=================
Python vs IPython
=================
This document is meant to highlight the main differences between the Python
language and what are the specific constructs you can do only in IPython.
Unless expressed otherwise all of the constructs you will see here will raise a
``SyntaxError`` if run in a pure Python shell, or if executing in a Python
script.
Each of these features is described more in detail in the further parts of the documentation.
Quick overview:
===============
All the following constructs are valid IPython syntax:
.. code-block:: ipython
In [1]: ?
.. code-block:: ipython
In [1]: ?object
.. code-block:: ipython
In [1]: object?
.. code-block:: ipython
In [1]: *pattern*?
.. code-block:: ipython
In [1]: %shell like --syntax
.. code-block:: ipython
In [1]: !ls
.. code-block:: ipython
In [1]: my_files = !ls ~/
In [1]: for i, file in enumerate(my_files):
...: raw = !echo $file
...: !echo {file[0].upper()} $raw
.. code-block:: ipython
In [1]: %%perl magic --function
...: @months = ("July", "August", "September");
...: print $months[0];
Each of these constructs is compiled by IPython into valid python code and will
do most of the time what you expect it will do. Let's see each of these examples
in more detail.
Accessing help
==============
As IPython is mostly an interactive shell, the question mark is a simple
shortcut to get help. A question mark alone will bring up the IPython help:
.. code-block:: ipython
In [1]: ?
IPython -- An enhanced Interactive Python
=========================================
IPython offers a combination of convenient shell features, special commands
and a history mechanism for both input (command history) and output (results
caching, similar to Mathematica). It is intended to be a fully compatible
replacement for the standard Python interpreter, while offering vastly
improved functionality and flexibility.
At your system command line, type 'ipython -h' to see the command line
options available. This document only describes interactive features.
MAIN FEATURES
-------------
...
A single question mark before or after an object available in the current
namespace will show help relative to this object:
.. code-block:: ipython
In [6]: object?
Docstring: The most base type
Type: type
A double question mark will try to pull out more information about the object,
and if possible display the python source code of this object.
.. code-block:: ipython
In[1]: import collections
In[2]: collections.Counter??
Init signature: collections.Counter(*args, **kwds)
Source:
class Counter(dict):
'''Dict subclass for counting hashable items. Sometimes called a bag
or multiset. Elements are stored as dictionary keys and their counts
are stored as dictionary values.
>>> c = Counter('abcdeabcdabcaba') # count elements from a string
>>> c.most_common(3) # three most common elements
[('a', 5), ('b', 4), ('c', 3)]
>>> sorted(c) # list all unique elements
['a', 'b', 'c', 'd', 'e']
>>> ''.join(sorted(c.elements())) # list elements with repetitions
'aaaaabbbbcccdde'
...
If you are looking for an object, the use of wildcards ``*`` in conjunction
with a question mark will allow you to search the current namespace for objects with
matching names:
.. code-block:: ipython
In [24]: *int*?
FloatingPointError
int
print
Shell Assignment
================
When doing interactive computing it is a common need to access the underlying shell.
This is doable through the use of the exclamation mark ``!`` (or bang).
This allows to execute simple commands when present in beginning of the line:
.. code-block:: ipython
In[1]: !pwd
/User/home/
Change directory:
.. code-block:: ipython
In[1]: !cd /var/etc
Or edit file:
.. code-block:: ipython
In[1]: !mvim myfile.txt
The line after the bang can call any program installed in the underlying
shell, and support variable expansion in the form of ``$variable`` or ``{variable}``.
The later form of expansion supports arbitrary python expressions:
.. code-block:: ipython
In[1]: file = 'myfile.txt'
In[2]: !mv $file {file.upper()}
The bang (``!``) can also be present on the right hand side of an assignment, just
after the equal sign, or separated from it by a white space. In this case the
standard output of the command after the bang will be split out into lines
in a list-like object and assigned to the left hand side.
This allows you, for example, to put the list of files of the current working directory in a variable:
.. code-block:: ipython
In[1]: my_files = !ls
You can combine the different possibilities in for loops, conditions, functions...:
.. code-block:: ipython
my_files = !ls ~/
for i, file in enumerate(my_files):
raw = !echo $backup $file
!cp $file {file.split('.')[0] + '.bak'}
Magics
------
Magic functions (magics) are often present in the form of shell-like syntax, but they are
python functions under the hood. The syntax and assignment possibilities are
similar to the one with the bang (``!``) syntax, but with more flexibility and
power. Magic functions start with a percent sign (``%``) or double percent signs (``%%``).
A magic call with a single percent sign will act only on one line:
.. code-block:: ipython
In[1]: %xmode
Exception reporting mode: Verbose
Magics support assignment:
.. code-block:: ipython
In [1]: results = %timeit -r1 -n1 -o list(range(1000))
62.1 µs ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each)
In [2]: results
<TimeitResult : 62.1 µs ± 0 ns per loop (mean ± std. dev. of 1 run, 1 loop each)>
Magics with double percent signs (``%%``) can spread over multiple lines, but they do not support assignments:
.. code-block:: ipython
In[1]: %%bash
... : echo "My shell is:" $SHELL
... : echo "My disk usage is:"
... : df -h
My shell is: /usr/local/bin/bash
My disk usage is:
Filesystem Size Used Avail Capacity iused ifree %iused Mounted on
/dev/disk1 233Gi 216Gi 16Gi 94% 56788108 4190706 93% /
devfs 190Ki 190Ki 0Bi 100% 656 0 100% /dev
map -hosts 0Bi 0Bi 0Bi 100% 0 0 100% /net
map auto_home 0Bi 0Bi 0Bi 100% 0 0 100% /hom