def _silent_idx(x, y):
    """
    Given a list x and an object y, returns the index of y in x. Otherwise
    return a None without raising any exceptions.
    >>> _silent_idx([1,2,3,4], 2)
    1
    >>> _silent_idx([1,2,3,4], 5)
    """
    if y in x:
        return x.index(y) 
    else:
        return None