[GitHub] szha commented on issue #13998: expand_dims() makes copy instead of simply reshaping

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szha commented on issue #13998: expand_dims() makes copy instead of simply reshaping
URL: https://github.com/apache/incubator-mxnet/issues/13998#issuecomment-459617256
 
 
   @stephenrawls you're right that `expand_dims` and `squeeze` for NDArray should behave consistently with `reshape`. Currently, `reshape` is done throught [a special C API](https://github.com/apache/incubator-mxnet/blob/master/python/mxnet/ndarray/ndarray.py#L1065-L1069) which returns a modified view of the data, while the `expand_dims` and `squeeze` are making a copy. For symbol (and thus the hybridized version), since in-place identity is possible it should not matter.
   ```
   In [1]: import mxnet as mx
   
   In [2]: a = mx.nd.arange(6)
   
   In [3]: b = a.reshape(2, 3)
   
   In [4]: c = a.expand_dims(1)
   
   In [5]: a[1] = 7
   
   In [6]: b
   Out[6]:
   
   [[0. 7. 2.]
    [3. 4. 5.]]
   <NDArray 2x3 @cpu(0)>
   
   In [7]: c
   Out[7]:
   
   [[0.]
    [1.]
    [2.]
    [3.]
    [4.]
    [5.]]
   <NDArray 6x1 @cpu(0)>
   ```
   
   It's straightforward to let `expand_dims`/`squeeze` return a view too (e.g. by simply calling `.shape` and `reshape` in `expand_dims` and `squeeze` in python/mxnet/ndarray/ndarray.py).

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