Provider

FastAPI XML provider example.

This module defines a simple provider implemented with fastapi which will be tested with Pact in the provider test. As Pact is a consumer-driven framework, the consumer defines the contract which the provider must then satisfy.

The provider receives requests from the consumer and returns XML responses built using the standard library xml.etree.ElementTree module. Serialisation is handled manually rather than via FastAPI's built-in JSON serialisation, since FastAPI does not natively support XML response bodies.

This also showcases how Pact tests differ from merely testing adherence to an OpenAPI specification. The Pact tests are concerned with the practical use of the API from the consumer's perspective. The User model here could contain additional fields in a real application; if the provider later adds or removes fields, Pact's consumer-driven testing will verify that the consumer remains compatible with those changes.

Note that the code in this module is agnostic of Pact (i.e., this would be your production code). The pact-python dependency only appears in the tests. This is because the provider is not concerned with Pact, only the tests are.

Attributes

app = FastAPI() module-attribute

logger = logging.getLogger(__name__) module-attribute

Classes

User(id: int, name: str) dataclass

Represents a user in the provider system.

This class uses a plain dataclass rather than Pydantic to keep the focus on the XML serialisation pattern. In a real FastAPI application you would typically use a Pydantic model to benefit from automatic validation and JSON serialisation; see the requests_and_fastapi example for that approach.

The provider's model may contain more fields than any single consumer requires. This demonstrates how provider-side data can be richer than what appears in the consumer contract.

Attributes

id: int instance-attribute

name: str instance-attribute

UserDb

A simple in-memory user database abstraction for demonstration purposes.

This class simulates a user database using a class-level dictionary. In a real application this would interface with a persistent database or external service. For testing, the state handlers in the provider test populate and clean up this database directly, ensuring each contract interaction runs against a predictable state.

Functions

create(user: User) -> None classmethod

Add a new user to the database.

PARAMETER	DESCRIPTION
user	The User instance to add. TYPE: User

Source code in examples/http/xml_example/provider.py

@classmethod
def create(cls, user: User) -> None:
    """
    Add a new user to the database.

    Args:
        user: The User instance to add.
    """
    cls._db[user.id] = user

delete(user_id: int) -> None classmethod

Delete a user from the database by their ID.

PARAMETER	DESCRIPTION
user_id	The ID of the user to delete. TYPE: int

RAISES	DESCRIPTION
KeyError	If the user does not exist.

Source code in examples/http/xml_example/provider.py

@classmethod
def delete(cls, user_id: int) -> None:
    """
    Delete a user from the database by their ID.

    Args:
        user_id: The ID of the user to delete.

    Raises:
        KeyError: If the user does not exist.
    """
    if user_id not in cls._db:
        msg = f"User {user_id} does not exist."
        raise KeyError(msg)
    del cls._db[user_id]

get(user_id: int) -> User | None classmethod

Retrieve a user by their ID.

PARAMETER	DESCRIPTION
user_id	The ID of the user to retrieve. TYPE: int

RETURNS	DESCRIPTION
User | None	The User instance if found, else None.

Source code in examples/http/xml_example/provider.py

@classmethod
def get(cls, user_id: int) -> User | None:
    """
    Retrieve a user by their ID.

    Args:
        user_id: The ID of the user to retrieve.

    Returns:
        The User instance if found, else None.
    """
    return cls._db.get(user_id)

Functions

get_user_by_id(uid: int) -> Response async

Retrieve a user by their ID, returning an XML response.

PARAMETER	DESCRIPTION
uid	The user ID to retrieve. TYPE: int

RAISES	DESCRIPTION
HTTPException	If the user is not found, a 404 error is raised.

Source code in examples/http/xml_example/provider.py

@app.get("/users/{uid}")
async def get_user_by_id(uid: int) -> Response:
    """
    Retrieve a user by their ID, returning an XML response.

    Args:
        uid:
            The user ID to retrieve.

    Raises:
        HTTPException: If the user is not found, a 404 error is raised.
    """
    logger.debug("GET /users/%s", uid)
    user = UserDb.get(uid)
    if not user:
        raise HTTPException(
            status_code=status.HTTP_404_NOT_FOUND,
            detail="User not found",
        )
    root = ET.Element("user")
    ET.SubElement(root, "id").text = str(user.id)
    ET.SubElement(root, "name").text = user.name
    return Response(
        content=ET.tostring(root, encoding="unicode"),
        media_type="application/xml",
    )