Coming from a finance background, my first love in Python was pandas. Naturally, when I started building web apps, I gravitated towards Flask. It’s simple, synchronous, and gets the job done.
But as my applications grew—specifically Taxpedited, which processes massive datasets for tax compliance—I hit a bottleneck. The blocking nature of WSGI servers meant that long-running calculation tasks were holding up the entire request queue.
Enter FastAPI.
01. The Async Advantage
The killer feature of FastAPI isn't just the speed (though it is fast); it's the native support for Python's async and await keywords.
In a traditional Flask app, if a user requests a heavy database report, the worker is blocked until that database returns. In FastAPI, we can define that route as async, allowing the server to handle other traffic while waiting for the database I/O.
from fastapi import FastAPI
import asyncio
app = FastAPI()
@app.get("/heavy-report")
async def generate_report():
# This non-blocking sleep simulates a DB call
await asyncio.sleep(5)
return {"status": "Report Generated"}02. Pydantic: Data Validation That Doesn't Suck
In accounting, accuracy is everything. Validating inputs in Flask usually involved custom decorators or Marshmallow schemas that felt disconnected from the code logic.
FastAPI uses Pydantic. You define your data shape as a Python class, and type hints handle the rest. If a client sends a string where an integer is expected, FastAPI rejects it automatically with a clear error message.
from pydantic import BaseModel
from typing import Optional
class TaxEntry(BaseModel):
company_name: str
fiscal_year: int
total_revenue: float
is_audited: bool = False
@app.post("/entries/")
async def create_entry(entry: TaxEntry):
# 'entry' is already validated & converted here
return entry03. Automatic Documentation
The feature that saved me the most time as a solo developer? Swagger UI.
FastAPI inspects your Pydantic models and route definitions to generate an interactive API documentation page at /docs. I didn't have to write a single line of YAML or HTML for it. It allows me to test endpoints directly from the browser, which accelerated my debugging process significantly.
Conclusion
Migrating to FastAPI didn't just make my applications faster; it made my development workflow cleaner. The strict type enforcement prevents "silent failures" typical in financial data processing, and the async capabilities allow my servers to punch way above their weight class.