widelog

widelog emits one structured event per operation instead of a line per step. Each event carries the context you attached during the operation, how long it took, and errors that say why they happened and what to do about them. It has no dependencies outside the standard library.

widelog is a Python implementation of the wide-event approach from evlog. contextvars takes the place of AsyncLocalStorage, and one ASGI adapter covers what evlog handles with fifteen framework integrations.

Install

uv add widelog-py     # or: pip install widelog-py

Python 3.10 or later. The distribution is widelog-py, the import is widelog.

Log a request

Add the middleware once at startup. Then call use_logger() anywhere in the request to attach fields to that request's event.

from widelog import WidelogMiddleware, init, use_logger

init(service="checkout")
app.add_middleware(WidelogMiddleware)


@app.post("/api/checkout")
async def checkout():
    log = use_logger()
    log.set(user={"id": user.id, "plan": "premium"})
    log.set(cart={"items": 3, "total": 9999})
    return {"ok": True}

One JSON line goes out when the response does:

{
  "timestamp": "2026-07-30T10:23:45.612Z",
  "level": "info",
  "service": "checkout",
  "environment": "production",
  "duration_ms": 1204.7,
  "method": "POST",
  "path": "/api/checkout",
  "status": 200,
  "user": { "id": "u_123", "plan": "premium" },
  "cart": { "items": 3, "total": 9999 }
}

log.set() merges. Dictionaries merge key by key, so a later log.set(user={"tier": "gold"}) adds to user rather than replacing it. Lists concatenate. Anything else overwrites.

Attach fields from anywhere in the call stack

use_logger() reads a contextvars slot, so a function several frames deep writes to the same event without taking a logger argument.

async def charge(cart):
    use_logger().set(payment={"method": "card"})

This is ambient context rather than dependency injection, and the choice is deliberate. FastAPI's Depends resolves only in a handler signature, so injecting the logger would mean passing log down through every helper that wants to add a field. If you want the dependency visible in the signature anyway, wrap it. You get the same object.

Log = Annotated[WideEvent, Depends(use_logger)]


@app.post("/api/checkout")
async def checkout(log: Log): ...

Raise errors that explain themselves

WidelogError carries an HTTP status and a machine-readable code, plus two fields written for whoever reads the log at 3am: why says what went wrong, fix says what to do next.

raise WidelogError(
    "Payment failed",
    code="CARD_DECLINED",
    status=402,
    why="Issuer declined the charge",
    fix="Try a different payment method or contact your bank",
    internal={"processor_ref": "ch_live_x9"},
)

to_dict() returns everything except internal, so you can put the error straight on the wire. FastAPI converts the exception into a response before the middleware sees it, so record it in an exception handler:

@app.exception_handler(WidelogError)
async def on_widelog_error(request, exc):
    use_logger().error(exc)
    return JSONResponse(status_code=exc.status, content=exc.to_dict())

Without that handler the client gets a 500 and the event has no error field.

Declare your errors once

Once a service has more than a handful of errors, writing why and fix at each raise means they drift. Declare them in a catalog instead. Each ErrorSpec becomes a factory whose code is prefix.ATTRIBUTE_NAME, so the code cannot fall out of step with the name.

from widelog import ErrorCatalog, ErrorSpec


class BillingErrors(ErrorCatalog, prefix="billing"):
    CART_EMPTY = ErrorSpec(status=400, message="Cart is empty")
    PAYMENT_DECLINED = ErrorSpec(
        status=402,
        message="Card declined",
        why="Issuer declined the charge",
        fix="Try a different payment method",
        link="https://docs.example.com/errors/payment-declined",
    )
    INSUFFICIENT_FUNDS = ErrorSpec(
        status=402,
        message=lambda available, required: f"Insufficient funds: ${available} of ${required}",
        fix="Add funds and retry",
    )

Raising one reads as the name of the thing that went wrong:

if not cart.items:
    raise BillingErrors.CART_EMPTY()

raise BillingErrors.INSUFFICIENT_FUNDS(available=balance, required=cart.total, cause=exc)

A message that is a function turns its parameters into required keyword arguments, so a templated error cannot be raised with a missing value. Any spec field can be overridden at the call site, and internal merges with the call site winning:

raise BillingErrors.PAYMENT_DECLINED(
    link="/support/payment-issues",  # overrides the spec
    internal={"processor_ref": "ch_x"},  # merged, stays server-side
    cause=stripe_error,
)

Branch on the code without repeating the string anywhere:

except WidelogError as exc:
    if exc.code == BillingErrors.PAYMENT_DECLINED.code:
        ...

BillingErrors.CART_EMPTY.status   # 400
BillingErrors.codes()             # ('billing.CART_EMPTY', 'billing.PAYMENT_DECLINED', ...)

For a single error with no group to join, bind a spec to a code directly:

from widelog import ErrorFactory

fraud_detected = ErrorFactory(
    "billing.FRAUD_DETECTED",
    ErrorSpec(status=403, message="Transaction flagged for review"),
)

Coming from evlog: ErrorCatalog is defineErrorCatalog, ErrorFactory is defineError. A class body replaces the entry map because it is what gives Python editors autocomplete on BillingErrors. and lets a type checker see the attributes. A dict would type as Any.

Log work that is not a request

from widelog import wide_event

with wide_event(job="nightly-reconcile") as log:
    log.set(rows=len(rows))

The event is emitted when the block exits, including when the block raises.

Deploy to AWS Lambda

from widelog import lambda_wide_event, use_logger


@lambda_wide_event
def handler(event, context):
    use_logger().set(order={"id": "o_1"})
    return {"statusCode": 200}

The decorator adds request_id, cold_start, function, remaining_ms, and the X-Ray trace_id. It reads method and path from Function URL, API Gateway v1, and API Gateway v2 payloads.

500ms before the invocation deadline, widelog emits the event with "timed_out": true. Lambda kills the process at the deadline, so otherwise a timed-out invocation leaves no record at all. emit() is idempotent, so a normal return after the guard has fired does nothing.

The default sink writes to stdout, which Lambda forwards to CloudWatch Logs. You do not need a drain or a waitUntil callback.

Examples

uv run examples/aws_lambda_handler.py
uv run --with fastapi --with uvicorn uvicorn examples.fastapi_app:app

The Lambda example needs no AWS account. It prints three events: one authorized, one declined, and one that hits the timeout guard. Both examples are covered by tests/test_examples.py, so they cannot drift out of date without CI noticing.

API

init(service=…, environment=…, redact=…, sink=…)	Configure once at startup.
use_logger()	The current event. Outside an operation, returns a standalone logger that emits on each call.
wide_event(**fields)	Context manager that scopes one event.
log.set(dict) or log.set(**kw)	Attach context. Dictionaries merge, lists concatenate.
log.info/warn/debug/error(…)	Record a message. The most severe level wins.
log.set_level(level)	Pin the level so later error() and warn() calls cannot raise it.
log.emit(**overrides)	Emit and seal the event. Idempotent.
WidelogError(msg, code=, status=, why=, fix=, link=, internal=)	Error with a status and an explanation.
ErrorSpec(message=, status=, why=, fix=, link=, tags=, internal=)	One error declared once. message may be a function of required params.
ErrorCatalog (subclass with prefix=)	Turns each ErrorSpec in the body into a factory coded prefix.NAME. codes() lists them.
ErrorFactory(code, spec)	A spec bound to a code, for an error with no catalog.
WidelogMiddleware	ASGI. Covers FastAPI, Starlette, Litestar, and Django-async.
lambda_wide_event	Decorator for an AWS Lambda handler.

sink takes a callable(dict). Point it at your backend, or leave it unset to write NDJSON to stdout.

Tracebacks

An error on the event carries stack, the innermost frames as path:line in function, and causes, the chain behind it. The chain follows __cause__ before __context__, so raise X from Y and WidelogError(cause=Y) win over an exception that happened to be in flight. raise X from None ends the chain. widelog filters its own frames out, so the first entry is always your code. init(stack_depth=N) changes how many frames are kept, five by default, and none of it reaches to_dict().

Redaction

Keys ending in password, token, secret, authorization, apikey, or cookie are replaced with [REDACTED] at any depth. Matching ignores case, underscores, and hyphens, and looks at the end of the key, so refresh_token, x-api-key, set-cookie, proxy-authorization, and apiKey all match. tokens_used does not, so metrics stay readable. Pass init(redact={…}) to replace the set with your own names. WidelogError(internal=…) is never serialized into the event or into to_dict().

Limits

widelog copies every field as it enters the event, so it never writes back into the dicts and lists you pass to set(). It keeps nesting to 32 levels and replaces anything deeper with [TRUNCATED], which also makes a self-referential payload safe to log.

Sealed events

emit() seals the event. A set() after that prints a warning to stderr and drops the data, so you can see the loss instead of wondering where a field went.

Failure

emit() never raises. If your sink is down, or a field cannot be serialized, widelog reports the dropped event on stderr and returns None. Logging is not allowed to fail the request it is describing, or to replace the exception the application is already handling.

Not implemented

Sampling, batched delivery to a backend, a pretty development terminal, audit hash-chaining, the CLI, SQS and SNS and EventBridge batch triggers, and WSGI for Flask and Django-sync. Open an issue if you need one of them.

Development

uv sync
uv run pytest
uv run ruff check . && uv run ruff format .
uv build

The docs site under docs/ is an Astro project built with Nimbus. It is a separate toolchain from the package, which has no Node dependency at all.

cd docs
bun install
bun run dev          # http://localhost:4321
bun run lint:docs    # frontmatter shape and internal links
bun run build

License

MIT, An Pham. The wide-event design comes from evlog (MIT, HugoRCD). This is an independent Python implementation of it.