10.9 将文件夹加入到sys.path

问题¶

You have Python code that can’t be imported because it’s not located in a directory listed in sys.path. You would like to add new directories to Python’s path, but don’t want to hardwire it into your code.

解决方案¶

There are two common ways to get new directories added to sys.path. First, you can add them through the use of the PYTHONPATH environment variable. For example:

bash % env PYTHONPATH=/some/dir:/other/dir python3
Python 3.3.0 (default, Oct 4 2012, 10:17:33)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import sys
>>> sys.path
['', '/some/dir', '/other/dir', ...]
>>>

In a custom application, this environment variable could be set at program startup or through a shell script of some kind.

The second approach is to create a .pth file that lists the directories like this:

# myapplication.pth
/some/dir
/other/dir

This .pth file needs to be placed into one of Python’s site-packages directories, which are typically located at /usr/local/lib/python3.3/site-packages or ~/.local/lib/python3.3/sitepackages. On interpreter startup, the directories listed in the .pth file will be added to sys.path as long as they exist on the filesystem. Installation of a .pth file might require administrator access if it’s being added to the system-wide Python interpreter.

讨论¶

Faced with trouble locating files, you might be inclined to write code that manually adjusts the value of sys.path. For example:

import sys
sys.path.insert(0, '/some/dir')
sys.path.insert(0, '/other/dir')

Although this “works,” it is extremely fragile in practice and should be avoided if possible. Part of the problem with this approach is that it adds hardcoded directory names to your source. This can cause maintenance problems if your code ever gets moved around to a new location. It’s usually much better to configure the path elsewhere in a manner that can be adjusted without making source code edits.

You can sometimes work around the problem of hardcoded directories if you carefully construct an appropriate absolute path using module-level variables, such as __file__. For example:

import sys
from os.path import abspath, join, dirname
sys.path.insert(0, abspath(dirname('__file__'), 'src'))

This adds an src directory to the path where that directory is located in the same directory as the code that’s executing the insertion step.

The site-packages directories are the locations where third-party modules and packages normally get installed. If your code was installed in that manner, that’s where it would be placed. Although .pth files for configuring the path must appear in site-packages, they can refer to any directories on the system that you wish. Thus, you can elect to have your code in a completely different set of directories as long as those directories are included in a .pth file.