def maxlevel(lst):
    """Return maximum nesting depth"""
    maxlev = 0
    def f(lst, level):
        nonlocal maxlev
        if isinstance(lst, list):
            level += 1
            maxlev = max(level, maxlev)
            for item in lst:
                f(item, level)
    f(lst, 0)
    return maxlev