Python dl_open() function (from dl module) calls dlopen() and check 
its result: if it's NULL, it's an error. This is correct if I read the 
man page. But with an invalid flag value (-1), dlopen() returns NULL 
but dlerror() also gives a NULL pointer. Example:

>>> import dl
>>> dl.open("/usr/lib/libm.so", -1)
Erreur de segmentation (core dumped)

Workaround: use a static error message if dlerror() returns NULL.

I wrote a patch for dl_open() but other functions (in ctypes module?) 
should also call dlerror().