3.3 Running Python Programs

Whatever tools you use to produce your Python application, you can see your application as a set of Python source files. A script is a file that you can run directly. A module is a file that you can import (as covered in Chapter 7) to provide functionality to other files or to interactive sessions. A Python file can be both a module and a script, exposing functionality when imported, but also suitable for being run directly. A useful and widespread convention is that Python files that are primarily meant to be imported as modules, when run directly, should execute self-test operations. Testing is covered in Chapter 17.

The Python interpreter automatically compiles Python source files as needed. Python source files normally have extension .py. Python saves the compiled bytecode file for each module in the same directory as the module's source, with the same basename and extension .pyc (or .pyo if Python is run with option -O). Python does not save the compiled bytecode form of a script when you run the script directly; rather, Python recompiles the script each time you run it. Python saves bytecode files only for modules you import. It automatically rebuilds each module's bytecode file whenever necessary, for example when you edit the module's source. Eventually, for deployment, you may package Python modules using tools covered in Chapter 26.

You can run Python code interactively, with the Python interpreter or an IDE. Normally, however, you initiate execution by running a top-level script. To run a script, you give its path as an argument to python, as covered earlier in this chapter. Depending on your operating system, you can invoke python directly, from a shell script, or in a command file. On Unix-like systems, you can make a Python script directly executable by setting the file's permission bits x and r and beginning the script with a so-called shebang line, which is a first line of the form:

#!/usr/bin/env python {options}

providing a path to the python program.

On Windows, you can associate file extensions .py, .pyc, and .pyo with the Python interpreter in the Windows registry. Most Python versions for Windows perform this association when installed. You can then run Python scripts with the usual Windows mechanisms, such as double-clicking on their icons. On Windows, when you run a Python script by double-clicking on the script's icon, Windows automatically closes the text-mode console associated with the script as soon as the script terminates. If you want the console to linger in order to allow the user to read the script's output on the screen, you need to ensure the script doesn't terminate too soon, for example by using the following as the script's last statement:

raw_input('Press Enter to terminate')

This is not necessary when you run the script from a pre-existing console (also known as a MS-DOS Prompt or Command Prompt window).

On Windows, you can also use extension .pyw and interpreter program pythonw.exe instead of .py and python.exe. The w variants run Python without a text-mode console, and thus without standard input and output. These variants are appropriate for scripts that rely on GUIs. You normally use them only when the script is fully debugged, to keep standard output and error available for information, warnings, and error messages during development.

Applications coded in other languages may embed Python, controlling the execution of Python code for their own purposes. We examine this subject further in Chapter 24.