g(y):
----> 6     return f(y+1)

/home/fperez/teach/astro-hack-week/day1/ipython/mod.py in f(x)
      2 def f(x):
----> 3     return 1.0/(x-1)
      5 def g(y):

ZeroDivisionError: float division by zero

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:

In [40]:

ZeroDivisionError                         Traceback (most recent call last)
<ipython-input-40-5e708f13c839> in <module>()
----> 1 mod.g(0)

/home/fperez/teach/astro-hack-week/day1/ipython/mod.py in g(y)
      5 def g(y):
----> 6     return f(y+1)

/home/fperez/teach/astro-hack-week/day1/ipython/mod.py in f(x)
      2 def f(x):
----> 3     return 1.0/(x-1)
      5 def g(y):

ZeroDivisionError: float division by zero

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> /home/fperez/teach/astro-hack-week/day1/ipython/mod.py(3)f()
      2 def f(x):
----> 3     return 1.0/(x-1)

ipdb> u
> /home/fperez/teach/astro-hack-week/day1/ipython/mod.py(6)g()
      5 def g(y):
----> 6     return f(y+1)

ipdb> q

Don't foget to exit your debugging session. Raw input can of course be use to ask for user input:

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enjoy = raw_input('Are you enjoying this tutorial ?')
print 'enjoy is :', enjoy

Are you enjoying this tutorial ?Yes !
enjoy is : Yes !

Running code in other languages with special %% magics

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@months = ("July", "August", "September");
print $months[0];


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name = "world"
puts "Hello #{name.capitalize}!"

Hello World!

The IPython kernel/client model

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  "stdin_port": 50023, 
  "ip": "", 
  "control_port": 50024, 
  "hb_port": 50025, 
  "signature_scheme": "hmac-sha256", 
  "key": "b54b8859-d64d-48bb-814a-909f9beb3316", 
  "shell_port": 50021, 
  "transport": "tcp", 
  "iopub_port": 50022

Paste the above JSON into a file, and connect with:
    $> ipython <app> --existing <file>
or, if you are local, you can connect with just:
    $> ipython <app> --existing kernel-30f00f4a-230c-4e64-bea5-0e5f6a52cb40.json 
or even just:
    $> ipython <app> --existing 
if this is the most recent IPython session you have started.

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:

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