gdementen

# 1) like others pointed out, you should use a defaultdict in this case

# 2) otherwise use setdefault
output = {}
for element in elements:
    output.setdefault(element["owner"], []).append(element["pet"])
    
# 3) if, for some reason, this does not work in your case, use a dict comprehension (if available for your version of Python, 2.7+ I think)
output = {e["owner"]: [] for e in elements}
for element in elements: