from typing import List, Optional, TypeVar


T = TypeVar("T")


def repeatingPattern(L: List[T]) -> Optional[List[T]]:
    """Returns the smallest repeating pattern of a list or None if no pattern exists."""
    if len(L) < 2:
        return None
    repeat_options = list(
        reversed([i for i in range(2, max(len(L) // 2 + 1, 3)) if len(L) % i == 0])
    )
    M = [L[0]]
    for repeat in repeat_options:
        M = L[: len(L) // repeat]
        if M * repeat == L:
            return M
        if len(M) > len(L):
            return None
    return None


print("Testing repeatingPattern()...", end="")
assert repeatingPattern([1, 2, 1, 2]) == [1, 2]
assert repeatingPattern([1, 2, 1, 2, 1, 2, 1, 2]) == [1, 2]
assert repeatingPattern([1, 2, 3, 1, 2, 3]) == [1, 2, 3]
assert repeatingPattern([1, 1]) == [1]
assert repeatingPattern([]) == None
assert repeatingPattern([42]) == None
assert repeatingPattern([1, 2, 3, 1, 2]) == None
assert repeatingPattern([121, 212, 12]) == None
L = [1, 2, 3, 4]
assert repeatingPattern(L * 20) == L

# Finally, verify that the function is non-mutating
L = [1, 2, 1, 2]
repeatingPattern(L)
assert L == [1, 2, 1, 2]
print("Passed!")