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
1
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().