What is this attribute syntax in Python? [duplicate]

Question

I am current following a tutorial in Pytorch and there is this expression:

grad_h[h < 0] = 0

How does this syntax work and what does it do?

before this q is shut, it is assigning all values in the grad_h array which are less than zero to zero — chris
– chris, Commented Aug 21, 2018 at 7:07
if grad_h is dict and h = 1 is int then: grad_h[h < 0] = 0 will result in grad_h = {False: 0} — dorintufar
– dorintufar, Commented Aug 21, 2018 at 7:07
This is almost certainly a duplicate of an existing question with a good answer; we just need to find it for you. — abarnert
– abarnert, Commented Aug 21, 2018 at 7:09
Meanwhile, here's the relevant section in the numpy user guide, which explains things pretty nicely. — abarnert
– abarnert, Commented Aug 21, 2018 at 7:12

Gabriel M · Accepted Answer · 2018-08-21 07:08:41Z

0

It means replace with zeros all the values in grad_h where its corresponding h is negative.

So it is implementing some kind of mask, to keep the gradient values only when h is negative

suppose that grad_h and h have the same shape.

grad_h.shape == h.shape

when you do h < 0 you obtain an array of booleans of the same shape that is set to True if h[i] < 0 for each i.

So then you apply this mask to do slicing on grad_h and finally you set all the sliced elements to zero

answered Aug 21, 2018 at 7:07

Gabriel M

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2 Comments

Can you explain how that syntax works? At a casual glance, I'd expect it to set the attribute True or False of grad_h to 0…

ok I'll edit to explain better

Sleepless · Accepted Answer · 2018-08-21 07:07:04Z

-2

It means that the variable grad_h is equal to 0 as long as h is less than 0.

answered Aug 21, 2018 at 7:07

Sleepless

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