# Tensor Views¶

PyTorch allows a tensor to be a `View`

of an existing tensor. View tensor shares the same underlying data
with its base tensor. Supporting `View`

avoids explicit data copy, thus allows us to do fast and memory efficient
reshaping, slicing and element-wise operations.

For example, to get a view of an existing tensor `t`

, you can call `t.view(...)`

.

```
>>> t = torch.rand(4, 4)
>>> b = t.view(2, 8)
>>> t.storage().data_ptr() == b.storage().data_ptr() # `t` and `b` share the same underlying data.
True
# Modifying view tensor changes base tensor as well.
>>> b[0][0] = 3.14
>>> t[0][0]
tensor(3.14)
```

Since views share underlying data with its base tensor, if you edit the data in the view, it will be reflected in the base tensor as well.

Typically a PyTorch op returns a new tensor as output, e.g. `add()`

.
But in case of view ops, outputs are views of input tensors to avoid unncessary data copy.
No data movement occurs when creating a view, view tensor just changes the way
it interprets the same data. Taking a view of contiguous tensor could potentially produce a non-contiguous tensor.
Users should be pay additional attention as contiguity might have implicit performance impact.
`transpose()`

is a common example.

```
>>> base = torch.tensor([[0, 1],[2, 3]])
>>> base.is_contiguous()
True
>>> t = base.transpose(0, 1) # `t` is a view of `base`. No data movement happened here.
# View tensors might be non-contiguous.
>>> t.is_contiguous()
False
# To get a contiguous tensor, call `.contiguous()` to enforce
# copying data when `t` is not contiguous.
>>> c = t.contiguous()
```

For reference, here’s a full list of view ops in PyTorch:

Basic slicing and indexing op, e.g.

`tensor[0, 2:, 1:7:2]`

returns a view of base`tensor`

, see note below.`movedim()`

`view_as_real()`

`view_as_imag()`

`split_with_sizes()`

`indices()`

(sparse tensor only)`values()`

(sparse tensor only)

Note

When accessing the contents of a tensor via indexing, PyTorch follows Numpy behaviors that basic indexing returns views, while advanced indexing returns a copy. Assignment via either basic or advanced indexing is in-place. See more examples in Numpy indexing documentation.

It’s also worth mentioning a few ops with special behaviors:

`reshape()`

and`reshape_as()`

can return either a view or new tensor, user code shouldn’t rely on whether it’s view or not.`contiguous()`

returns**itself**if input tensor is already contiguous, otherwise it returns a new contiguous tensor by copying data.

For a more detailed walk-through of PyTorch internal implementation, please refer to ezyang’s blogpost about PyTorch Internals.