Set up output first

petowner_comprehension1.py

# 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:
    output[element["owner"]].append(element["pet"])

# 4) if not available, use dict on a generator of tuples: this is a lot more readable/idiomatic IMO.
output = dict((e["owner"]: []) for e in elements)
for element in elements:
    output[element["owner"]].append(element["pet"])
    
# 5) in last resort, use a dict on list comprenhension. But in that case, please use a list of *tuples*, this is still more readable/idiomatic than a list of lists IMO.
output = dict([(e["owner"]: []) for e in elements])
for element in elements:
    output[element["owner"]].append(element["pet"])