Python sys.path,PYTHONPATH,PATH和LD_LIBRARY_PATH的关系

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本文详细解析了Python运行时如何查找包和模块，包括sys.path的构成与优先级，以及动态链接库查找路径LD_LIBRARY_PATH的作用。通过实例展示了Python在不同路径下查找模块和库的过程。

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简单来说，Python运行时的包会去sys.path找，而sys.path这个数组会从前到后找，优先级以此是：

文件本身的路径
系统环境变量PYTHONPATH的路径
系统环境变量PATH里指定的anaconda的安装路径

可以用以下脚本进行一下测试：

import os
import sys
print("os.environ['PYTHONPATH']={0}".format(os.environ['PYTHONPATH']))
print("os.environ['PATH']={0}".format(os.environ['PATH']))
print("sys.path={0}".format(sys.path))

-----------------------------------------------------------------------

LD_LIBRARY_PATH是用来寻找Python的加载binary文件的。sys.path is only searched for Python modules. For dynamic linked libraries, the paths searched must be in LD_LIBRARY_PATH. Check if your LD_LIBRARY_PATH includes /usr/local/lib, and if it doesn't, add it and try again.

An example:

I'm trying to import pycurl:

$ python -c "import pycurl"
Traceback (most recent call last):
File "<string>", line 1, in <module>
ImportError: libcurl.so.4: cannot open shared object file: No such file or directory

Now, libcurl.so.4 is in /usr/local/lib. As you can see, this is in sys.path:

$ python -c "import sys; print(sys.path)"
['', '/usr/local/lib/python2.5/site-packages/setuptools-0.6c9-py2.5.egg', 
'/usr/local/lib/python25.zip', '/usr/local/lib/python2.5', 
'/usr/local/lib/python2.5/plat-linux2', '/usr/local/lib/python2.5/lib-tk', 
'/usr/local/lib/python2.5/lib-dynload', 
'/usr/local/lib/python2.5/sitepackages', '/usr/local/lib', 
'/usr/local/lib/python2.5/site-packages']

Any help will be greatly appreciated.

------------------------------

Answer:

sys.path is only searched for Python modules. For dynamic linked libraries, the paths searched must be in LD_LIBRARY_PATH. Check if your LD_LIBRARY_PATH includes /usr/local/lib, and if it doesn't, add it and try again.

Some more information (source):

In Linux, the environment variable LD_LIBRARY_PATH is a colon-separated set of directories where libraries should be searched for first, before the standard set of directories; this is useful when debugging a new library or using a nonstandard library for special purposes. The environment variable LD_PRELOAD lists shared libraries with functions that override the standard set, just as /etc/ld.so.preload does. These are implemented by the loader /lib/ld-linux.so. I should note that, while LD_LIBRARY_PATH works on many Unix-like systems, it doesn't work on all; for example, this functionality is available on HP-UX but as the environment variable SHLIB_PATH, and on AIX this functionality is through the variable LIBPATH (with the same syntax, a colon-separated list).

Update: to set LD_LIBRARY_PATH, use one of the following, ideally in your ~/.bashrc or equivalent file:

export LD_LIBRARY_PATH=/usr/local/lib

export LD_LIBRARY_PATH=/usr/local/lib:$LD_LIBRARY_PATH

Use the first form if it's empty (equivalent to the empty string, or not present at all), and the second form if it isn't. Note the use of export.