Python descriptor object for representing a url parameter

param.py

class Param:
  '''Python descriptor for representing a query param within a url'''
  def __init__(self, default=None, name=None, required=False):
    self.default = str(default)
    self.name = name
    self.required = required
    self.values = {} # in a real impl you'd use a WeakRefDict

  def __set_name__(self, owner, name):
    if not self.name:
      self.name = name

  def __get__(self, obj, objtype=None):
    if obj is None:
      return self
    else:
      value = self.value(obj)
      if value =! self.default or self.required:
        return ParamProxy(self.name, value)
      else:
        return EmptyParam
  
  def value(self, obj):
    #can't do .get(obj) or default because empty strings are 'falsey'
    return self.values[obj] if obj in self.values else self.default

  def __set__(self, obj, value):
    self.values[obj] = str(value)


class ParamProxy:
  def __init__(self, name, value):
    self.name = name
    self.value = value
  
  def as_pair(self):
    return (self.name, self.value)
  
  def as_param(self):
    return self.__str__()
  
  def __str__(self):
    return self.name + '=' + urlencode(self.value)