Creating PyTorch Tensors - Best Options
Welcome back to this series on neural network programming with PyTorch. In this post, we will look closely at the differences between the primary ways of transforming data into PyTorch tensors.
By the end of this post, we’ll know the differences between the primary options as well as which options should be used and when. Without further ado, let’s get started.
PyTorch tensors as we have seen are instances of the
torch.Tensor PyTorch class. The difference between the abstract concept of a tensor and a PyTorch tensor is that PyTorch tensors give
us a concrete implementation that we can work with in code.
last post, we saw how to create tensors in PyTorch using data like Python lists, sequences and NumPy ndarrays. Given a
numpy.ndarray, we found that there are four ways to create a
Here is a quick recap:
> data = np.array([1,2,3]) > type(data) numpy.ndarray > o1 = torch.Tensor(data) > o2 = torch.tensor(data) > o3 = torch.as_tensor(data) > o4 = torch.from_numpy(data) > print(o1) > print(o2) > print(o3) > print(o4) tensor([1., 2., 3.]) tensor([1, 2, 3], dtype=torch.int32) tensor([1, 2, 3], dtype=torch.int32) tensor([1, 2, 3], dtype=torch.int32)
Our task in this post is to explore the difference between these options and to suggest a best option for our tensor creation needs.
Tensor creation operations: What's the difference?
Let’s get started and figure out what these differences are all about.
Notice how the first option
torch.Tensor() has an uppercase
T while the second option
torch.tensor() has a lowercase
t. What’s up with this difference?
The first option with the uppercase
T is the constructor of the
torch.Tensor class, and the second option is what we call a
factory function that constructs
torch.Tensor objects and returns them to the caller.
You can think of the
torch.tensor() function as a factory that builds tensors given some parameter inputs. Factory functions are a software design pattern for creating objects. If you want
to read more about it check
Okay. That’s the difference between the uppercase
T and the lower case
t, but which way is better between these two? The answer is that it’s fine to use either one. However, the factory function
torch.tensor() has better documentation and more configuration options, so it gets the winning spot at the moment.
dtype vs inferred
Alright, before we knock the
The difference is in the
dtype of each tensor. Let’s have a look:
> print(o1.dtype) > print(o2.dtype) > print(o3.dtype) > print(o4.dtype) torch.float32 torch.int32 torch.int32 torch.int32
The difference here arises in the fact that the
torch.Tensor() constructor uses the default
dtype when building the tensor. We can verify the default
> torch.get_default_dtype() torch.float32
To verify with code, we can do this:
> o1.dtype == torch.get_default_dtype() True
The other calls choose a dtype based on the incoming data. This is called
type inference. The
dtype is inferred based on the incoming data. Note that the
dtype can also be explicitly set for these calls by specifying the
> torch.tensor(data, dtype=torch.float32) > torch.as_tensor(data, dtype=torch.float32)
torch.Tensor(), we are unable to pass a
dtype to the constructor. This is an example of the
torch.Tensor() constructor lacking in configuration options. This
is one of the reasons to go with the
torch.tensor() factory function for creating our tensors.
Let’s look at the last hidden difference between these alternative creation methods.
Sharing memory for performance: copy vs share
The third difference is lurking behind the scenes or underneath the hood. To reveal the difference, we need to make a change to the original input data in the
numpy.ndarray after using the
ndarray to create our tensors.
Let’s do this and see what we get:
> print('old:', data) old: [1 2 3] > data = 0 > print('new:', data) new: [0 2 3] > print(o1) > print(o2) > print(o3) > print(o4) tensor([1., 2., 3.]) tensor([1, 2, 3], dtype=torch.int32) tensor([0, 2, 3], dtype=torch.int32) tensor([0, 2, 3], dtype=torch.int32)
Note that originally, we had
data=1, and also note that we only changed the data in the original
numpy.ndarray. Notice we didn't explicity make any changes to our tensors (
However, after setting
data=0, we can see some of our tensors have changes. The first two
o2 still have the original value of
1 for index
0, while the second two
o4 have the new value of
0 for index
This happens because
copy their input data while
their input data in memory with the original input object.
|Share Data||Copy Data|
This sharing just means that the actual data in memory exists in a single place. As a result, any changes that occur in the underlying data will be reflected in both objects, the
torch.Tensor and the
Sharing data is more efficient and uses less memory than copying data because the data is not written to two locations in memory.
If we have a
torch.Tensor and we want to convert it to a
numpy.ndarray, we do it like so:
> print(o3.numpy()) > print(o4.numpy()) [0 2 3] [0 2 3]
> print(type(o3.numpy())) > print(type(o4.numpy())) <class 'numpy.ndarray'> <class 'numpy.ndarray'>
This establishes that
torch.from_numpy() both share memory with their input data. However, which one should we use, and how are they different?
torch.from_numpy() function only accepts
numpy.ndarrays, while the
torch.as_tensor() function accepts a wide variety of
array-like objects including other PyTorch tensors. For this reason,
torch.as_tensor() is the winning choice in the memory sharing game.
Best options for creating tensors in PyTorch
Given all of these details, these two are the best options:
torch.tensor() call is the sort of go-to call, while
torch.as_tensor() should be employed when tuning our code for performance.
Some things to keep in mind about memory sharing (it works where it can):
numpy.ndarrayobjects are allocated on the CPU, the
as_tensor()function must copy the data from the CPU to the GPU when a GPU is being used.
The memory sharing of
as_tensor()doesn’t work with built-in Python data structures like lists.
as_tensor()call requires developer knowledge of the sharing feature. This is necessary so we don’t inadvertently make an unwanted change in the underlying data without realizing the change impacts multiple objects.
as_tensor()performance improvement will be greater if there are a lot of back and forth operations between
numpy.ndarrayobjects and tensor objects. However, if there is just a single load operation, there shouldn’t be much impact from a performance perspective.
At this point, we should now have a better understanding of the PyTorch
tensor creation options. We’ve learned about factory functions and we’ve seen how memory
sharing vs copying
can impact performance and program behavior. I’ll see you in the