input: Write a function to calculate the mean of a list of numbers. If the list is empty, return 0.
  assert calculate_mean([10, 20, 30])==20
  assert calculate_mean([0, 0, 0, 0, 0])==0
  assert calculate_mean([1])==1
  assert calculate_mean([-10, 10])==0
  assert calculate_mean([])==0
output: def calculate_mean(numbers):
      if not numbers:
          return 0
      return sum(numbers) / len(numbers)