def normalizeInterpolationFactor(value):
    """
    Normalizes interpolation factor.

    * **value** must be an :ref:`type-int-float`, ``tuple`` or ``list``.
    * If **value** is a ``tuple`` or ``list``, it must have exactly two items.
      These items must be instances of :ref:`type-int-float`.
    * Returned value is a ``tuple`` of two ``float``.
    """
    if not isinstance(value, (int, float, list, tuple)):
        raise TypeError("Interpolation factor must be an int, float, or tuple "
                        "instances, not %s." % type(value).__name__)
    if isinstance(value, (int, float)):
        value = (float(value), float(value))
    else:
        if not len(value) == 2:
            raise ValueError("Interpolation factor tuple must contain two "
                             "values, not %d." % len(value))
        for v in value:
            if not isinstance(v, (int, float)):
                raise TypeError("Interpolation factor tuple values must be an "
                                ":ref:`type-int-float`, not %s."
                                % type(value).__name__)
        value = tuple([float(v) for v in value])
    return value