def collatz_sequence(n):

"""Collatz conjecture: start with any positive integer n.Next termis obtained from the previous term as follows:

if the previous term is even, the next term is one half the previous term.

If the previous term is odd, the next term is 3 times the previous term plus 1.

The conjecture states the sequence will always reach 1 regaardess of starting n."""

sequence = [n]

while n != 1:

if n % 2 == 0:# even

n //= 2

else:

n = 3*n +1

sequence.append(n)

return sequence

answer = max([(len(collatz_sequence(i)), i) for i in range(1,1000000)])

print("Longest Collatz sequence under one million is %d with length %d" % (answer[1],answer[0]))