================= 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)) 1 loops, best of 1: 21.1 µs per loop In [2]: results Out[2]: 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