import datetime
result = [{'start_date': datetime.date(2019, 1, 20)}, {'start_date': datetime.date(2019, 1, 21)}, {'start_date': datetime.date(2019, 1, 22)}]
# Map function
res = map(lambda d: str(d['start_date']), result)
print(res)
# List-comprehension
res = [ str(d['start_date']) for d in result ]
print(res)