PyTorch set_grad_enabled(False) vs with no_grad():

Actually no, there no difference in the way used in the question. When you take a look at the source code of no_grad. You see that it is actually using torch.set_grad_enabled to archive this behaviour:

class no_grad(object):
    r"""Context-manager that disabled gradient calculation.

    Disabling gradient calculation is useful for inference, when you are sure
    that you will not call :meth:`Tensor.backward()`. It will reduce memory
    consumption for computations that would otherwise have `requires_grad=True`.
    In this mode, the result of every computation will have
    `requires_grad=False`, even when the inputs have `requires_grad=True`.

    Also functions as a decorator.


    Example::

        >>> x = torch.tensor([1], requires_grad=True)
        >>> with torch.no_grad():
        ...   y = x * 2
        >>> y.requires_grad
        False
        >>> @torch.no_grad()
        ... def doubler(x):
        ...     return x * 2
        >>> z = doubler(x)
        >>> z.requires_grad
        False
    """

    def __init__(self):
        self.prev = torch.is_grad_enabled()

    def __enter__(self):
        torch._C.set_grad_enabled(False)

    def __exit__(self, *args):
        torch.set_grad_enabled(self.prev)
        return False

    def __call__(self, func):
        @functools.wraps(func)
        def decorate_no_grad(*args, **kwargs):
            with self:
                return func(*args, **kwargs)
        return decorate_no_grad

However there is an additional functionality of torch.set_grad_enabled over torch.no_grad when used in a with-statement which lets you control to switch on or off gradient computation:

    >>> x = torch.tensor([1], requires_grad=True)
    >>> is_train = False
    >>> with torch.set_grad_enabled(is_train):
    ...   y = x * 2
    >>> y.requires_grad

https://pytorch.org/docs/stable/_modules/torch/autograd/grad_mode.html

---

Edit:

@TomHale Regarding your comment. I just made a short test with PyTorch 1.0 and it turned out that the gradient will be active:

import torch
w = torch.rand(5, requires_grad=True)
print('Grad Before:', w.grad)
torch.set_grad_enabled(False)
with torch.enable_grad():
    scalar = w.sum()
    scalar.backward()
    # Gradient tracking will be enabled here.
torch.set_grad_enabled(True)

print('Grad After:', w.grad)

Output:

Grad Before: None
Grad After: tensor([1., 1., 1., 1., 1.])

So gradients will be computed in this setting.

The other setting you posted in your answer also yields to the same result:

import torch
w = torch.rand(5, requires_grad=True)
print('Grad Before:', w.grad)
with torch.no_grad():
    with torch.enable_grad():
        # Gradient tracking IS enabled here.
        scalar = w.sum()
        scalar.backward()

print('Grad After:', w.grad)

Output:

Grad Before: None
Grad After: tensor([1., 1., 1., 1., 1.])

The torch.autograd.enable_grad documentation says:

Enables gradient calculation inside a no_grad context. This has no effect outside of no_grad.

Given this wording, the following is expected:

torch.set_grad_enabled(False)
with torch.enable_grad:
    # Gradient tracking will NOT be enabled here.
torch.set_grad_enabled(True)

vs:

with torch.no_grad():
    with torch.enable_grad:
        # Gradient tracking IS enabled here.

But as blue-phoenix shows, this is not the case.

I raised an issue here.