Interactive Python

Gregor von Laszewski (laszewski@gmail.com)

Python can be used interactively. You can enter the interactive mode by entering the interactive loop by executing the command:

$ python

You will see something like the following:

$ python
Python 3.9.5 (v3.9.5:0a7dcbdb13, May  3 2021, 13:17:02) 
[Clang 6.0 (clang-600.0.57)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>

The >>> is the prompt used by the interpreter. This is similar to bash where commonly $ is used.

Sometimes it is convenient to show the prompt when illustrating an example. This is to provide some context for what we are doing. If you are following along you will not need to type in the prompt.

This interactive python process does the following:

• read your input commands
• evaluate your command
• print the result of the evaluation
• loop back to the beginning.

This is why you may see the interactive loop referred to as a REPL: Read-Evaluate-Print-Loop.

REPL (Read Eval Print Loop)

There are many different types beyond what we have seen so far, such as dictionariess, lists, sets. One handy way of using the interactive python is to get the type of a value using type():

>>> type(42)
<type 'int'>
>>> type('hello')
<type 'str'>
>>> type(3.14)
<type 'float'>

You can also ask for help about something using help():

>>> help(int)
>>> help(list)
>>> help(str)

Using help() opens up a help message within a pager. To navigate you can use the spacebar to go down a page w to go up a page, the arrow keys to go up/down line-by-line, or q to exit.

Interpreter

Although the interactive mode provides a convenient tool to test things out you will see quickly that for our class we want to use the python interpreter from the command line. Let us assume the program is called prg.py. Once you have written it in that file you simply can call it with

$ python prg.py

It is important to name the program with meaningful names.