Given a list, determine if the list is monotonic

monotonic.py

# 1, 2, 3, 4, 5 -> True
# 64, 32, 16, 8 -> True
# 1, 1, 2, 3, 5 -> True
# 10, 20, 15 -> False
# 0, 0, 0, 0 -> True
# 1 -> True
# -> True

def is_monotonic(nums):
    is_ascending = None

    # Single item list is monotonic by definition   
    if len(nums) == 1:
        return True
    
    # Let's compare each pair of numbers to check and ensure uni-directionality
    for i in xrange(len(nums)-1):
        if nums[i] > nums[i+1]:
            if is_ascending is None:
                is_ascending = False
            if is_ascending is True:
                return False
        if nums[i] < nums[i+1]:
            if is_ascending is None:
                is_ascending = True
            if is_ascending is False:
                return False
            
    return True

assert is_monotonic([1, 2, 3, 4, 5])
assert is_monotonic([64, 32, 16, 8])
assert is_monotonic([1, 1, 2, 3, 5])
assert not is_monotonic([10, 20, 15])
assert is_monotonic([0, 0, 0, 0])
assert is_monotonic([1])
assert is_monotonic([])