Running Python code

Many people, when they think of Python, they think of the interactive Jupyter environment in a browser. We will come to Jupyter soon. But it is important to understand that Python itself is a more fundamental concept: it is a programming language, and is not dependent on any browser or environment as such.

The most basic way to use Python would be from the Linux command line (called a 'shell'), if you use the Linux operating system. For users of a Macbook, also MacOS has a command line interface (a 'terminal'). If you use Windows it is less obvious, but it, too, has a command line interface.

Here we will discuss how to use Python interactively from the command-line.

Interactively from the command line (demo)

To run Python code interactively, one can use the standard Python prompt, which can be launched by typing python in your standard shell:

$ python
Python 3.4.1 (default, May 21 2014, 21:17:51) 
[GCC 4.2.1 Compatible Apple Clang 4.1 ((tags/Apple/clang-421.11.66))] on darwin
Type "help", "copyright", "credits" or "license" for more information.

The >>> indicates that Python is ready to accept commands. If you type a = 1 then press enter, this will assign the value 1 to a. If you then type a you will see the value of a (this is equivalent to print a):

>>> a = 1
>>> a

The Python shell can execute any Python code, even multi-line statements, though it is often more convenient to use Python non-interactively for such cases.

The default Python shell is limited, so we will use the IPython (or interactive Python) shell here. This is an add-on package that adds many features to the default Python shell, including the ability to edit and navigate the history of previous commands, as well as the ability to tab-complete variable and function names. To start up IPython, type:

$ ipython
Python 3.4.1 (default, May 21 2014, 21:17:51) 
Type "copyright", "credits" or "license" for more information.

IPython 2.1.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]:

The first time you start up IPython, it will display a message which you can skip over by pressing ENTER. The >>> symbols are now replaced by In [x], and output, when present, is prepended with Out [x]. If we now type the same commands as before, we get:

In [1]: a = 1

In [2]: a
Out[2]: 1

If you now type the up arrow twice, you will get back to a = 1.

To exit the Python shell at any time and return to the command prompt of the terminal, type exit().