Learning Python - Mark Lutz [321]
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Whether you create package directories or not, you will probably import from them eventually.
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[53] Yes, there will be a 2.7 release, and possibly 2.8 and later releases, in parallel with new releases in the 3.X line. As described in the Preface, both the Python 2 and Python 3 lines are expected to be fully supported for years to come, to accommodate the large existing Python 2 user and code bases.
Chapter Summary
This chapter introduced Python’s package import model—an optional but useful way to explicitly list part of the directory path leading up to your modules. Package imports are still relative to a directory on your module import search path, but rather than relying on Python to traverse the search path manually, your script gives the rest of the path to the module explicitly.
As we’ve seen, packages not only make imports more meaningful in larger systems, but also simplify import search path settings (if all cross-directory imports are relative to a common root directory) and resolve ambiguities when there is more than one module of the same name (including the name of the enclosing directory in a package import helps distinguish between them).
Because it’s relevant only to code in packages, we also explored the newer relative import model here—a way for imports in package files to select modules in the same package using leading dots in a from, instead of relying on an older implicit package search rule.
In the next chapter, we will survey a handful of more advanced module-related topics, such as relative import syntax and the __name__ usage mode variable. As usual, though, we’ll close out this chapter with a short quiz to test what you’ve learned here.
Test Your Knowledge: Quiz
What is the purpose of an __init__.py file in a module package directory?
How can you avoid repeating the full package path every time you reference a package’s content?
Which directories require __init__.py files?
When must you use import instead of from with packages?
What is the difference between from mypkg import spam and from . import spam?
Test Your Knowledge: Answers
The __init__.py file serves to declare and initialize a module package; Python automatically runs its code the first time you import through a directory in a process. Its assigned variables become the attributes of the module object created in memory to correspond to that directory. It is also not optional—you can’t import through a directory with package syntax unless it contains this file.
Use the from statement with a package to copy names out of the package directly, or use the as extension with the import statement to rename the path to a shorter synonym. In both cases, the path is listed in only one place, in the from or import statement.
Each directory listed in an import or from statement must contain an __init__.py file. Other directories, including the directory containing the leftmost component of a package path, do not need to include this file.
You must use import instead of from with packages only if you need to access the same name defined in more than one path. With import, the path makes the references unique, but from allows only one version of any given name.
from mypkg import spam is an absolute import—the search for mypkg skips the package directory and the module is located in an absolute directory in sys.path. A statement from . import spam, on the other hand, is a relative import—spam is looked up relative to the package in which this statement is contained before sys.path is searched.