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talking about to show how initialization files and paths come into play. The following three files are coded in a directory dir1 and its subdirectory dir2—comments give the path names of these files:

# dir1\__init__.py

print('dir1 init')

x = 1

# dir1\dir2\__init__.py

print('dir2 init')

y = 2

# dir1\dir2\mod.py

print('in mod.py')

z = 3

Here, dir1 will be either a subdirectory of the one we’re working in (i.e., the home directory), or a subdirectory of a directory that is listed on the module search path (technically, on sys.path). Either way, dir1’s container does not need an __init__.py file.

import statements run each directory’s initialization file the first time that directory is traversed, as Python descends the path; print statements are included here to trace their execution. As with module files, an already imported directory may be passed to reload to force reexecution of that single item. As shown here, reload accepts a dotted pathname to reload nested directories and files:

% python

>>> import dir1.dir2.mod # First imports run init files

dir1 init

dir2 init

in mod.py

>>>

>>> import dir1.dir2.mod # Later imports do not

>>>

>>> from imp import reload # Needed in 3.0

>>> reload(dir1)

dir1 init

>>>

>>> reload(dir1.dir2)

dir2 init

Once imported, the path in your import statement becomes a nested object path in your script. Here, mod is an object nested in the object dir2, which in turn is nested in the object dir1:

>>> dir1

>>> dir1.dir2

>>> dir1.dir2.mod

In fact, each directory name in the path becomes a variable assigned to a module object whose namespace is initialized by all the assignments in that directory’s __init__.py file. dir1.x refers to the variable x assigned in dir1\__init__.py, much as mod.z refers to the variable z assigned in mod.py:

>>> dir1.x

1

>>> dir1.dir2.y

2

>>> dir1.dir2.mod.z

3

from Versus import with Packages

import statements can be somewhat inconvenient to use with packages, because you may have to retype the paths frequently in your program. In the prior section’s example, for instance, you must retype and rerun the full path from dir1 each time you want to reach z. If you try to access dir2 or mod directly, you’ll get an error:

>>> dir2.mod

NameError: name 'dir2' is not defined

>>> mod.z

NameError: name 'mod' is not defined

It’s often more convenient, therefore, to use the from statement with packages to avoid retyping the paths at each access. Perhaps more importantly, if you ever restructure your directory tree, the from statement requires just one path update in your code, whereas imports may require many. The import as extension, discussed formally in the next chapter, can also help here by providing a shorter synonym for the full path:

% python

>>> from dir1.dir2 import mod # Code path here only

dir1 init

dir2 init

in mod.py

>>> mod.z # Don't repeat path

3

>>> from dir1.dir2.mod import z

>>> z

3

>>> import dir1.dir2.mod as mod # Use shorter name (see Chapter 24)

>>> mod.z

3

Why Use Package Imports?

If you’re new to Python, make sure that you’ve mastered simple modules before stepping up to packages, as they are a somewhat advanced feature. They do serve useful roles, though, especially in larger programs: they make imports more informative, serve as an organizational tool, simplify your module search path, and can resolve ambiguities.

First of all, because package imports give some directory information in program files, they both make it easier to locate your files and serve as an organizational tool. Without package paths, you must often resort to consulting the module search path to find files. Moreover, if you organize your files into subdirectories for functional areas, package imports make it more obvious what role a module plays, and so make your code more