Better python service

query.py

''' Responsible for querying remote services and validating the result. '''

from simplejson import JSONDecodeError
import requests

# Exceptions
class ValidationError(Exception): pass
class RequestError(Exception): pass

# Types for representing results from backend services.
# Data from the backend service will be passed to the constructor
# using dictionary unpacking (the ** operator). Any parameters
# specified in the constructor will be captured and any others
# will end up in the ignored dictionary, which will be ignored.
# This means that we only need to handle those parts of the data
# that we care about. Besides saving some keystrokes, it also makes
# our code less brittle if somebody wants to make changes to what
# fields the other service exposes.
class Bill(object):
    def __init__(self, amount, **ignored):
        self.amount = amount

class Merchant(object):
    def __init__(self, name, **ignored):
        self.name = name

# Fetching and validation.
def get_merchant(merchant_id):
    data = _query('http://merchant/merchant/' + merchant_id)
    try:
        return Merchant(**data)
    except:
        raise ValidationError('Could not validate Merchant')

def get_billing(merchant_id):
    data = _query('http://billing/merchant/' + merchant_id)
    try:
        return [Bill(**item) for item in data]
    except:
        raise ValidationError('Could not validate Bills')

# Helper for making the request and checking the status code.
def _query(url):
    res = requests.get(url)
    if res.status_code != 200:
        raise RequestError('Status code != 200')
    try:
        return res.json()
    except JSONDecodeError:
        raise RequestError('Could not decode JSON')

server.py

from bottle import route, run
from mako.template import Template
import query

template = Template('''
<dl>
  <dd>Name</dd>
  <dt>${merchant.name}</dt>

  <dd>Number of bills</dd>
  <dt>${merchant.number_of_bills}</dt>

  <dd>Total of bills</dd>
  <dt>${merchant.total_of_bills}</dt>
</div>
''')

class MerchantWithBilling(object):
    def __init__(self, merchant, bills):
        self.name = merchant.name
        self.number_of_bills = len(bills)
        self.total_of_bills = sum(x.amount for x in bills)

@route('/merchants/<merchant_id>')
def get_merchant(merchant_id):
    query = Query()
    merchant = query.get_merchant(merchant_id)
    billing = query.get_billing(merchant_id)
    aggregate = MerchantWithBilling(merchant, billing)
    return template.render(merchant=aggregate)

if __name__ == '__main__':
    run()