YATL — Yet Another Testing Language

YATL is a declarative, YAML‑based testing language for API testing. If you know HTTP and YAML, you know YATL.

Quick start

pip install yatl-testing

Create your first test file ping.yatl.yaml:

name: ping
base_url: google.com

steps:
  - name: simple_test
    request:
      method: GET
    expect:
      status: 200

Run it:

yatl .

That’s it!

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Consider another example, let's try to test a simple POST request:

We will use FastAPI to create a simple API:

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class User(BaseModel):
    name: str
    age: int

db = [
    User(name="John", age=30),
    User(name="Jane", age=25),
]

@app.post("/users", status_code=201)
def create_user(user: User):
    db.append(user)
    return {"status": "ok"}

Create a test file users.yatl.yaml:

name: User API
base_url: https://localhost:8000

steps:
  - name: create_user
    description: Create a new user with name=John and age=30
    request:
      method: POST
      url: /users
      body:
        json:
          name: John
          age: 30
    expect:
      status: 201
      body:
        json:
          status: ok

Run it:

yatl .

That’s it!

Why YATL?

Writing API tests in code is cumbersome. YATL turns tests into pure data — declarative, readable, and accessible to every team member.

The Problem

• You have to write code – even for a simple GET request
• High entry barrier – need to know programming languages well
• Complex dependencies – chaining requests becomes spaghetti code
• Hard to maintain – tests become unreadable over time

The Solution

YATL is a domain‑specific language that lets you describe API tests in clean YAML. No imperative code, no hidden magic.

If you know HTTP and YAML, you know YATL.

Key Features

• Declarative syntax – describe what to test, not how
• Data extraction & templating – use Jinja2 to reuse response data
• Multiple data formats – JSON, XML, form data, multipart files
• Parallel execution – run tests in parallel with --workers
• Skip tests & steps – disable tests without deleting them
• Advanced validation – validate with rules like gt, regex, type

Example

name: User API
base_url: https://api.example.com

steps:
  - name: login
    request:
      method: POST
      url: /auth/login
      body:
        json:
          username: "test"
          password: "secret"
    expect:
      status: 200
    extract:
      token: "response.access_token"

  - name: get_profile
    request:
      method: GET
      url: /profile
      headers:
        Authorization: "Bearer {{ token }}"
    expect:
      status: 200

Usage

Running Tests

# Run all `.yatl.yaml` files in a directory
yatl .


# Run with 5 parallel workers on `tests/` directory
yatl tests/ --workers 5

Writing Tests

Every YATL test is a YAML file with a .yatl.yaml extension. The structure is simple:

name: Test Suite Name
base_url: https://api.example.com

steps:
  - name: step_one
    request:
      method: GET
      url: /endpoint
    expect:
      status: 200
      body:
        json:
          field: "expected_value"

See the full documentation for all available options.

Documentation

Full documentation is available in the docs/ directory:

• Usage Guide (English)
• Руководство по использованию (Russian)

CI/CD Integration

YATL fits seamlessly into CI pipelines. Example GitHub Actions workflow:

testing_api:
  runs-on: ubuntu-latest
  steps:
    - uses: actions/checkout@v4
    - name: Setup Python
      uses: actions/setup-python@v4
      with:
        python-version: 3.14

    - name: install yatl
      run: pip install yatl-testing

    - name: run tests
      run: yatl ./api_tests

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If you find this project useful, please star it on GitHub. It really motivates me! ⭐️