Assume this script runs on a Epic Workflow.
Requirement:
Search for the “Story Points” custom field on your instance.
sum = 0
for issue in api.search_issues(f'parent = {issue.key}'):
sum += issue.fields.<your_story_points_customfield_id>
issue.set('<your_story_points_customfield_id>', sum)
To test it, get an Epic from the target project, this Epic should have child issues that contain story points != null.
# For testing purposes in the Console, do:
issue = Issue('Your Epic - Issue Key')
sum = 0
for issue in api.search_issues(f'parent = {issue.key}'):
sum += issue.fields.<your_story_points_customfield_id>
issue.set('<your_story_points_customfield_id>', sum)
After your testing is done, remove or comment the issue = Issue('Your Epic - Issue Key') part and plug it into a listener/postfunction.