VOWEL

YAML-based evaluation framework for testing Python functions with AI-powered test generation, function healing and TDD approach.

vowel makes it easy to define test cases in YAML and run them against your Python functions. It also provides AI-powered generators that can automatically create test specs, generate implementations, and fix buggy functions.

Installation

pip install vowel

# Or with uv
uv add vowel

Optional Dependencies

Vowel supports several optional dependency groups for enhanced functionality:

Group	Install Command	Purpose / Extras
all	pip install vowel[all]	All optional features
dev	pip install vowel[dev]	Development & testing tools
mcp	pip install vowel[mcp]	MCP server
optimization	pip install vowel[optimize]	Performance optimizations
monty	pip install vowel[monty]	Monty runtime support
logfire	pip install vowel[logfire]	Logfire integration

Tip:
You can install multiple extras at once, e.g.
pip install vowel[dev,mcp] Recommended: pip install vowel[all]

Development

git clone https://github.com/fswair/vowel.git
cd vowel
pip install -e ".[all]"

---

Quick Start

Note:
For a deeper understanding of how vowel handles fixtures, see the examples in db_fixture.yml and db.py. These files demonstrate the underlying mechanics of fixture setup and usage.

Tip:
To enable YAML schema validation in your editor, place vowel-schema.json in your project directory.
Then, add the following directive at the top of your YAML file to activate schema support and instructions:

# yaml-language-server: $schema=<path/to/vowel-schema.json>

Replace <path/to/vowel-schema.json> with the actual path to your schema file.

1. Create a YAML spec

# evals.yml
add:
  dataset:
    - case:
        inputs: { x: 2, y: 2 }
        expected: 4
    - case:
        inputs: { x: -5, y: 5 }
        expected: 0

divide:
  evals:
    Type:
      type: "float"
  dataset:
    - case:
        inputs: { a: 10, b: 2 }
        expected: 5.0
    - case:
        inputs: { a: 1, b: 0 }
        raises: ZeroDivisionError

2. Run from CLI

vowel evals.yml

3. Or programmatically

from vowel import run_evals

def add(x: int, y: int) -> int:
    return x + y

def divide(a: float, b: float) -> float:
    return a / b

summary = run_evals("evals.yml", functions={"add": add, "divide": divide})
print(f"All passed: {summary.all_passed}")
print(f"Coverage: {summary.coverage * 100:.1f}%")

4. Or use the fluent API

from vowel import RunEvals

summary = (
    RunEvals.from_file("evals.yml")
    .with_functions({"add": add, "divide": divide})
    .filter(["add"])
    .debug()
    .run()
)

summary.print()

---

Features

Evaluators

8 built-in evaluators for flexible testing:

Evaluator	Purpose
Expected	Exact value matching
Type	Return type checking (strict/lenient)
Assertion	Custom Python expressions (output > 0, output == input * 2)
Duration	Performance constraints (function-level & case-level)
Pattern	Regex validation on output
ContainsInput	Verify output contains the input
Raises	Exception class + optional message matching
LLMJudge	AI-powered rubric evaluation

factorial:
  evals:
    Assertion:
      assertion: "output > 0"
    Type:
      type: "int"
    Duration:
      duration: 1.0
  dataset:
    - case: { input: 0, expected: 1 }
    - case: { input: 5, expected: 120 }

Full reference: docs/EVALUATORS.md

Fixtures (Dependency Injection)

Inject databases, temp files, caches into functions under test. Three patterns: generator (yield), tuple (setup/teardown), simple (setup only).

fixtures:
  db:
    setup: myapp.setup_db
    teardown: myapp.close_db
    scope: module

query_user:
  fixture: [db]
  dataset:
    - case:
        inputs: { user_id: 1 }
        expected: { name: "Alice" }

def query_user(user_id: int, *, db: dict) -> dict | None:
    return db["users"].get(user_id)

Full reference: docs/FIXTURES.md

Input Serializers

Transform YAML inputs into Pydantic models, dates, or custom types:

summary = (
    RunEvals.from_file("evals.yml")
    .with_functions({"get_user": get_user})
    .with_serializer({"get_user": User})      # Schema mode
    .run()
)

Full reference: docs/SERIALIZERS.md

AI-Powered Generation

EvalGenerator — test existing functions

from vowel import EvalGenerator, Function

generator = EvalGenerator(model="openai:gpt-4o", load_env=True)
func = Function.from_callable(my_function)

result = generator.generate_and_run(func, auto_retry=True, heal_function=True)
print(f"Coverage: {result.summary.coverage * 100:.1f}%")

TDDGenerator — generate everything from a description

from vowel.tdd import TDDGenerator

generator = TDDGenerator(model="gemini-3-flash-preview", load_env=True)

result = generator.generate_all(
    description="Binary search for target in sorted list. Returns index or -1.",
    name="binary_search"
)

result.print()  # Shows: signature → tests → code → results

Step-by-step control:

description = "Calculate factorial of a non-negative integer"
signature = generator.generate_signature(description=description, name="factorial")
runner, yaml_spec = generator.generate_evals_from_signature(signature, description=description)
func = generator.generate_implementation(signature, yaml_spec, description=description)
summary = runner.with_functions({"factorial": func.impl}).run()

Full reference: docs/AI_GENERATION.md

MCP Server

Expose vowel's capabilities to AI assistants like Claude Desktop via Model Context Protocol.

Setup guide: docs/MCP.md

---

CLI

vowel evals.yml                          # Run single file
vowel -d ./tests                         # Run directory
vowel evals.yml -f add,divide            # Filter functions
vowel evals.yml --ci --cov 90         # CI mode
vowel evals.yml --watch                  # Watch mode
vowel evals.yml --dry-run                # Show plan without running
vowel evals.yml --export-json out.json   # Export results
vowel evals.yml -v                       # Verbose summary
vowel evals.yml -v --hide-report         # Verbose, hide pydantic_evals report

Full reference: docs/CLI.md

---

EvalSummary

summary = run_evals("evals.yml", functions={...})

summary.all_passed       # bool
summary.success_count    # int
summary.failed_count     # int
summary.total_count      # int
summary.coverage         # float (0.0-1.0)
summary.failed_results   # list[EvalResult]

summary.meets_coverage(0.9)    # Check threshold
summary.print()                # Rich formatted output
summary.to_json()              # Export as dict
summary.xml()                  # Export as XML

---

Documentation

Document	Description
YAML Spec	Complete YAML format reference
Evaluators	All 8 evaluator types
Fixtures	Dependency injection guide
Serializers	Input serializer patterns
AI Generation	EvalGenerator & TDDGenerator
CLI	Command-line reference
MCP Server	AI assistant integration
Troubleshooting	Common errors & solutions

---

License

Apache License 2.0