Starter script to set up proper code ligatures in a GlyphsApp font source

LICENSE.txt

MIT License

Copyright 2025 ArrowType / Stephen Nixon

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prep-code-ligature-glyphs-and-fea-code.py

#MenuTitle: Generate calt feature for code ligatures
__doc__="""
Generates a 'calt' feature for code ligatures in a font.
- Sets up a "filler" glyph that is used to ensure the ligatures have a consistent width.
- The filler glyph is based on the 'space' glyph, but has a width of the monospace unit.
- Generates ligature feature code, and copies it to the clipboard.
- The ligature feature code is based on the glyph names, which should follow a specific naming convention.
- The naming convention is: <first_part>_<middle_part>_<last_part>.code
- The generated feature code will ignore the ligature sequence in both directions, and then substitute the ligature with the filler glyph.
"""

font = Glyphs.font

filler = "codeligfiller"

feaCode = """"""

code_ligatures = []

for glyph in font.glyphs:
    if ".code" in glyph.name:
        code_ligatures.append(glyph.name)

# sort code_ligatures by number of underscores in the name
code_ligatures.sort(key=lambda x: x.count("_"), reverse=True)


# get basic unit width for the monospace font, from the "n" glyph
unit_width = [layer.width for layer in font["n"].layers if layer.width > 0][0]

# make new glyph for the code ligature filler
if filler not in font.glyphs:
    newGlyph = font.glyphs['space'].copy()
    newGlyph.name = filler
    font.glyphs.append(newGlyph)
else:
    for layer in font.glyphs[filler].layers:
        layer.width = unit_width

# make sure code ligature glyphs keep right margin, but become one unit in width
for glyphname in code_ligatures:
    glyph = font.glyphs[glyphname]

    # set the width of the code ligature glyph to the unit width, with overflow to the left
    for layer in glyph.layers:
        for comp in layer.components:
            comp.automaticAlignment = False

        right_margin = layer.RSB

        glyph_left_overflow = layer.width - unit_width
        layer.LSB = layer.LSB - glyph_left_overflow
        layer.RSB = right_margin

    glyph.widthMetricsKey = "n"


# generate feature code for each ligature

for glyphname in code_ligatures:
    glyph = font.glyphs[glyphname]
    parts = glyph.name.replace(".code","").split("_")

    lookup_name = glyphname.replace('.code', '')
    lig_sequence = lookup_name.replace("_","' ") + "'"
    first_part = parts[0]
    last_part = parts[-1]

    feaCode += f"lookup {lookup_name} {{\n"

    feaCode += f"    ignore sub {first_part} {lig_sequence};\n"
    feaCode += f"    ignore sub {lig_sequence} {first_part};\n"

    for i in range(len(parts) -1, -1, -1):
        feaCode += "    sub " + (f"{filler} " * i) + " ".join([parts[i] + "'"] + parts[i + 1:])
        feaCode += f" by {glyphname};\n" if i == len(parts) -1 else f" by {filler};\n"


    feaCode += f"}} {lookup_name};\n\n"


# copy feaCode to clipboard
import AppKit
pb = AppKit.NSPasteboard.generalPasteboard()
pb.declareTypes_owner_([AppKit.NSStringPboardType], None)
pb.setString_forType_(feaCode, AppKit.NSStringPboardType)

print(feaCode)

print(f"Copied {len(code_ligatures)} code ligature feature definitions to clipboard.")
print("Paste them into the font's 'calt' feature in Font Info > Features.")