input: Write a python function to check if a number is an Armstrong number.
  assert is_armstrong_number(153) == True
  assert is_armstrong_number(123) == False
  assert is_armstrong_number(9474) == True
  assert is_armstrong_number(9475) == False
output: def is_armstrong_number(n):
      digits = list(map(int, str(n)))
      power = len(digits)
      return sum(digit ** power for digit in digits) == n