def normalize_bound(bound):
    """
    Replace ``None`` with + or - inf in bound tuples.

    Examples
    --------
    >>> normalize_bound((2.6, 7.2))
    (2.6, 7.2)

    >>> normalize_bound((None, 7.2))
    (-inf, 7.2)

    >>> normalize_bound((2.6, None))
    (2.6, inf)

    >>> normalize_bound((None, None))
    (-inf, inf)

    This operation is idempotent:

    >>> normalize_bound((-float("inf"), float("inf")))
    (-inf, inf)
    """
    min_, max_ = bound
    if min_ is None:
        min_ = -float('inf')
    if max_ is None:
        max_ = float('inf')
    return min_, max_
