Web Frameworks — FastAPI and Flask Integration with Slomo

Web frameworks catch your exceptions and turn them into 500 responses before sys.excepthook ever sees them. That’s exactly the failure mode slomo’s auto-tracing solves: the exception is recorded the moment it escapes your handler function, before the framework swallows it — with arguments, locals, and the SQL/HTTP calls that led up to it. The integration for every framework is the same line:

import slomo

slomo.enable()

FastAPI

from fastapi import FastAPI
import slomo

slomo.enable()   # that's the whole integration
app = FastAPI()

@app.get("/checkout/{sku}")
async def checkout(sku: str):
    ...

Call enable() at import time of your app module (before the routes run). With multiple workers (uvicorn/gunicorn), each worker process records its own session — forked children are labeled forked_from in their metadata. A 500 in the checkout route lands in slomo issues like any crash: fingerprinted, classified, with the route function’s args (sku), its locals, and any database or HTTP activity from the request.

The repository’s examples/fastapi_app.py is a complete runnable app showing lifespan integration, tracked async routes, per-request middleware events, and background-task failures.

Flask

from flask import Flask
import slomo

slomo.enable()
app = Flask(__name__)

@app.route("/checkout/<sku>")
def checkout(sku):
    ...

Same story: view functions are project code, so they’re auto-traced; escaping exceptions are recorded before Flask converts them to error responses. See examples/flask_app.py.

Per-request context

Two useful patterns for request-scoped visibility:

from slomo import event, snapshot

@app.middleware("http")                       # FastAPI
async def request_events(request, call_next):
    event("request.started", method=request.method, path=request.url.path)
    response = await call_next(request)
    event("request.finished", status=response.status_code)
    return response

snapshot("before-payment", user_id=user.id, cart_total=total)

Custom events make slomo timeline read like an access log interleaved with your traces, and slomo search path=/checkout finds every request to a route.

Diagnosing a production-shaped 500

$ slomo issues                       # the 500 is an issue now
$ slomo doctor SM-1a2b3c4d           # root-cause heuristic
$ slomo timeline SM-1a2b3c4d         # the request: middleware events, SQL, the crash
$ slomo http SM-1a2b3c4d             # outbound HTTP made during the request
$ slomo replay SM-1a2b3c4d           # step through it

Live-tailing during development

Run your dev server in one terminal and follow the recording in another:

slomo timeline --follow          # live event feed of the running session
slomo timeline --follow --errors # only warnings and errors

Every request, query, and warning streams by as it happens — a structured alternative to print debugging that you can search afterwards.

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Core Concepts

Recording

Investigating

Configuration

Web Frameworks — FastAPI and Flask Integration with Slomo

FastAPI

Flask

Per-request context

Diagnosing a production-shaped 500

Live-tailing during development

​FastAPI

​Flask

​Per-request context

​Diagnosing a production-shaped 500

​Live-tailing during development

FastAPI

Flask

Per-request context

Diagnosing a production-shaped 500

Live-tailing during development