Async

Fastapi-icontract works out-of-the-box with the async functions in conditions and snapshot captures. If a condition or a capture returns a coroutine, the coroutine is awaited first and then tested for truthiness or captured, respectively.

However, Python 3 does not allow async lambdas (see this Python issue), so you need to use a third-party library such as asyncstdlib.

For example, note how we transform each book in the result in the following post-condition and verify that the author of the book exists in the system using an async function has_author:

@app.get("/books_in_category", response_model=List[Book])
@fastapi_icontract.require(
    has_category,
    status_code=404,
    description="The category must exist."
)
@fastapi_icontract.ensure(
    lambda result: a.all(await_each(has_author(book.author) for book in result)),
    description="One ore more authors of the resulting books do not exist."
)
async def books_in_category(category: str) -> Any:
    """Retrieve the books of the given category from the database."""

The full example is available at tests.example.books_in_category().