Advanced Module Selection Concepts

Normally, imports work as described in this section—they find and load files on your machine. However, it is possible to redefine much of what an import operation does in Python, using what are known as import hooks. These hooks can be used to make imports do various useful things, such as loading files from archives, performing decryption, and so on.

In fact, Python itself makes use of these hooks to enable files to be directly imported from ZIP archives: archived files are automatically extracted at import time when a .zip file is selected from the module import search path. One of the standard library directories in the earlier sys.path display, for example, is a .zip file today. For more details, see the Python standard library manual’s description of the built-in __import__ function, the customizable tool that import statements actually run.

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Python also supports the notion of .pyo optimized byte code files, created and run with the -O Python command-line flag; because these run only slightly faster than normal .pyc files (typically 5 percent faster), however, they are infrequently used. The Psyco system (see Chapter 2) provides more substantial speedups.

Third-Party Software: distutils

This chapter’s description of module search path settings is targeted mainly at user-defined source code that you write on your own. Third-party extensions for Python typically use the distutils tools in the standard library to automatically install themselves, so no path configuration is required to use their code.

Systems that use distutils generally come with a setup.py script, which is run to install them; this script imports and uses distutils modules to place such systems in a directory that is automatically part of the module search path (usually in the Lib\site-pack⁠ages subdirectory of the Python install tree, wherever that resides on the target machine).

For more details on distributing and installing with distutils, see the Python standard manual set; its use is beyond the scope of this book (for instance, it also provides ways to automatically compile C-coded extensions on the target machine). Also check out the emerging third-party open source eggs system, which adds dependency checking for installed Python software.

 

[49] See also Chapter 23’s discussion of the new relative import syntax in Python 3.0; this modifies the search path for from statements in files inside packages when “.” characters are used (e.g., from . import string). By default, a package’s own directory is not automatically searched by imports in Python 3.0, unless relative imports are used by files in the package itself.

[50] Some programs really need to change sys.path, though. Scripts that run on web servers, for example, often run as the user “nobody” to limit machine access. Because such scripts cannot usually depend on “nobody” to have set PYTHONPATH in any particular way, they often set sys.path manually to include required source directories, prior to running any import statements. A sys.path.append(dirname) will often suffice.