expressions for plugin and granular deployment

"""Expression parser based on some templater
Pros:
- it is not a new language
- it has very strict api, so atleast we can try to guarantee its stability
- complex abstract logic can be hidden in simple python methods

Stements will be expressed in the form of:

api.cluster_status == 'operational'
api.role_in_deployment('ceph-osd')
api.role_in_cluster('zabbix-server')
api.cluster_status == 'new' and api.nodes_count() > 10
"""

import mock
import jinja2

class ExpressionApi(object):
    
    def __init__(self, cluster, nodes):
        self.cluster = cluster
        self.nodes = nodes
        
    def role_in_deployment(self, role):
        for node in self.nodes:
            if role in node.roles:
                return True
        return False
        
    def role_in_cluster(self, role):
        for node in self.cluser.nodes:
            if role in node.roles:
                return True
        return False
    
    def nodes_count(self):
        return len(self.nodes)
    
    @property
    def cluster_status(self):
        return self.cluster.status


cluster = mock.Mock()
cluster.status = 'operational'
nodes = [mock.Mock()]*8
API = ExpressionApi(cluster, nodes)
env = jinja2.Environment()
expr = env.compile_expression("api.cluster_status == 'operational' and api.nodes_count() < 9")
print expr(api=API)

api.cluster_status == 'operational' and api.role_in_deployment('compute')