import json
from collections import defaultdict
from functools import wraps

VALID_HTTP_METHODS = ['GET', 'POST']


class User:
    def __init__(self, name):
        self.name = name
        self.owes = defaultdict(float)
        self.owed_by = defaultdict(float)
        self.balance = 0.0

    def lend(self, lender_name, amount):
        self.owed_by[lender_name] += amount
        if not self.owed_by[lender_name]:
            del self.owed_by[lender_name]

    def borrow(self, borrower_name, amount):
        self.owes[borrower_name] += amount
        if not self.owed_by[borrower_name]:
            del self.owed_by[borrower_name]

    def to_json(self):
        return json.dumps(self.__dict__)

class Registry:
    def __init__(self):
        self.methods = {k: {} for k in VALID_HTTP_METHODS }

    def register_url(self, url, http_method):

        def actual_decorator(f):

            self.methods[http_method][url] = f

            @wraps(f)
            def _impl(f_self, *f_args, **f_kwargs):
                return f(f_self, *f_args, **f_kwargs)
        return actual_decorator

class RestAPI:

    registry = Registry()

    def __init__(self, database=None):
        self._database = database if database else {}
        self._users = {}

    @registry.register_url('/users', 'GET')
    def users(self, payload=None):
        pass

    @registry.register_url('/add', 'POST')
    def add(self, payload=None):
        try:
            payload = json.loads(payload)
            user = User(payload['user'])
            self._users[user.name] = user
            return user.to_json()
        except:
            pass

    @registry.register_url('/iou', 'POST')
    def iou(self, payload=None):
        pass

    def get(self, url, payload=None):
        methods = self.registry.methods['GET']
        if url in methods.keys():
            return methods[url](self, payload)
        return ''
#        else:
#            raise Exception('Error 404')

    def post(self, url, payload=None):
        methods = self.registry.methods['POST']
        if url in methods.keys():
            return methods[url](self, payload)
        return ''
#        else:
#            raise Exception('Error 404')