How to return 400 (Bad Request) on Flask?

You have a variety of options:

The most basic:

@app.route('/')def index():    return "Record not found", 400

If you want to access the headers, you can grab the response object:

@app.route('/')def index():    resp = make_response("Record not found", 400)    resp.headers['X-Something'] = 'A value'    return resp

Or you can make it more explicit, and not just return a number, but return a status code object

from flask_api import status@app.route('/')def index():    return "Record not found", status.HTTP_400_BAD_REQUEST

Further reading:

You can read more about the first two here: About Responses (Flask quickstart)
And the third here: Status codes (Flask API Guide)

I like to use the flask.Response class:

from flask import Response@app.route("/")def index():    return Response(        "The response body goes here",        status=400,    )

flask.abort is a wrapper around werkzeug.exceptions.abort which is really just a helper method to make it easier to raise HTTP exceptions. That's fine in most cases, but for restful APIs, I think it may be better to be explicit with return responses.

Here's some snippets from a Flask app I wrote years ago. It has an example of a 400 response

import werkzeugfrom flask import Flask, Response, jsonfrom flask_restplus import reqparse, Api, Resource, abortfrom flask_restful import requestfrom flask_cors import CORSapp = Flask(__name__)CORS(app)api = Api(app)parser = reqparse.RequestParser()parser.add_argument('address_to_score', type=werkzeug.datastructures.FileStorage, location='files')class MissingColumnException(Exception):    passclass InvalidDateFormatException(Exception):    pass@api.route('/project')class project(Resource):    @api.expect(parser)    @api.response(200, 'Success')    @api.response(400, 'Validation Error')    def post(self):        """        Takes in an excel file of addresses and outputs a JSON with scores and rankings.        """        try:            df, input_trees, needed_zones = data.parse_incoming_file(request)        except MissingColumnException as e:            abort(400, 'Excel File Missing Mandatory Column(s):', columns=str(e))        except Exception as e:            abort(400, str(e))        project_trees = data.load_needed_trees(needed_zones, settings['directories']['current_tree_folder'])        df = data.multiprocess_query(df, input_trees, project_trees)        df = data.score_locations(df)        df = data.rank_locations(df)        df = data.replace_null(df)        output_file = df.to_dict('index')        resp = Response(json.dumps(output_file), mimetype='application/json')        resp.status_code = 200    return resp@api.route('/project/health')class projectHealth(Resource):    @api.response(200, 'Success')    def get(self):        """        Returns the status of the server if it's still running.        """        resp = Response(json.dumps('OK'), mimetype='application/json')        resp.status_code = 200    return resp