Setting LD_LIBRARY_PATH from inside Python

Question

Is there a way to set specify during runtime where Python looks for shared libraries?

I have fontforge.so located in fontforge_bin and tried the following

os.environ['LD_LIBRARY_PATH']='fontforge_bin'
sys.path.append('fontforge_bin')
import fontforge

and get

ImportError: fontforge_bin/fontforge.so: cannot open shared object file: No such file or directory

Doing ldd on fontforge_bin/fontforge.so gives the following

linux-vdso.so.1 =>  (0x00007fff2050c000)
libpthread.so.0 => /lib/libpthread.so.0 (0x00007f10ffdef000)
libc.so.6 => /lib/libc.so.6 (0x00007f10ffa6c000)
/lib64/ld-linux-x86-64.so.2 (0x00007f110022d000)

Answer 1

Your script can check for the existence/properness of the environment variable before you import your module, then set it in os.environ if it is missing, and then call os.execv() to restart the python interpreter using the same command line arguments but an updated set of environment variables.

This is only advisable before any other imports (other than os and sys), because of potential module-import side-effects, like opened file descriptors or sockets, which may be challenging to close cleanly.

This code sets LD_LIBRARY_PATH and ORACLE_HOME:

#!/usr/bin/python
import os, sys
if 'LD_LIBRARY_PATH' not in os.environ:
    os.environ['LD_LIBRARY_PATH'] = '/usr/lib/oracle/XX.Y/client64/lib'
    os.environ['ORACLE_HOME'] = '/usr/lib/oracle/XX.Y/client64'
    try:
        os.execv(sys.argv[0], sys.argv)
    except Exception, exc:
        print 'Failed re-exec:', exc
        sys.exit(1)
#
# import yourmodule
print 'Success:', os.environ['LD_LIBRARY_PATH']
# your program goes here

It's probably cleaner to set that environment variable as part of the starting environment (in the parent process or systemd/etc job file).

Answer 2

...well sort of you could load all libraries from some folder of your choosing via ctypes and thus make them available for you regardless of the LD_LIBRARY_PATH.

from ctypes import *
lib1 = cdll.LoadLibrary('/home/username/lib/some_library.so')

or iterate through the files in that dir... you get the idea, once it is loaded it is there for you [if the dependencies are also out of the default path you should load them too...].

Answer 3

LD_LIBRARY_PATH sets the dynamic linker path; that generally can't be changed at runtime, since it's usually cached by the dynamic linker.

That's not where Python looks for imports, though, including module imports. Changing sys.path is correct.

# ls foo/
_csv.so
# python
Python 2.6.6 (r266:84292, Dec 26 2010, 22:31:48)
>>> import sys
>>> sys.path.insert(0, "foo")
>>> import _csv
>>> _csv.__file__
'foo/_csv.so'

(By the way, you may want to ldd the library to see if you have any odd import paths in the library. "ImportError: fontforge_bin/fontforge.so" looks strange.)