Handling Errors in FontForge Python Scripts

Automating font operations with FontForge's Python extension often leads to runtime failures caused by corrupt font files, missing glyphs, or invalid OpenType tables. This article explains how to effectively catch and manage exceptions in FontForge scripts, maintain stability during batch operations, and ensure system resources are safely released.

Standard Exception Handling in FontForge

FontForge functions generally raise standard Python built-in exceptions rather than a wide array of proprietary error classes. Operations such as opening an invalid file typically raise an EnvironmentError or generic OSError, while invalid parameters raise ValueError or TypeError.

Use standard try-except blocks around native FontForge calls to prevent unexpected termination:

import fontforge

font_path = "problematic_font.ttf"

try:
    font = fontforge.open(font_path)
except EnvironmentError as e:
    print(f"Failed to open font file {font_path}: {e}")
except Exception as e:
    print(f"An unexpected error occurred: {e}")

Handling Missing Glyphs Safely

Attempting to access a glyph that does not exist in a font via direct indexing will raise an exception. Instead of relying on exceptions for normal control flow, verify glyph existence using the in operator, or handle the TypeError and KeyError properly.

glyph_name = "uni0041"

# Preferred method: check membership before access
if glyph_name in font:
    glyph = font[glyph_name]
    glyph.clear()
else:
    print(f"Glyph '{glyph_name}' does not exist in the font.")

If querying glyphs dynamically by codepoint or name where availability is uncertain, structure your retrieval inside an explicit exception handler:

try:
    glyph = font[0x1234]
except (TypeError, KeyError, ValueError) as err:
    print(f"Could not retrieve glyph: {err}")

Ensuring Resource Cleanup with Finally

FontForge loads entire font binaries into memory. In batch processing scripts, unclosed fonts cause memory leaks that degrade performance and crash the Python runtime. Always ensure font.close() is invoked using a try...finally block.

import fontforge

font = None
try:
    font = fontforge.open("MyFont.otf")
    # Perform modifications
    font.generate("MyFont-Modified.otf")
except Exception as err:
    print(f"Processing failed: {err}")
finally:
    if font is not None:
        font.close()

Managing Batch Processing Failures

When processing directories containing hundreds of fonts, a single corrupt font should not stop the entire pipeline. Combine error handling with Python's built-in logging module to track failed files while allowing the loop to continue.

import fontforge
import logging
import os

logging.basicConfig(
    filename="font_processing_errors.log",
    level=logging.ERROR,
    format="%(asctime)s - %(levelname)s - %(message)s"
)

font_files = ["font1.ttf", "corrupted.ttf", "font2.ttf"]

for file_name in font_files:
    font = None
    try:
        font = fontforge.open(file_name)
        font.version = "1.1"
        font.generate(file_name)
    except Exception as exc:
        logging.error(f"Error processing {file_name}: {exc}", exc_info=True)
    finally:
        if font is not None:
            font.close()

Handling Generation Warnings and Errors

The font.generate() method can fail if glyph contours are self-intersecting or if metadata is invalid. FontForge may display C-level warnings to standard output without throwing a Python exception. To catch generation errors:

1. Validate outlines before saving using glyph.isWorthOutputting() and glyph.selfIntersects().
2. Wrap font.generate() inside an exception handler to catch filesystem write errors, permission issues, and critical OpenType table creation failures.