/usr/share/pam-external/auithenticate.py:

#!/usr/bin/python

from sys import exit

print ")Username: "
username = raw_input()

if username != 'pamtest':
    print '[PAM_USER_UNKNOWN]'
    exit(1)

for i in range(0, 3):
    print ":Password: "
    password = raw_input()
    if password == 'boogaloo':
        print '[PAM_SUCCESS]'
        exit(0)
    print "!Password incorrect. Please try again."

print '!Authentication failed.'
print '[PAM_AUTH_ERR]'
exit(1)

ls -al /etc/pam-external:

drwxr-xr-x   2 root root  4096 May 16 13:30 .
drwxr-xr-x 145 root root 12288 May 16 13:30 ..
lrwxrwxrwx   1 root root    39 May 16 13:30 authenticate -> /usr/share/pam-external/authenticate.py

pamtester:

root@westeros:~# pamtester flarp pamtest authenticate
Password:
Password incorrect. Please try again.
Password:
pamtester: successfully authenticated


/var/log/auth.log:

May 17 14:12:59 westeros pamtester: pam_external(flarp:auth): Debug activated
May 17 14:12:59 westeros pamtester: pam_external(flarp:auth): <-- )Username:
May 17 14:12:59 westeros pamtester: pam_external(flarp:auth): --> pamtest
May 17 14:12:59 westeros pamtester: pam_external(flarp:auth): <-- :Password:
May 17 14:13:02 westeros pamtester: pam_external(flarp:auth): --> Sending password
May 17 14:13:02 westeros pamtester: pam_external(flarp:auth): <-- !Password incorrect. Please try again.
May 17 14:13:02 westeros pamtester: pam_external(flarp:auth): <-- :Password:
May 17 14:13:06 westeros pamtester: pam_external(flarp:auth): --> Sending password
May 17 14:13:06 westeros pamtester: pam_external(flarp:auth): <-- [PAM_SUCCESS]