Flask · Lesson 7 of 15

JSON APIs

Build a small JSON API with status codes, error handlers and marshmallow or pydantic.

Intermediate
15 min read
3 objectives

Before this lessonLesson 6: Authentication with Flask-Login

What you will learn

Return JSON
Validate a body
Map errors to status codes

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Flask can serve a JSON API. You do the validation and status codes yourself (or with marshmallow / pydantic). FastAPI automates more of this; Flask stays explicit.

JSON views

from flask import Blueprint, jsonify, request, abort

api = Blueprint("api", __name__, url_prefix="/api")

@api.route("/tasks", methods=["GET"])
def list_tasks():
    tasks = Task.query.order_by(Task.id).all()
    return jsonify([{"id": t.id, "title": t.title, "done": t.done} for t in tasks])

@api.route("/tasks", methods=["POST"])
def create_task():
    data = request.get_json(silent=True) or {}
    title = (data.get("title") or "").strip()
    if not title:
        return jsonify(error="title is required"), 400
    t = Task(title=title)
    db.session.add(t)
    db.session.commit()
    return jsonify(id=t.id, title=t.title, done=t.done), 201

Validation with pydantic

from pydantic import BaseModel, ValidationError, Field

class TaskIn(BaseModel):
    title: str = Field(min_length=1, max_length=100)
    done: bool = False

@api.route("/tasks", methods=["POST"])
def create_task():
    try:
        body = TaskIn.model_validate(request.get_json())
    except ValidationError as e:
        return jsonify(errors=e.errors()), 422
    ...

Error handler

@api.errorhandler(404)
def not_found(e):
    return jsonify(error="Not found"), 404

Up next · Lesson 8Error Handlers and LoggingCatch 404/500 consistently and log with a request id.