Remove instances with a non-default opsz location from a variable font, as a workaround to a current Safari bug (https://bugs.webkit.org/show_bug.cgi?id=247987)

limit-fvar-to-default-opsz-instances.py

"""
    Remove instances with a non-default opsz location from a variable font, 
    in order to avoid Safari v16 bug (https://bugs.webkit.org/show_bug.cgi?id=247987)

    NOTE: seems to trigger another Safari issue for wght + opsz fonts, so I’m currently only
    using this on an opsz-only font. Do your own testing!

    Assumes:
    - exactly one font path is being fed in
    - it’s a variable font
    - the font includes an opsz axis
    - you wish to save the changes directly in place

    Tested on AT Kyrios (a font with only an opsz axis) and AT Name Sans (with wght plus opsz axes).

    Requirements:
    - fonttools
    - brotli (if you want to feed in a woff2)

    Usage:
    python3 limit-fvar-to-default-opsz-instances.py YOUR_FONT_PATH_HERE.woff2

    Helpful reference code found in the FontTools Instancer, written by Cosimo Lupo, found at:
    https://github.com/fonttools/fonttools/blob/ded595f03c9718082921c57cb8b4ff161df48a4f/Lib/fontTools/varLib/instancer/__init__.py#L963-L972
"""

import sys
from fontTools.ttLib import TTFont

# pass in font path from command line
filepath = sys.argv[1]

# open font in memory with TTFont
font = TTFont(filepath)

# determine default opsz value
for axis in font["fvar"].axes:
    if axis.axisTag == "opsz":
        defaultOpsz = axis.defaultValue

# make blank list for adding instances we want to keep
newInstances = []

# find instances with default opsz, add them to the list
for instance in font["fvar"].instances:
    if instance.coordinates["opsz"] == defaultOpsz:
        newInstances.append(instance)

# set the font’s fvar instances to our new, simplified list
font["fvar"].instances = newInstances

# save font to the same path as before
font.save(filepath)