Learning Python - Mark Lutz [296]
[48] As described earlier, Python keeps already imported modules in the built-in sys.modules dictionary so it can keep track of what’s been loaded. In fact, if you want to see which modules are loaded, you can import sys and print list(sys.modules.keys()). More on other uses for this internal table in Chapter 24.
The Module Search Path
As mentioned earlier, the part of the import procedure that is most important to programmers is usually the first—locating the file to be imported (the “find it” part). Because you may need to tell Python where to look to find files to import, you need to know how to tap into its search path in order to extend it.
In many cases, you can rely on the automatic nature of the module import search path and won’t need to configure this path at all. If you want to be able to import files across directory boundaries, though, you will need to know how the search path works in order to customize it. Roughly, Python’s module search path is composed of the concatenation of these major components, some of which are preset for you and some of which you can tailor to tell Python where to look:
The home directory of the program
PYTHONPATH directories (if set)
Standard library directories
The contents of any .pth files (if present)
Ultimately, the concatenation of these four components becomes sys.path, a list of directory name strings that I’ll expand upon later in this section. The first and third elements of the search path are defined automatically. Because Python searches the concatenation of these components from first to last, though, the second and fourth elements can be used to extend the path to include your own source code directories. Here is how Python uses each of these path components:
Home directory
Python first looks for the imported file in the home directory. The meaning of this entry depends on how you are running the code. When you’re running a program, this entry is the directory containing your program’s top-level script file. When you’re working interactively, this entry is the directory in which you are working (i.e., the current working directory).
Because this directory is always searched first, if a program is located entirely in a single directory, all of its imports will work automatically with no path configuration required. On the other hand, because this directory is searched first, its files will also override modules of the same name in directories elsewhere on the path; be careful not to accidentally hide library modules this way if you need them in your program.
PYTHONPATH directories
Next, Python searches all directories listed in your PYTHONPATH environment variable setting, from left to right (assuming you have set this at all). In brief, PYTHONPATH is simply set to a list of user-defined and platform-specific names of directories that contain Python code files. You can add all the directories from which you wish to be able to import, and Python will extend the module search path to include all the directories your PYTHONPATH lists.
Because Python searches the home directory first, this setting is only important when importing files across directory boundaries—that is, if you need to import a file that is stored in a different directory from the file that imports it. You’ll probably want to set your PYTHONPATH variable once you start writing substantial programs, but when you’re first starting out, as long as you save all your module files in the directory in which you’re working (i.e., the home directory, described earlier) your imports will work without you needing to worry about this setting at all.
Standard