1. What FastAPI Is

FastAPI vs Flask vs Django

FastAPI, Flask, and Django address different problem spaces within Python web development. Flask prioritizes minimalism and developer freedom, Django emphasizes batteries-included full-stack development, while FastAPI focuses on correctness, typing, and API contracts. FastAPI is best suited for systems where explicit interfaces, validation, and performance under I/O load are critical.

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Why FastAPI Is API-First, Not Template-First

FastAPI is designed primarily for building APIs rather than server-rendered web pages. Its core abstractions revolve around request schemas, response models, and HTTP semantics instead of templates and views. HTML rendering exists as an integration, but APIs remain the central design concern.

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ASGI, Starlette, and Pydantic Roles

FastAPI is built on top of Starlette, which provides the ASGI-based web layer, routing, and middleware support. Pydantic handles data validation, parsing, and schema generation using Python type hints. Together, they enable FastAPI to deliver high concurrency, strict validation, and accurate OpenAPI documentation.

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()

class Item(BaseModel):
    name: str
    price: float

@app.post("/items")
def create_item(item: Item):
    return item

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When FastAPI Is the Wrong Choice

FastAPI may not be ideal for applications that require a full-stack framework with built-in authentication, admin panels, and ORM integrations. It is also a poor fit for teams uncomfortable with strong typing or projects dominated by CPU-bound workloads. In such cases, frameworks like Django or simpler synchronous tools may be more appropriate.

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Installation

FastAPI is installed using standard Python package management tools and is typically paired with an ASGI server for production use. The framework itself is lightweight, with performance and scalability determined largely by application design.

pip install fastapi
pip install "uvicorn[standard]"