from typing import Sequence


def almostEqual(x, y):
    return abs(x - y) < 10**-9


def median(L: Sequence[float]) -> float | None:
    """Return the median of a sequence of numbers."""
    L = list(L)
    if not len(L):
        return None
    L.sort()
    mid = len(L) // 2
    if len(L) % 2 == 1:
        return L[mid]
    else:
        return (L[mid - 1] + L[mid]) / 2


print("Testing median()...", end="")
assert median([42]) == 42
assert almostEqual(median([1, 2]), 1.5)
assert median([-1.7, 3, -2, 4.2, 2]) == 2
assert median([2, 3, 2, 4, 2]) == 2
assert almostEqual(median([2, 3, 2, 4, 2, 3]), 2.5)
assert median([]) == None

# Verify function is non-mutating
L = [1, 3, 2]
assert median(L) == 2
assert L == [1, 3, 2]

print("Passed!")