The Programmer’s View

In its simplest form, a Python program is just a text file containing Python statements. For example, the following file, named script0.py, is one of the simplest Python scripts I could dream up, but it passes for a fully functional Python program:

print('hello world')
print(2 ** 100)

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This file contains two Python print statements, which simply print a string (the text in quotes) and a numeric expression result (2 to the power 100) to the output stream. Don’t worry about the syntax of this code yet—for this chapter, we’re interested only in getting it to run. I’ll explain the print statement, and why you can raise 2 to the power 100 in Python without overflowing, in the next parts of this book.

You can create such a file of statements with any text editor you like. By convention, Python program files are given names that end in .py; technically, this naming scheme is required only for files that are “imported,” as shown later in this book, but most Python files have .py names for consistency.

After you’ve typed these statements into a text file, you must tell Python to execute the file—which simply means to run all the statements in the file from top to bottom, one after another. As you’ll see in the next chapter, you can launch Python program files by shell command lines, by clicking their icons, from within IDEs, and with other standard techniques. If all goes well, when you execute the file, you’ll see the results of the two print statements show up somewhere on your computer—by default, usually in the same window you were in when you ran the program:

hello world
1267650600228229401496703205376

For example, here’s what happened when I ran this script from a DOS command line on a Windows laptop (typically called a Command Prompt window, found in the Accessories program menu), to make sure it didn’t have any silly typos:

C:\temp> python script0.py
hello world
1267650600228229401496703205376

We’ve just run a Python script that prints a string and a number. We probably won’t win any programming awards with this code, but it’s enough to capture the basics of program execution.