1.9 Writing and executing Python program

Writing and executing Python program

·         Python code can be written using any plain text editor that can load and save text using either the ASCII or the UTF-8 Unicode encoding.

·         By default, Python files are assumed to use the UTF-8-character encoding, a superset of ASCII that can represent pretty well every character in every language.

·         Python files normally have an extension of .py, although on some Unix-like systems (e.g., Linux and Mac OS X) some Python applications have no extension, and Python GUI (Graphical User Interface) programs usually have an extension of. pyw, particularly on Windows and Mac OSX. In this book we always use an extension of .py for Python console programs and Python modules, and. pyw for GUI programs.

·         Just to make sure that everything is set up correctly, and to show the classical first example, create a file called hello.py in a plain text editor (Windows Notepad is fine—we’ll use a better editor shortly), with the following contents:

#!/usr/bin/env python3

print("Hello", "World!")

·         The first line is a comment. In Python, comments begin with a # and continue to the end of the line. (We will explain the rather cryptic comment in a moment.)

·         The second line is blank—outside quoted strings, Python ignores blank lines, but they are often useful to humans to break up large blocks of code to make them easier to read.

·         The third line is Python code. Here, the print() function is called with two arguments, each of type str (string; i.e., a sequence of characters).

·         Each statement encountered in a .py file is executed in turn, starting with the first one and progressing line by line. This is different from some other languages, for example, C++ and Java, which have a particular function or method with a special name where they start from.

·         The flow of control can of course be diverted as we will see when we discuss Python’s control structures in the next section.

Now that we have a program, we can run it.

·         Python programs are executed by the Python interpreter, and normally this is done inside a console window. On Windows the console is called “Console”, or “DOS Prompt”, or “MS-DOS Prompt”.

·         Start up a console, and on Windows enter the following commands (which assume that Python is installed in the default location)—the console’s output is shown in lightface; what you type is shown in bold:

C:\>cd c:\py3eg

C:\py3eg\>c:\python31\python.exe hello.py

·         Since the cd (change directory) command has an absolute path, it doesn’t matter which directory you start out from. Unix users enter this instead (assuming that Python 3 is in the PATH):

$ cd $HOME/py3eg

$ python3 hello.py

In both cases the output should be the same:

Hello World!