There is an example to get user by USER_ID on the keystone-api documentation.(https://keystoneclient.readthedocs.org/en/latest/api.html) But usually we want to get user by USER_NAME. Here's the way to do that.

keystone_user_get.py

#!/usr/bin/env python
# vim: tabstop=8 expandtab shiftwidth=4 softtabstop=4

from keystoneclient.v2_0 import client
from keystoneclient import utils

keystone = client.Client(username='admin',
                         password='admin_password',
                         tenant_name='admin_tenant',
                         cacert='/path/to/cacert.pem',
                         auth_url='https://keystone.domain.org:35357/v2.0/')

# Get user by USER_NAME instead of USER_ID
user = utils.find_resource(keystone.users, USER_NAME)

# Referred links
# https://keystoneclient.readthedocs.org/en/latest/api.html
# http://stackoverflow.com/questions/15525297/openstack-keystoneclient-get-user-by-name