formatters.html

HTML formatter for Rosettes.

Generates HTML output with semantic or Pygments-compatible CSS classes. Thread-safe by design — no mutable shared state.

Features:

• Dual CSS class output: semantic (.syntax-function) or Pygments (.nf)
• CSS custom properties for runtime theming
• Line highlighting (hl_lines parameter)
• Streaming output (generator-based)

Design Philosophy:

The HTML formatter is optimized for the common case while supporting advanced features when needed:

1. Fast path:format_fast()for simple highlighting (~50µs/block)
2. Slow path:format()for line highlighting, line numbers
3. Immutable: Frozen dataclass ensures thread-safety
4. Streaming: Yields chunks for memory-efficient processing

CSS Class Styles:

• semantic (default): Human-readable class names like.syntax-function, .syntax-keyword. Better for custom themes and debugging.
• pygments: Pygments-compatible names like.nf,.k. Works with existing Pygments CSS themes.

Performance Optimizations:

1. Fast path when no line highlighting needed
2. Pre-computed escape table (C-levelstr.translate)
3. Pre-built span templates (avoid f-string in loop)
4. Direct token type value access (StrEnum)
5. Streaming output (generator, no intermediate list)

Benchmarks (100-line Python file):

• format_fast(): ~50µs
• format()with hl_lines: ~80µs
• format()with line numbers: ~100µs

Common Mistakes:

# ❌ WRONG: Calling format() and ignoring streaming
html = ""
for chunk in formatter.format(tokens):
    html += chunk  # O(n²) string concatenation

# ✅ CORRECT: Use format_string() or join()
html = formatter.format_string(tokens)
# or
html = "".join(formatter.format(tokens))

See Also:

• rosettes.formatters.terminal: ANSI terminal output formatter
• rosettes.formatters.null: No-op formatter for testing
• rosettes.themes: CSS generation for HTML themes
• rosettes._escape: HTML entity escaping (used internally)