Returns the value of `key` if it exists, else `default`.

def get(self, key, default=None):
    """Returns the value of `key` if it exists, else `default`."""
    if key in self._hparam_types:
      # Ensure that default is compatible with the parameter type.
      if default is not None:
        param_type, is_param_list = self._hparam_types[key]
        type_str = 'list<%s>' % param_type if is_param_list else str(param_type)
        fail_msg = ("Hparam '%s' of type '%s' is incompatible with "
                    'default=%s' % (key, type_str, default))

        is_default_list = isinstance(default, list)
        if is_param_list != is_default_list:
          raise ValueError(fail_msg)

        try:
          if is_default_list:
            for value in default:
              _cast_to_type_if_compatible(key, param_type, value)
          else:
            _cast_to_type_if_compatible(key, param_type, default)
        except ValueError as e:
          raise ValueError('%s. %s' % (fail_msg, e))

      return getattr(self, key)

    return default