import sys as _sys

_seen = {}
_here = _sys.modules[__name__]
for _name in dir():
    if _name.startswith('_'):
        continue
    _val = getattr(_here, _name)
    if _val in _seen:
        raise ValueError("Duplicate value '%s' for names '%s' and '%s'." %
                         (_val, _seen[_val], _name))
    _seen[_val] = _name