A module’s source code file automatically becomes a module object when that module is imported. Technically, the module’s source code is run during the import, one statement at a time, and all the names assigned in the process become attributes of the module object.
You only need to set PYTHONPATH to import from directories other than the one in which you are working (i.e., the current directory when working interactively, or the directory containing your top-level file). In practice, this will be a common case for nontrivial programs.
The five major components of the module import search path are the top-level script’s home directory (the directory containing it), all directories listed in the PYTHONPATH environment variable, the standard library directories, all directories listed in .pth path files located in standard places, and the site-packages root directory for third-party extension installs. Of these, programmers can customize PYTHONPATH and .pth files.
Python might load a source code (.py) file, a byte code (.pyc or .pyo) file, a C extension module (e.g., a .so file on Linux or a .dll or .pyd file on Windows), or a directory of the same name for package imports. Imports may also load more exotic things such as ZIP file components, Java classes under the Jython version of Python, .NET components under IronPython, and statically linked C extensions that have no files present at all. In fact, with import hooks, imports can load arbitrary items.
A namespace is a self-contained package of variables, which are known as the attributes of the namespace object. A module’s namespace contains all the names assigned by code at the top level of the module file (i.e., not nested in def or class statements). Technically, a module’s global scope morphs into the module object’s attributes namespace. A module’s namespace may also be altered by assignments from other files that import it, though this is generally frowned upon (see Chapter 17 for more on the downsides of cross-file changes).