def string_to_num(s: str):
    """Converts a string to an int/float

    Returns ``None`` if it can't be converted

    >>> string_to_num('5')
    5
    >>> string_to_num('5.2')
    5.2
    >>> string_to_num(10)
    10
    >>> string_to_num(10.1)
    10.1
    >>> string_to_num('this is not a string') is None
    True
    """
    if isinstance(s, (int, float)):
        return s
    if s.isdigit():
        return int(s)
    try:
        return float(s)
    except ValueError:
        return None