code: def put(self, key, value):
          
          mem = sys.getsizeof(value)
          if self._mem + mem > self._maxmem:
              self.delete(self.last())
          LRUCache.put(self, key, (value, mem))
          self._mem += mem
docs: >>> c = MemSizeLRUCache(maxmem=24*4)
          >>> c.put(1, 1)
          >>> c.mem() # 24-bytes per integer
          24
          >>> c.put(2, 2)
          >>> c.put(3, 3)
          >>> c.put(4, 4)
          >>> c.get(1)
          1
          >>> c.mem()
          96
          >>> c.size()
          4
          >>> c.put(5, 5)
          >>> c.size()
          4
          >>> c.get(2)
          Traceback (most recent call last):
              ...
          KeyError: 2