When can "s != s" occur in a method?

Question

I found a code snippet, which is a custom metric for tensorboard (pytorch training)

def specificity(output, target, t=0.5):
    
    tp, tn, fp, fn = tp_tn_fp_fn(output, target, t)

    if fp == 0:
        return 1
    s = tn / (tn + fp)

    if s != s:
        s = 1

    return s

def tp_tn_fp_fn(output, target, t):
    with torch.no_grad():
        preds = output > t  # torch.argmax(output, dim=1)
        preds = preds.long()
        num_true_neg = torch.sum((preds == target) & (target == 0), dtype=torch.float).item()
        num_true_pos = torch.sum((preds == target) & (target == 1), dtype=torch.float).item()
        num_false_pos = torch.sum((preds != target) & (target == 1), dtype=torch.float).item()
        num_false_neg = torch.sum((preds != target) & (target == 0), dtype=torch.float).item()

    return num_true_pos, num_true_neg, num_false_pos, num_false_neg

In terms of the calculation itself it is easy enough to understand.

What I don't understand is s != s. What does that check do, how can the two s even be different?

Answer 1

Since it's ML-related, I'll assume the data are all numbers. The only number where s != s is true is the special not-a-number value nan. Any comparison with nan is always false, so from that follows that nan is not equal to itself.