How To Think Like A Computer ITSC 1212 - Functions First Edition

The programming language you will be learning is Python. Python is an interpreted high level programming language.

There are two ways to use the Python interpreter: shell mode and program mode. In shell mode, you type Python expressions into the Python shell, and the interpreter immediately shows the result. The example below shows the Python shell at work.

$ python3
Python 3.2 (r32:88445, Mar 25 2011, 19:28:28)
[GCC 4.5.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> 2 + 3
5
>>>

The >>> is called the Python prompt. The interpreter uses the prompt to indicate that it is ready for instructions. We typed 2 + 3. The interpreter evaluated our expression and replied 5. On the next line it gave a new prompt indicating that it is ready for more input.

Working directly in the interpreter is convenient for testing short bits of code because you get immediate feedback. Think of it as scratch paper used to help you work out problems.

Alternatively, you can write an entire program by placing lines of Python instructions in a file and then use the interpreter to execute the contents of the file as a whole. Such a file is often referred to as source code. For example, we used a text editor to create a source code file named firstprogram.py with the following contents:

print("My first program adds two numbers, 2 and 3:")
print(2 + 3)

By convention, files that contain Python programs have names that end with .py . Following this convention will help your operating system and other programs identify a file as containing python code.

$ python firstprogram.py
My first program adds two numbers, 2 and 3:
5

These examples show Python being run from a Unix command line. In other development environments, the details of executing programs may differ. Also, most programs are more interesting than this one.