Example of responding to a data request using a fixed template.

Template Engine

from collections import defaultdict
import copy
import json

simulated_release = {
    "artist":"The Beatles",
    "title":"Collection of Random Hits",
    "mbid":3,
    }

simulated_recordings = [
    {
        "artist":"The Beatles",
        "title":"Get Back",
        "length":20386,
        "mbid":1,
        "release":3
    },
    {
        "artist":"The Beatles",
        "title":"Hey Jude",
        "length":20385,
        "mbid":2,
        "release":3
    }
]

class ReleaseTemplate():
    def __init__(self, template):
        self.template = template
        self.saved_template = copy.deepcopy(self.template)
        self.data = {}
        self.keywords = {
            "recordings":self.recording,
            }

        self.requested = defaultdict(list)
        self.request_mbid = None

    def process_template(self,data = None):
        for k,v in self.template.iteritems():
            self.process_part(self.template,k)

    def process_part(self, parent, key):
        value = parent[key]
        if type(value) is dict:
            for k,v in value.iteritems():
                self.process_part(value,k)
        else:
            parent[key] = self.keywords[key]()

    def recording(self):
        if self.data is not None:
            self.recordings = [r['mbid'] for r in simulated_recordings if r['release'] == self.request_mbid]

            temp = RecordingTemplate(self.saved_template['release']['recordings'][0])
            temp.process_template()
            result = []
            for r in self.recordings:
                result.append(temp.get_data(r))

            return result
        else:
            return None
                
            
    def get_data(self, mbid):
        self.request_mbid = mbid
        for db, params in self.requested.iteritems():
            #Query db for params
            self.data[db] = [r for r in simulated_recordings if r['mbid'] == mbid][0]

        for k,v in self.template.iteritems():
            self.process_part(self.template,k)

        return self.template
            

class RecordingTemplate():
    def __init__(self, template):
        self.template = template
        self.saved_template = copy.deepcopy(self.template)
        self.data = {}
        self.keywords = {
            "title":self.title,
            "artist":self.artist,
            "length":self.length
            }

        self.requested = defaultdict(list)
        self.request_mbid = None

    def process_part(self, parent, key):
        value = parent[key]
        if type(value) is dict:
            for k,v in value.iteritems():
                self.process_part(value,k)
        else:
            parent[key] = self.keywords[key]()
            
    def get_data(self, mbid):
        self.request_mbid = mbid
        for db, params in self.requested.iteritems():
            #Query db for params
            self.data[db] = [r for r in simulated_recordings if r['mbid'] == mbid][0]

        for k,v in self.template.iteritems():
            self.process_part(self.template,k)

        return copy.deepcopy(self.template)

        

    def process_template(self,data = None):
        for k,v in self.template.iteritems():
            self.process_part(self.template,k)
            
    def title(self):
        if not self.data:
            self.requested['recording'].append('title')
            return None
        else:
            return self.data['recording']['title']

    def artist(self):
        if not self.data:
            self.requested['recording'].append('artist')
            return None
        else:
            return self.data['recording']['artist']

    def length(self):
        if not self.data:
            self.requested['recording'].append('length')
            return None
        else:
            return self.data['recording']['length']

template = {
        "release": {
        "recordings": 
            [{
                "artist" : None,
                "length" : None
            },]
        }
    }


my_template = ReleaseTemplate(template)

my_template.process_template()
print json.dumps(my_template.get_data(3), sort_keys=True, indent=4, separators=(',', ': '))

P.S. Try editing the template to add "title": None, between "artist": None, and "length": None to see why this is useful.