def __len__(self):
        return len(self._headers)
Install trait callbacks on deep-copied VariableTree.
dataset/ETHPy150Open kleientertainment/ds_mod_tools/pkg/win32/Python27/Lib/email/message.py/Message.__len__--crossover with--dataset/ETHPy150Open OpenMDAO/OpenMDAO-Framework/openmdao.main/src/openmdao/main/vartree.py/VariableTree.install_callbacks