Learning Python - Mark Lutz [313]
import utilities
offers much less information than an import that includes the path:
import database.client.utilities
Package imports can also greatly simplify your PYTHONPATH and .pth file search path settings. In fact, if you use explicit package imports for all your cross-directory imports, and you make those package imports relative to a common root directory where all your Python code is stored, you really only need a single entry on your search path: the common root. Finally, package imports serve to resolve ambiguities by making explicit exactly which files you want to import. The next section explores this role in more detail.
A Tale of Three Systems
The only time package imports are actually required is to resolve ambiguities that may arise when multiple programs with same-named files are installed on a single machine. This is something of an install issue, but it can also become a concern in general practice. Let’s turn to a hypothetical scenario to illustrate.
Suppose that a programmer develops a Python program that contains a file called utilities.py for common utility code and a top-level file named main.py that users launch to start the program. All over this program, its files say import utilities to load and use the common code. When the program is shipped, it arrives as a single .tar or .zip file containing all the program’s files, and when it is installed, it unpacks all its files into a single directory named system1 on the target machine:
system1\
utilities.py # Common utility functions, classes
main.py # Launch this to start the program
other.py # Import utilities to load my tools
Now, suppose that a second programmer develops a different program with files also called utilities.py and main.py, and again uses import utilities throughout the program to load the common code file. When this second system is fetched and installed on the same computer as the first system, its files will unpack into a new directory called system2 somewhere on the receiving machine (ensuring that they do not overwrite same-named files from the first system):
system2\
utilities.py # Common utilities
main.py # Launch this to run
other.py # Imports utilities
So far, there’s no problem: both systems can coexist and run on the same machine. In fact, you won’t even need to configure the module search path to use these programs on your computer—because Python always searches the home directory first (that is, the directory containing the top-level file), imports in either system’s files will automatically see all the files in that system’s directory. For instance, if you click on system1\main.py, all imports will search system1 first. Similarly, if you launch system2\main.py, system2 will be searched first instead. Remember, module search path settings are only needed to import across directory boundaries.
However, suppose that after you’ve installed these two programs on your machine, you decide that you’d like to use some of the code in each of the utilities.py files in a system of your own. It’s common utility code, after all, and Python code by nature wants to be reused. In this case, you want to be able to say the following from code that you’re writing in a third directory to load one of the two files:
import utilities
utilities.func('spam')
Now the problem starts to materialize. To make this work at all, you’ll have to set the module search path to include the directories containing the utilities.py files. But which directory do you put first in the path—system1 or system2?
The problem is the linear nature of the search path. It is always scanned from left to right, so no matter how long you ponder this dilemma, you will always get utilities.py from the directory listed first (leftmost) on the search path. As is, you’ll never be able to import it from the other directory at all. You could try changing sys.path within your script before each import operation, but that’s both