FastAPI¶

Logfire combines custom and third-party instrumentation for FastAPI with the logfire.instrument_fastapi() method.

Installation¶

Install logfire with the fastapi extra:

pipuvpoetry

pip install 'logfire[fastapi]'

uv add 'logfire[fastapi]'

poetry add 'logfire[fastapi]'

Usage¶

We have a minimal example below. Please install Uvicorn to run it:

pip install uvicorn

You can run it with python main.py:

main.py

import logfire
from fastapi import FastAPI

app = FastAPI()

logfire.configure()
logfire.instrument_fastapi(app)


@app.get("/hello")
async def hello(name: str):
    return {"message": f"hello {name}"}


if __name__ == "__main__":
    import uvicorn

    uvicorn.run(app)

Then visit http://localhost:8000/hello?name=world and check the logs.

logfire.instrument_fastapi() accepts arbitrary additional keyword arguments and passes them to the OpenTelemetry FastAPIInstrumentor.instrument_app() method. See their documentation for more details.

Endpoint arguments and validation errors¶

logfire.instrument_fastapi() adds the following attributes to the request spans:

fastapi.arguments.values: A dictionary mapping argument names of the endpoint function to parsed and validated values.
fastapi.arguments.errors: A list of validation errors for any invalid inputs.

You can customize these attributes by passing a request_attributes_mapper function to instrument_fastapi. This function will be called with the Request or WebSocket object and a dictionary containing keys values and errors corresponding to the attributes above. It should return a new dictionary of attributes. For example:

import logfire

app = ...


def request_attributes_mapper(request, attributes):
    if attributes["errors"]:
        # Only log validation errors, not valid arguments
        return {
            # This will become the `fastapi.arguments.errors` attribute
            "errors": attributes["errors"],
            # Arbitrary custom attributes can also be added here
            "my_custom_attribute": ...,
        }
    else:
        # Don't log anything for valid requests
        return {}


logfire.configure()
logfire.instrument_fastapi(app, request_attributes_mapper=request_attributes_mapper)

Note

The request_attributes_mapper function mustn't mutate the contents of values or errors, but it can safely replace them with new values.

Timestamps of argument parsing and endpoint execution¶

logfire.instrument_fastapi() also adds the following attributes to the request spans:

The times when parsing arguments and resolving dependencies started and ended:
- fastapi.arguments.start_timestamp
- fastapi.arguments.end_timestamp
The times when the actual endpoint function started and ended executing, leaving out the time spent on dependencies and middleware:
- fastapi.endpoint_function.start_timestamp
- fastapi.endpoint_function.end_timestamp

Spans for argument parsing and endpoint execution¶

You can also enable spans for argument parsing and endpoint execution with logfire.instrument_fastapi(app, extra_spans=True). The main request span will still have the attributes described above, but it will also have two extra child spans. This is mostly redundant now and is mainly provided for backwards compatibility. It can also be useful for grouping together child logs and spans produced by the request.