storage.py

import os
import os.path
import json
import time
import copy

class Storage:
 ''' Storage simulation for both server and client. '''

 def __init__(self, filename):
 self.filename = filename
 if os.path.exists(filename):
 with open(filename, 'r') as f:
 self.objects = json.loads(f.read())
 else:
 self.objects = { }

 def next_key(self):
 keys = self.objects.keys()
 max_key = (keys and max(keys)) or 0
 return max_key + 1

 def create(self, obj):
 ''' key, ts might be originated from server. '''
 key = self.next_key()
 obj['key'] = key
 self.objects[key] = obj
 self.store_locally()
 return key

 def read(self, **kwargs):
 ''' not responsible for futher operations on the object. '''
 if 'key' in kwargs:
 return copy.deepcopy(self.objects[kwargs['key']])
 for obj in self.objects.values():
 matches = True
 for k, v in kwargs.items():
 if k not in obj or obj[k]!=v:
 matches = False
 break
 if matches:
 return copy.deepcopy(obj)
 raise KeyError

 def update(self, key, obj):
 obj['key'] = key
 self.objects[key] = obj
 self.store_locally()

 def store_locally(self):
 with open(self.filename, 'w') as f:
 f.write(json.dumps(self.objects, indent=4))

 def read_all(self):
 return copy.deepcopy(self.objects.values())