checking shared libraries for non default loaders

ldd is a good simple way to check for shared libraries a given executable is or will be using. However it does not always work as expected. For example, see the following shell snippet that demonstrates how it "fails" to found the libreadline "dependency" into the python binary

I've tried many other distributions, but I'm copying from Tikanga

$ lsb_release -a
LSB Version:    :core-4.0-amd64:core-4.0-ia32:core-4.0-noarch:graphics-4.0-amd64:graphics-4.0-ia32:graphics-4.0-noarch:printing-4.0-amd64:printing-4.0-ia32:printing-4.0-noarch
Distributor ID: RedHatEnterpriseServer
Description:    Red Hat Enterprise Linux Server release 5.6 (Tikanga)
Release:        5.6
Codename:       Tikanga

See what ldd does on the default installed python (from official repositories).

$ which python
/usr/bin/python
$ ldd `which python`
    libpython2.4.so.1.0 => /usr/lib64/libpython2.4.so.1.0 (0x00000030e6200000)
    libpthread.so.0 => /lib64/libpthread.so.0 (0x00000030e0e00000)
    libdl.so.2 => /lib64/libdl.so.2 (0x00000030e0a00000)
    libutil.so.1 => /lib64/libutil.so.1 (0x00000030ee800000)
    libm.so.6 => /lib64/libm.so.6 (0x00000030e0600000)
    libc.so.6 => /lib64/libc.so.6 (0x00000030e0200000)
    /lib64/ld-linux-x86-64.so.2 (0x00000030dfe00000)
$ ldd `which python` | grep readline
$

Nothing found about readline. Now I do know from interactive usage that this binary does have realine functionality, so let't try to see where it's coming from.

$ python &
[1] 21003
$ Python 2.4.3 (#1, Dec 10 2010, 17:24:35) 
[GCC 4.1.2 20080704 (Red Hat 4.1.2-50)] on linux2
Type "help", "copyright", "credits" or "license" for more information.

[1]+  Stopped                 python

Started an interactive python session in background (pid 21003)

$ lsof -p 21003
COMMAND   PID    USER   FD   TYPE DEVICE     SIZE    NOD