Python in a Nutshell, 3rd Edition

Chapter 13. Controlling Execution

Python directly exposes, supports, and documents many of its internal mechanisms. This may help you understand Python at an advanced level, and lets you hook your own code into such Python mechanisms, controlling them to some extent. For example, “Python built-ins” covers the way Python arranges for built-ins to be visible. This chapter covers some other advanced Python techniques; Chapter 16 covers issues specific to testing, debugging, and profiling. Other issues related to controlling execution are about using multiple threads and processes, covered in Chapter 14, and about asynchronous processing, covered in Chapter 18.

Site and User Customization

Python provides a specific “hook” to let each site customize some aspects of Python’s behavior at the start of each run. Customization by each single user is not enabled by default, but Python specifies how programs that want to run user-provided code at startup can explicitly support such customization (a rarely used facility).

The site and sitecustomize Modules

Python loads the standard module site just before the main script. If Python is run with option -S, Python does not load site. -S allows faster startup but saddles the main script with initialization chores. site’s tasks are:

• Putting sys.path in standard form (absolute paths, no duplicates).

• Interpreting each .pth file found in the Python home directory, adding entries to sys.path, and/or importing modules, as each .pth file indicates. ...