What is the use of the yield keyword in Python? What does it do?

For example, I'm trying to understand this code1:

def _get_child_candidates(self, distance, min_dist, max_dist):
if self._leftchild and distance - max_dist < self._median:
yield self._leftchild
if self._rightchild and distance + max_dist >= self._median:
yield self._rightchild

And this is the caller:

result, candidates = [], [self]
while candidates:
node = candidates.pop()
distance = node._get_dist(obj)
if distance <= max_dist and distance >= min_dist:
result.extend(node._values)
candidates.extend(node._get_child_candidates(distance, min_dist, max_dist))
return result

What happens when the method _get_child_candidates is called? Is a list returned? A single element? Is it called again? When will subsequent calls stop?

python3 keywords code

Ram KasarlaDec 24, 2022, 02:20 AM

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Shortcut to understanding yield

When you see a function with yield statements, apply this easy trick to understand what will happen:

Insert a line result = [] at the start of the function.
Replace each yield expr with result.append(expr).
Insert a line return result at the bottom of the function.
Yay - no more yield statements! Read and figure out the code.
Compare function to the original definition.

This trick may give you an idea of the logic behind the function, but what actually happens with yield is significantly different than what happens in the list-based approach. In many cases, the yield approach will be a lot more memory efficient and faster too. In other cases, this trick will get you stuck in an infinite loop, even though the original function works just fine. Read on to learn more...
Don't confuse your Iterables, Iterators, and Generators

Revanth KDec 24, 2022, 02:21 AM