How do I solve this narcistic formula

def isNarcissistic(x):
    """Returns whether or not a given number is Narcissistic.

    A positive integer is called a narcissistic number if it
    is equal to the sum of its own digits each raised to the
    power of the number of digits.

    Example: 153 is narcissistic because 1^3 + 5^3 + 3^3 = 1 + 125 + 27 = 153.
    Note that by this definition all single digit numbers are narcissistic.
    """
narcissistic_numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407]

for i in narcissistic_numbers:
    assert_true(isNarcissistic(i), str(i) + ' is narcissistic')

non_narcissistic_numbers = [10, 11, 12, 13, 152, 154, 369, 372, 406]

for i in non_narcissistic_numbers:
    assert_true(not(isNarcissistic(i)), str(i) + ' is not narcissistic')

#test existence of docstring
assert_true(len(isNarcissistic.__doc__) > 1, "there is no docstring for isNarcissistic")
print("Success!")

y= 0
for i in range(1, x):
y+=i
if y==x:
return True
else:return False
print(isNarcissistic(153))