One of the benefits of the Python programming language is that it is interpreted, not compiled. This means that Python code is processed when it is called, rather than having to be pre-compiled before use. Because of this, interpreted languages generally have an interactive shell, allowing users to test code and otherwise have immediate feedback without having to create a separate source code file.

Of course, to get the most functionality out of a programming language, having permanent code files is necessary. When using an interactive prompt, the code lives in RAM; once the interactive session is closed, that code is lost. Thus, using an interactive prompt is a great way to quickly test programming ideas, but you wouldn't want to run a full-blown program from it.

This chapter will talk about using the Command Prompt to launch programs, as well as Python&apos...

By default, Python is installed on a computer with the Python interpreter included on the system path. This means that the interpreter will monitor the Command Prompt for any call to python. 

The most common usage for Python is to run a script. However, it may be desirable to launch a specific version of Python for a specific program.

1. The most basic command to execute a Python program is as follows:

      $ python <script_name>.py

2. The following examples show how to launch specific versions of Python, as needed:

      $ python2 some_script.py # Use the latest version of Python 2
      $ python2.7 ... # Specifically use Python 2.7
      $ python3 ... # Use...

When used non-interactively, the Python interpreter monitors the command line and parses all input before the command is actually executed. The following snippet shows all the possible options available when calling Python from the command line:

python [-bBdEhiIOqsSuvVWx?] [-c command | -m module-name | script | - ] [args] 

When working with a command line interface (CLI), examples of shell commands often show square brackets [] to indicate optional instructions. In this case, there are three groups of optional input that can be provided to the python command: generic options, interface options, and arguments.

1. A number of options are available for the Python command-line...

Environment variables are part of operating systems and affect system operations. Python has Python-specific variables that affect how Python functions, that is, the behavior of the Python interpreter. While they are processed before command-line options, the command-line switches will override environment variables if there is a conflict.

1. Environment variables are accessed via Python's os.environ.
2. Because the environ object is a dictionary, you can specify a particular variable to view:

      >>> import os
      >>> print(os.environ["PATH"])
      /home/cody/anaconda3/bin:/home/cody/bin:/home/cody/
      .local/bin:/usr/local/sbin:/usr/local...

Normally, executing a Python program requires typing python <program>.py. However, it is possible to make Python programs self-executing so they don't require typing python as the calling command.

1. On *NIX systems, putting #!/usr/bin/env python as the first line of a program allows the program to be executable by referencing the location of Python on the user's PATH. Of course, this assumes Python is on the PATH; if not, then the program will have to be invoked like normal.
2. After this has been added to the program, the file itself needs to be modified to make it executable, that is, $ chmod +x <program>.py.
3. If you are using a terminal program that displays...

As mentioned in the Working with environment variables recipe, the PYTHONSTARTUP environment variable can be set to point to a file that contains commands that run prior to the Python interpreter starting up. This functionality is similar to .profile on *NIX shells.

As this startup file is only examined when interactive mode is used, there is no need to worry about trying to set configurations for running scripts (though later on we will show how to include the startup file in a script). The commands in this file are executed within the same namespace as the interactive interpreter, so there is no need to qualify functions or other imports with dot-nomenclature. This file is also responsible for making changes to interactive prompts: >>> (sys.ps1) and ... (sys.ps2).

Python has been ported to a number of other environments, such as Java and .NET. This means Python can be used in these environments like normal, but gains access to the APIs and code underpinnings for these environments.

Jython is used for Java integration, IronPython is used for the .NET framework, Stackless Python is available for enhanced threading performance, and MicroPython is for use with microcontrollers.

1. To use Jython, a Java .jar file provides the installation executable. Two options are available for installation.
2. Normal GUI installation is available by using the following:

      java -jar jython_installer-2.7.1.jar

3. For console-based systems, such as headless...