CODEWITHSUNDEEP
python
kiran kumar
kiran kumar

Reputation: 27

What does comma in Python mean?

I am trying to learn backward propagation in pytorch, where I saw this code:

def backward(ctx, grad_output):
        """
        In the backward pass we receive a Tensor containing the gradient of the loss
        with respect to the output, and we need to compute the gradient of the loss
        with respect to the input.
        """
        i, = ctx.saved_tensors
        grad_output = 2*i
        return grad_output

I cannot understand what i, is here.

Upvotes: 2

Views: 215

Answers (2)

Krateng
Krateng

Reputation: 451

Python can unpack values, e.g if we have a tuple

t = (2,3,'wat')

we could assign its values to variables like this

coolnumber, othernumber, word = t

which would lead to word being set to 'wat'.

If our tuple has length one, we need to distinguish between assigning the whole tuple to a variable

a = (1,)
# a is now (1,)

and unpacking that one value

a, = (1,)
# a is now 1

Upvotes: 5

Amadan
Amadan

Reputation: 198324

i, is a 1-tuple whose sole element is i, just like i, j is a 2-tuple (obviously, can't just write i because that's not a tuple, but just i). When on the right side of an assignment, such syntax is used to construct a tuple; when on the left side, the assignment will deconstruct a tuple (or another sequence) into components. See this example:

# construction
scalar = 5          # => 5
two_tuple = 6, 7    # => (6, 7)
one_tuple = 8,      # => (8)

# deconstruction
five = scalar
six, seven = two_tuple
eight, = one_tuple

So, if ctx.saved_tensor is a 1-tuple, your code will assign the content of ctx.saved_tensor to i.

(Note that you will often read that the tuple syntax includes parentheses — (6, 7) instead of 6, 7. This is half-correct; tuple syntax does not include the parentheses, but they are necessary in many contexts because the comma has a very low priority, or because you want to delimit the commas inside a tuple and those outside of it, like in a function parameter list, or within another sequence. That said, the extra parentheses do not hurt.)

Upvotes: 1

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