Use setdefault to force an empty list in case you need to iterate over existing values / keys.

setdefault.py

my_dict = {}

my_dict['one'] = [1, 2, 3]
my_dict.setdefault('one', []).append(4)
my_dict.setdefault('two', []).append(4)
my_dict.setdefault('three', [])

# In: my_dict
# Out: {'one': [1, 2, 3, 4], 'three': [], 'two': [4]}