def lookupKerningValue(
    pair, kerning, groups, fallback=0, glyphToFirstGroup=None, glyphToSecondGroup=None
):
    """Retrieve the kerning value (if any) between a pair of elements.

    The elments can be either individual glyphs (by name) or kerning
    groups (by name), or any combination of the two.

    Args:
      pair:
          A tuple, in logical order (first, second) with respect
          to the reading direction, to query the font for kerning
          information on. Each element in the tuple can be either
          a glyph name or a kerning group name.
      kerning:
          A dictionary of kerning pairs.
      groups:
          A set of kerning groups.
      fallback:
          The fallback value to return if no kern is found between
          the elements in ``pair``. Defaults to 0.
      glyphToFirstGroup:
          A dictionary mapping glyph names to the first-glyph kerning
          groups to which they belong. Defaults to ``None``.
      glyphToSecondGroup:
          A dictionary mapping glyph names to the second-glyph kerning
          groups to which they belong. Defaults to ``None``.

    Returns:
      The kerning value between the element pair. If no kerning for
      the pair is found, the fallback value is returned.

    Note: This function expects the ``kerning`` argument to be a flat
    dictionary of kerning pairs, not the nested structure used in a
    kerning.plist file.

    Examples::

      >>> groups = {
      ...     "public.kern1.O" : ["O", "D", "Q"],
      ...     "public.kern2.E" : ["E", "F"]
      ... }
      >>> kerning = {
      ...     ("public.kern1.O", "public.kern2.E") : -100,
      ...     ("public.kern1.O", "F") : -200,
      ...     ("D", "F") : -300
      ... }
      >>> lookupKerningValue(("D", "F"), kerning, groups)
      -300
      >>> lookupKerningValue(("O", "F"), kerning, groups)
      -200
      >>> lookupKerningValue(("O", "E"), kerning, groups)
      -100
      >>> lookupKerningValue(("O", "O"), kerning, groups)
      0
      >>> lookupKerningValue(("E", "E"), kerning, groups)
      0
      >>> lookupKerningValue(("E", "O"), kerning, groups)
      0
      >>> lookupKerningValue(("X", "X"), kerning, groups)
      0
      >>> lookupKerningValue(("public.kern1.O", "public.kern2.E"),
      ...     kerning, groups)
      -100
      >>> lookupKerningValue(("public.kern1.O", "F"), kerning, groups)
      -200
      >>> lookupKerningValue(("O", "public.kern2.E"), kerning, groups)
      -100
      >>> lookupKerningValue(("public.kern1.X", "public.kern2.X"), kerning, groups)
      0
    """
    # quickly check to see if the pair is in the kerning dictionary
    if pair in kerning:
        return kerning[pair]
    # create glyph to group mapping
    if glyphToFirstGroup is not None:
        assert glyphToSecondGroup is not None
    if glyphToSecondGroup is not None:
        assert glyphToFirstGroup is not None
    if glyphToFirstGroup is None:
        glyphToFirstGroup = {}
        glyphToSecondGroup = {}
        for group, groupMembers in groups.items():
            if group.startswith("public.kern1."):
                for glyph in groupMembers:
                    glyphToFirstGroup[glyph] = group
            elif group.startswith("public.kern2."):
                for glyph in groupMembers:
                    glyphToSecondGroup[glyph] = group
    # get group names and make sure first and second are glyph names
    first, second = pair
    firstGroup = secondGroup = None
    if first.startswith("public.kern1."):
        firstGroup = first
        first = None
    else:
        firstGroup = glyphToFirstGroup.get(first)
    if second.startswith("public.kern2."):
        secondGroup = second
        second = None
    else:
        secondGroup = glyphToSecondGroup.get(second)
    # make an ordered list of pairs to look up
    pairs = [
        (first, second),
        (first, secondGroup),
        (firstGroup, second),
        (firstGroup, secondGroup),
    ]
    # look up the pairs and return any matches
    for pair in pairs:
        if pair in kerning:
            return kerning[pair]
    # use the fallback value
    return fallback


if __name__ == "__main__":
    import doctest

    doctest.testmod()
