freetypy.TT_HoriHeader.descender

TT_HoriHeader.descender

The font’s descender, i.e., the distance from the baseline to the bottom-most of all glyph points found in the font. It is negative.

This value is invalid in many fonts, as it is usually set by the font designer, and often reflects only a portion of the glyphs found in the font (maybe ASCII).

You should use the Face.get_os2_table().s_typo_descender if you want the correct one.