On Linux gcc and ld are used to implement 'find_library' and 'dlopen' is used to implement 'CDLL'.  ld searches /usr/local/lib.  'dlopen' might not if the LD_LIBRARY_PATH isn't set up to do so.  For example:


[meadori@li589-207 cpython]$ ls /usr/local/lib
libfoo.so  libfoo.so.1  libfoo.so.1.0  linode
[meadori@li589-207 cpython]$ ./python 
Python 3.4.0a0 (default:d6213012d87b, Jul 23 2013, 01:00:24) 
[GCC 4.7.2 20120921 (Red Hat 4.7.2-2)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from ctypes import *
>>> import ctypes.util
>>> ctypes.util.find_library('foo')
'libfoo.so.1'
>>> CDLL(_)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/meadori/src/cpython/Lib/ctypes/__init__.py", line 351, in __init__
    self._handle = _dlopen(self._name, mode)OSError: libfoo.so.1: cannot open shared object file: No such file or directory
>>> quit()
[meadori@li589-207 cpython]$ echo $LD_LIBRARY_PATH

[meadori@li589-207 cpython]$ export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/local/lib
[meadori@li589-207 cpython]$ ./python 
Python 3.4.0a0 (default:d6213012d87b, Jul 23 2013, 01:00:24) 
[GCC 4.7.2 20120921 (Red Hat 4.7.2-2)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from ctypes import *
>>> import ctypes.util
>>> ctypes.util.find_library('foo')
'libfoo.so.1'
>>> CDLL(_)
<CDLL 'libfoo.so.1', handle 28e4b50 at 7f31e062b608>


I am OK with this behavior in ctypes since it matches the system behavior with 'gcc' and 'dlopen'.  So, I would fixup LD_LIBRARY_PATH or /etc/ld.so.conf if you want /usr/local/lib to be searched at load time.