pyfastyaml

pyfastyaml — Fast YAML parser for Python with C++/SIMD backend. 12–18× faster than PyYAML (CSafeLoader/libyaml), 100–170× faster than PyYAML (SafeLoader), 150–220× faster than ruamel.yaml. API compatible with PyYAML for typical config use cases (Kubernetes, Ansible, CI, Docker Compose).

Features

• High performance: C++17 implementation with AVX2 SIMD on x86 — orders of magnitude faster than pure-Python parsers
• PyYAML compatible API: loads(s), load(fp), load_all(s), dump(), dump_all() — drop-in replacement for config loading
• Types: dict, list, str, int, float, bool, null, nested structures
• Indentation: 2-space (preferred), 4-space, tabs
• Flow syntax: [a, b, c], {a: 1, b: 2} (incl. nested)
• Multiline strings: | (literal), > (folded)
• Anchors and aliases: &anchor, *alias
• Merge key: <<: *anchor and <<: [*a, *b]
• Document markers: ---, ..., %YAML directive

Installation

pip install pyfastyaml

Requires Python 3.10+ and a C++ compiler with C++17 support.

Usage

import pyfastyaml

# Parse string
data = pyfastyaml.loads("""
key: value
nested:
  inner: 42
items:
  - a
  - b
""")
# {'key': 'value', 'nested': {'inner': 42}, 'items': ['a', 'b']}

# Parse file (path or file-like object)
data = pyfastyaml.load("config.yaml")

# Serialize to YAML
yaml_str = pyfastyaml.dump(data)
pyfastyaml.dump(data, stream=open("out.yaml", "w"))

API matches PyYAML for common cases:

# Drop-in replacement
import pyfastyaml as yaml  # instead of import yaml
data = yaml.loads(yaml_string)
data = yaml.load("config.yaml")

Benchmark

Parsing time (μs, lower is better):

Size	pyfastyaml	PyYAML (C)	PyYAML	ruamel
Small (~200 B)	~2.3	~32	~229	~376
Medium (~1.5 KB)	~11	~134	~1,014	~1,689
Large (~15 KB)	~83	~1,483	~11,169	~17,954
Realworld	~6.4	~98	~833	~1,388

PyYAML (C) = CSafeLoader (libyaml extension). PyYAML = SafeLoader (pure Python).

Run benchmarks:

pip install pytest-benchmark PyYAML ruamel.yaml
pytest tests/test_benchmark.py -v --benchmark-only

PyYAML Compatibility

pyfastyaml produces the same result as yaml.safe_load() for block-style YAML typical of configs. Compatibility tests included in tests/test_pyyaml_compat.py.

Serialization (dump)

# Single document
yaml_str = pyfastyaml.dump({"a": 1, "b": [1, 2, 3]})

# Write to file (path or stream)
pyfastyaml.dump(data, "output.yaml")
pyfastyaml.dump(data, stream=open("out.yaml", "w"))

# Deterministic output (sorted keys)
yaml_str = pyfastyaml.dump(data, sort_keys=True)

# Multiple documents
yaml_str = pyfastyaml.dump_all([{"a": 1}, {"b": 2}])

Multi-document streams

docs = pyfastyaml.load_all("""
---
a: 1
---
b: 2
""")
# [{'a': 1}, {'b': 2}]

Limitations

Not yet supported:

• Custom tags
• Complex flow keys (? syntax)

Development

pip install -e ".[dev]"
python setup.py build_ext --inplace
pytest tests/

ConfigLint (experimental)

configlint is an experimental YAML dev-tool (lint / fix / format) built on top of FastYAML.

Configuration (optional): create .configlint.yaml in your repo root (auto-discovered).

Install (from PyPI, configlint ships with pyfastyaml):

pip install pyfastyaml

Run:

# If configlint is on PATH
configlint check path/to/configs
configlint fix path/to/configs --keep last
configlint format path/to/configs
configlint format path/to/configs --check

# Or always works without PATH changes
python -m configlint check path/to/configs
python -m configlint fix path/to/configs --keep last
python -m configlint format path/to/configs

# stdin modes (useful in CI / editors)
type config.yaml | python -m configlint check --stdin --path config.yaml
type config.yaml | python -m configlint fix --stdin --path config.yaml > config.yaml.fixed
type config.yaml | python -m configlint format --stdin > config.yaml.formatted

Benchmark commands:

pytest tests/test_benchmark.py -v --benchmark-only
pytest tests/test_benchmark.py -v --benchmark-only --benchmark-autosave
pytest tests/test_benchmark.py -v --benchmark-only --benchmark-compare

Publishing (maintainers)

pip install build twine
python -m build
python -m twine upload dist/*

License

MIT