In [26]: mlist = nn.ModuleList([nn.ReLU(), nn.Conv2d(10, 10, 3, 1)])

In [27]: mlist
Out[27]: 
ModuleList(
  (0): ReLU()
  (1): Conv2d(10, 10, kernel_size=(3, 3), stride=(1, 1))
)

In [28]: mlist[-1] = nn.ReLU()

In [29]: mlist
Out[29]: 
ModuleList(
  (0): ReLU()
  (1): Conv2d(10, 10, kernel_size=(3, 3), stride=(1, 1))
  (-1): ReLU()
)

In [30]: mlist[-1]
---------------------------------------------------------------------------
KeyError                                  Traceback (most recent call last)
<ipython-input-30-229d1b6823a0> in <module>()
----> 1 mlist[-1]

~/anaconda3/lib/python3.6/site-packages/torch/nn/modules/container.py in __getitem__(self, idx)
    134             return ModuleList(list(self._modules.values())[idx])
    135         else:
--> 136             return self._modules[self._get_abs_string_index(idx)]
    137 
    138     def __setitem__(self, idx, module):

KeyError: '2'

    def __setitem__(self, idx, module):
        idx = self._get_abs_string_index(idx)
        return setattr(self, str(idx), module)

In [31]: class NewModuleList(nn.ModuleList):
    ...:     def __setitem__(self, idx, module):
    ...:         idx = self._get_abs_string_index(idx)
    ...:         return setattr(self, str(idx), module)
    ...:     

In [32]: mlist = NewModuleList([nn.ReLU(), nn.Conv2d(10, 10, 2, 1)])

In [33]: mlist[-1] = nn.ReLU()

In [34]: mlist
Out[34]: 
NewModuleList(
  (0): ReLU()
  (1): ReLU()
)