IPython - an enhanced interactive Python REPL.

Back in the Hello-world chapter we saw the vanilla python REPL. For what it is, it gets the job done.

IPython takes the idea of a REPL and adds better features such as tab completion*, multi-line editing, history, and other really powerful features to the table.

While it is the python kernel backing Jupyter Notebook, it can be used standalone without the rest of the scientific stack installed.

This particular library is difficult to express in text, for the full experience watch the presentation.

Launching IPython

IPython, just like the standard Python repl, can be launched from the command line:

# using the installed alias
ipython3
# Or, to launch ipython witin a specific interpreter
python3 -m IPython

Anatomy of the IPython REPL

After launching IPython, you will be greeted with the following display:

~ ⌚ 12:37:04
$ ipython3
Python 3.8.10 (default, Jun  2 2021, 10:49:15) 
Type 'copyright', 'credits' or 'license' for more information
IPython 7.24.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]:

In[n]: is akin to a line number, it is essentialy the >>> prompt in the standard REPL.

Let's type some basic python code!

In[1]: x = 7

In[2]: x + 3
Out[2]: 10

In[3]: Out[2] - 4
Out[3]: 6

In[4]:
  • In[1] did't produce an Out[1], this is because assignment operations evaluate to None.
  • In[2] produced an Out[2], which represents the output of In[2].
  • In[3] uses the value Out[2] in a further calculation.
    • In and Out are special variables IPython provides!

IPython remembers

IPython keeps a history of inputted commands, which can be retrieved at a later point in time.

This feature can be very useful when the user is many lines down into some knapkin math and needs to find a calculation done many lines prior.

There are two ways of finding previous inputs:

  • Route 1: start typing the first couple characters of the input, then hit up-arrow
  • Route 2: Ctrl+r

IPython allows for editing lines already inserted, but not yet executed.

In the standard python REPL, after a line of code has been submitted, it cannot be changed. For basic arithmetic, this is not a problem. When writing loops or more complicated expressions, this becomes a problem.

  • For writing any significant length of code, an IDE should be used and not the REPL.

For example:

$ python3
Python 3.8.10 (default, Jun  2 2021, 10:49:15) 
[GCC 9.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.

>>> x = 7
>>> while z > 6:
    ...  # well now i can't go and fix that while statement without starting over again....
KeyboardInterrupt
>>>
  • IPython allows the user to edit lines

Magics

IPython also brings some unique syntax to the table in the form of magic commands.

One such magic is %timeit, which allows the user to benchmark a block of code. This can be useful when comparing two logically equivalent implementations.