Python in a Nutshell, 2nd Edition

Running Python Programs

Whatever tools you use to produce your Python application, you can see your application as a set of Python source files, which are normal text 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 is also suitable for being run directly. A useful and widespread convention is that Python files that are primarily intended to be imported as modules, when run directly, should execute some simple self-test operations, as covered in Testing.

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 27.

You can run Python code interactively with the Python interpreter or an IDE. Normally, ...