input: Write a python function to check if a number is an Armstrong number.
  assert is_armstrong_number(153) == True
  assert is_armstrong_number(9474) == True
  assert is_armstrong_number(123) == False
output: def is_armstrong_number(number):
      digits = [int(d) for d in str(number)]
      num_digits = len(digits)
      armstrong_sum = sum(d ** num_digits for d in digits)
      return armstrong_sum == number