pyvial 0.15.0

A micro web framework for AWS Lambda.

Vial

Vial is an unopinionated micro web framework for AWS Lambda. It's main purpose is to provide an easy to use interface while also making the core functionality of the framework as modular as possible.

Installation

To add vial to your project, run the following command:

poetry add pyvial

Usage

Entry Point

The main entry point of the application is always the Vial#__call__ function. When deploying to AWS Lambda, the Lambda handler should point to the Vial object in whichever file it's defined in. As an example:

from vial.app import Vial

app = Vial(__name__)

If this code snippet is defined in an app.py file, the handler would be app.app.

Basic API

from vial.app import Vial

app = Vial(__name__)


@app.get("/hello-world")
def hello_world() -> dict[str, str]:
    return {"hello": "world"}

Basic serverless.yml file to deploy the project with the serverless framework:

service: my-function
provider:
  name: aws
  runtime: python3.9
  memorySize: 128
  region: us-west-2

package:
  patterns:
    - app.py

functions:
  api:
    handler: app.app
    events:
      - http: get /hello-world

custom:
  pythonRequirements:
    usePoetry: true

plugins:
  - serverless-python-requirements

You can now deploy the project with serverless deploy.

Current Request

The current request is tracked within a contextual object that wraps the lambda request. It can be accessed like so:

from vial import request
from vial.app import Vial
from vial.types import Request

app = Vial(__name__)


@app.get("/hello-world")
def hello_world() -> dict[str, list[str]]:
    request: Request = request.get()
    query_params = request.query_parameters
    if not query_params:
        raise ValueError("Must provide at least one query parameter")
    return dict(query_params)

The request.get() function is only available during a lambda request and will raise an error if called outside of one.

Path Parameters

You can define path parameters like this:

@app.get("/users/{user_id}")
def get_user(user_id: str) -> User:
    return user_service.get(user_id)

Vial supports some path parameter parsing as part of the invocation process. For example when using a UUID as a path parameter, Vial can convert it from a string to a UUID automatically:

from uuid import UUID

@app.get("/users/{user_id:uuid}")
def get_user(user_id: UUID) -> User:
    return user_service.get(user_id)

The following parsers are supported by default:

Parser	Type
str	str
bool	bool
int	int
float	float
decimal	decimal.Decimal
uuid	uuid.UUID

You can register your own parser like this:

@app.parser("list")
def list_parser(value: str) -> list[str]:
    return [value]


@app.get("/users/{user_id:list}")
def get_user(user_ids: list[str]) -> list[User]:
    return [user_service.get(user_id) for user_id in user_ids]

As parsers are bound directly to the registered route function, they have to be defined before the route function that uses one is registered.

Resources

As your application grows, you may want to split certain functionality amongst resources and files, similar to blueprints of other popular frameworks like Flask.

You can define a resource like this:

from dataclasses import dataclass

from vial.app import Resource, Vial

stores_app = Resource(__name__)


@dataclass
class Store:
    store_id: str


@stores_app.get("/stores/{store_id}")
def get_store(store_id: str) -> Store:
    return Store(store_id)


app = Vial(__name__)

app.register_resource(stores_app)

A test case with this example is available in tests/samples/test_with_resources.py.

Middleware

You can register middleware functions to be executed before / after route invocations. All middleware is scoped to where it's registered. A middleware function registered with the Vial instance is scoped to all routes within the application, but a function registered with a Resource instance will only be invoked for routes defined in that specific resource.

Below is an example of registering a middleware to log route invocation:

from __future__ import annotations

from vial import request
from vial.app import Vial
from vial.middleware import CallChain
from vial.types import Request, Response

app = Vial(__name__)


@app.middleware
def log_events(event: Request, chain: CallChain) -> Response:
    app.logger.info("Began execution of %s", event.context)
    event.headers["custom-injected-header"] = "hello there"
    try:
        return chain(event)
    finally:
        app.logger.info("Completed execution of %s", event.context)


@app.get("/hello-world")
def hello_world() -> dict[str, str | list[str]]:
    return {"hello": "world", **request.get().headers}

A test case with this example is available in tests/samples/test_with_middleware.py.

Json Encoding

You can customize how Vial serializes / deserializes JSON objects by passing a custom encoder. The below example shows how to substitute the native JSON module with another library like simplejson:

from decimal import Decimal
from typing import Any

import simplejson

from vial.app import Vial
from vial.json import Json


class SimpleJson(Json):
    @staticmethod
    def dumps(value: Any) -> str:
        return simplejson.dumps(value)

    @staticmethod
    def loads(value: str) -> Any:
        return simplejson.loads(value)


class SimpleJsonVial(Vial):
    json_class = SimpleJson


app = SimpleJsonVial(__name__)


@app.get("/prices")
def get_prices() -> dict[str, Decimal]:
    # Decimal is not supported natively by the json module, but is by simplejson.
    return {"bread": Decimal("42.24"), "cheese": Decimal("129.34")}

A test case with this example is available in tests/samples/test_with_json_encoding.py.

Testing

The vial.gateway.Gateway class provides functionality to interact with the Vial application locally, without deploying to AWS Lambda. It can be constructed using the original Vial application instance, exposing the application endpoints with basic URL path matching.

Here is an example test case using pytest:

from http import HTTPStatus

import pytest

from vial import request
from vial.app import Vial
from vial.errors import BadRequestError
from vial.gateway import Gateway

app = Vial(__name__)


@app.get("/health")
def health() -> dict[str, str]:
    return {"status": "OK"}


@app.post("/stores/{store_id}")
def create_store(store_id: str) -> dict[str, str]:
    if not (body := request.get().body):
        raise BadRequestError("Bad request")
    return {"store_id": store_id, **app.json.loads(body)}


@pytest.fixture(name="gateway")
def gateway_fixture() -> Gateway:
    return Gateway(app)


def test_health(gateway: Gateway) -> None:
    response = gateway.get("/health")
    assert response.status == HTTPStatus.OK
    assert response.body == {"status": "OK"}


def test_create_store(gateway: Gateway) -> None:
    response = gateway.post("/stores/my-cool-store", app.json.dumps({"store_name": "My very cool store"}))
    assert response.status == HTTPStatus.OK
    assert response.body == {"store_id": "my-cool-store", "store_name": "My very cool store"}

This code is also available in tests/samples/test_with_gateway.py.