第520页 | Learning Python | 阅读 ‧ 电子书库

Package Import Example

Let’s actually code the example we’ve been 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
<module 'dir1' from 'dir1\__init__.pyc'>
>>>
>>> reload(dir1.dir2)
dir2 init
<module 'dir1.dir2' from 'dir1\dir2\__init__.pyc'>

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
<module 'dir1' from 'dir1\__init__.pyc'>
>>> dir1.dir2
<module 'dir1.dir2' from 'dir1\dir2\__init__.pyc'>
>>> dir1.dir2.mod
<module 'dir1.dir2.mod' from 'dir1\dir2\mod.pyc'>

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